BASIC STUDY MATERIAL FOR SWITCHGEAR

SELECTION, MAINTENANCE AND

PROTECTION

Electrical system training centre, lallaguda

South central railway

1

Index

Sr. No Topic Page

1 LV switchgear terminology 1

2 Introduction to LV Switchgear 7

3 Contactors 8

4 Protection of LV motors 15

5 Starter 20

6 Fuse 22

7 Switch Disconnector Fuse 27

8 Moulded Case Circuit Breaker (MCCB) 29

9 Motor Protection Circuit Breaker (MPCB) 36

10 . Miniature Circuit Breaker (MCB) 38

11 Residual Current Circuit Breaker (RCCB) 40

2

LOW VOLTAGE SWITCHGEAR TERMINOLOGY

Switchgear: General term covering switching devices and their combination with associated control, measuring, protective and regulating equipment, also assemblies of such devices and equipments with associated interconnections, accessories, enclosures and supporting structures, intended in principle for use in connection with generation, transmission, distribution and conversion of electric energy.

What is switchgear? A device or a combination of devices, primarily intended for the purpose of making, carrying

and breaking electric currents in circuits under normal conditions as well as under abnormal (faulty) conditions.

Controlgear: General term covering switching devices and their combination with associated control, measuring, protective and regulating equipment, also assemblies of such devices and equipments- with associated interconnections, accessories, enclosures and supporting structures, intended in principle for the control of electric energy consuming equipment

The LV Switchgear Terminologies are generally with respect to the following parameters:

• Voltages• Currents• behavior,under Normal & Overload Conditions• Behavior under Short Circuit Conditions• Utilization categories

TERMINOLOGIES RELATED TO VOLTAGE

Rated Operational Voltage (Ue): This is the voltage that available continuously across the terminals of different phases of the switchgear. This rating will be 415V for LV Switchgear @50Hz is present between the terminals, there will not be any flash-over (i.e.) the insulation between phases is sufficient to tolerate a potential difference of 415V continuously.Rated Insulation Withstand Voltage (Ui): This is thevalue of voltage to which dielectric tests and creepage distances are referred. Normally this would be 690V for LV Switchgear. Even under the worst condition , the 415V system voltage would not go beyond 595V(the permissible fluctuations in the supply voltage (+10%) vide IER 54 - and the worst probable tap ratios in transformers (+/- 12.5%)).. So, switchgear rated for a Ui of 690V would very well withstand the worst condition.

Rated Impulse Withstand Voltage (Uimp): There could be impulse voltages striking the system - either a Lightning Impulse (LI) or a Switching Impulse (SI). These are very high magnitude but very short duration voltage waves. Typically, they are high frequency. They can strike our system and our switchgear might fail. So, this rating is also given for switchgear.

TERMINOLOGIES RELATED TO CURRENT

Rated Free Air Thermal Current (Ith):The maximum current that the switchgear can carry continuously for a period of 8 hours, without the temperature rise of any part reaching beyond permissible limits by conducting test in open-air conditions.

3

Rated Enclosed Thermal Current (Ithe): The maximum current that the switchgear can carry continuously for aperiod of 8 hours, without the temperature rise of any part reaching beyond permissibleconducting test in enclosed conditions.

Rated Uninterrupted Current (lu): The maximum current that switchgear can carrycontinuously for periods extending to even hours, weeks & months, without the temperaturerise of any part reaching beyond permissible limits.

Rated Operational Current (le):The continuous current of a switchgear would vary depending upon the making & breaking capacities expected for an application. Such a rating is the 'le' rating.

Rated Current (In): This rating is similar to the Rated Operational Current Rating, but is applied to Fuses & Circuit Breakers. BEHAVIOR UNDER NORMAL & OVERLOAD CONDITIONS

Rated Normal Condition Making Current: The maximum current with a multiple of 'le', that the switchgear can safely make, without causing any damage to itself or to the installation or to the operator.

Rated Normal Condition Breaking Current: The maximum current with a multiple of 'le', that the switchgear can safely break, without causing any damage to itself or td the installation or tothe operator.

BEHAVIOUR UNDER SHORT CIRCUIT CONDITIONS

Rated Short Circuit Making Current(lcm): The maximum current that the switchgear can safely make, under short circuit conditions, without causing any damage to itself or to the installation or to the operator is called the rated S/C making capacity or a Peak value of short circuit current.

AC part of the fault current is fed by the supply. DC part

comes from stored energy in inductances and capacitance of

network 'components. An AC component is symmetrical and

sinusoidal whereas DC component is exponentially decaying. The decay depends on the time constant of the system which can be given by ratio L/R. AC components is superimposed on DC component. Thus short circuit current wave is initially disposed asymmetrically about the zero axis. A DC component dies out in 3 to 4 cycles. This gives AC sinusoidal current as sustained fault current.

If the switchgear is making on peak value of S/C current then huge electro dynamic force the switchgear has to withstand, & switchgear should not fail. So the making capacity is peak value of current.

Rated Short Circuit Breaking Current (lcs): The maximum current that the switchgear can safely break, under short circuit conditions, without causing any damage to itself or to the installation or to the operator is called the rated S/C breaking capacity or a RMS value of short circuit current.

Rated Short time withstand Current (lcw):.The ability of the switchgear to carry the short circuit current for the co-ordinated short circuit

time is called 'lcw'. Obviously, the more 'lcw', that switchgear has got, the more co-ordination

4

WAVEFORM

CURRENT, TOPENVELOPE

■ DECAYING DC COMPONENT

TIME;

.BOTTMENVELOP'E

interval ,normally specified as, Say, 50kA for 1 sec. Many switchgear may have making and breaking capacities of, say, 50kA, but their one-second 'lcw' will be lower at, say, 35kA only.

Generally if the switchgear closes on a short circuit fault, the stored energy in the system tries to discharge through the switchgear. So along with the AC sinusoidal short circuit currents some DC components get added to it. But this component is decaying in nature so it dies down after some msec. While switchgear closing on fault it will have to make the resultant current of both. When making capacity is specified it is assumed that the switchgear is closed in the worst case i.e.max peak value of current. So making capacity is always a peak value of current. But the braking capacity is RMS value of current, Also while doing fault level calculation we calculateRMS&Peakfault level.

UTILIZATION CATEGORIES

Switchgear is a switching device. As such, it could be employed for any kind of switching operation. But, its characteristics like the operational current, the making & breaking currents will greatly depend on the application. Based on the applications, switchgears are categorized under different "Utilization categories".

NORMAL SERVICE CONDITION

Ambient air temperature:• Maximum+ 40°C and Minimum-5°C• Average over a period of 24 hours does not exceed + 35°C

Altitude• The altitude of the site of installation does not exceed 2 000 m.

Humidity• Relative humidity of the air does not exceed 50 % at a maximumtemperature of+40 °C.• Higher humidities permitted at lower temperatures, e.g. 90 % at +20 °C.• The pollution degree refers to the environmental conditions for which the equipment is

intended.• For equipment intended for use within an enclosure or provided with an integral enclosure,

the pollution degree of the environment in the enclosure is applicable.• For the purpose of evaluating clearances and creepage distances, the following four

degrees of pollution of the micro-environment are established

Pollution Degree 1 - No Pollution or only dry non-conducting pollutionPollution Degree 2 - Normally non-conductive pollution, but occasionally

Temporaryconductivity caused by condensation must be expectedPollution Degree 3 - Conductive pollution or dry non-conductive pollution which becomes

conducting due to condensationPollution Degree 4 - Pollution generates persistent conductivity caused by conductive dust

and/or rain or snowStandard pollution degree of industrial applications: Unless otherwise stated by the relevant product standard, equipment for industrial applications is generally for use in pollution degree 3 environment.Standard pollution degree of household and similar applications: Unless otherwise-stated by the relevant product standard, equipment for household and similar applications is generally for1 use in pollution degree 2 environment.

GENERAL SELECTION PARAMETERS5

• Normal Condition Performance• Abnormal Condition performance• Life expectancy (if any)• Accessories

Normal Condition Performance:

Operational Voltage- Selection of switchgear mainly based on the system voltage (AC or DC). But generally the system voltage is standard \.e. 415V AC.

Operational Current-Operational current has to be greater than the load current.

Operational Temperature- Temperature in which switchgear is going to be used plays an important roleas the heat dissipation will be affected by high ambient temperature. Normally the ambient temperature is considered as 40 °C. if the ambient is high the derating factor has to be considered.

Product specific parameters (if any)- Depending on type of switchgear & application; as per standard different specifications are specified. For selection all those specifications are to be considered.

Abnormal Condition performance:

Switchgear is supposed to operate not only in normal condition but also, in abnormal conditions like S/C. So while selecting switchgear the suitability of switchgears for such conditions has to be considered.• Breaking Capacity- It is maximum current which the SCPD can break safely at the event ofS/C fault. This rating has to be greater than max. fault current calculated for that location i.efault level.

Making Capacity- Similar to breaking some times SCPD makes contact under S/C conditions. So the S/C making capacity is specified for switchgears.

• Withstand Capacity- When time based discrimination between various levels is provided then SCPD has to withstand S/C fault current for some time. During this time switchgear should not fail. So S/C withstandcapacity is given for CB.

• Product Specific Parameters (if any):

Life expectancy (if any):

For different applications different life is expected. For ex. if a contactor is used for frequent operations like in crane duty then the life expected is quite high. But if it is used for normal duty general application then not much life is expected from contactor. So for life of switchgear application understanding is required.

Other selection criteria:

■ Control Circuit voltage- Depending on the control circuit voltage rating (AC or DC) the suitability of switchgear has to be considered.

■ Accessories- As per the applications & availability accessories are selected. Most of the accessories are product specific.

6

■ Maintenance- If the switchgear is suitable for maintenance; it should be easy & less time consuming so as to reduce down time. Now a day the trend is towards the no' maintenance or less maintenance technology.

■ Altitude- If the switchgear is located at very* high altitude then the air density reduces causing the less heat dissipation. So for such applications the de-rating factor has to be considered.

■ Availability of spares

■ Size- if the size of switchgear is compact the overall size of panel also reduces. So the trend is towards compact product.

■ VA Consumption of coils■ Suitability for annunciation

Introduction to LV SWITCHGEAR system

In most of the industries primary source of power supply is from electricity board (through transformer) & generator is used as standby power source in case supply from EB fails or power quality from EB is not in acceptable limits.

So the separate switching & protection device is used for each source." As the fault levels at these locations are very high, protection device should have high breaking capacity & switching operations at these locations is very infrequent. Hence Air Circuit Breaker (ACB) is used at these locations. (Location: Tr&Genr). Purpose of these breakers is to protect busbars and provide isolation for whole system.

Generally the capacity, of generator is smaller than the transformer rating due to economic reasons, hence only critical loads are operated when generator is providing the power. Please note that generator is used as stand by source, hence Transformer & generator are not working in synchronism. If breaker of Tr&Genrare closed simultaneously there can be circulating current flowing through them or in worst case there can be short circuit. To avoid this condition bus coupler is used. By this arrangement all the critical loads are connected to busbar under generator set and noncritical loads to busbar under transformer. When EB supply is healthy breaker of Tr&Genris closed and supply is given to all critical & noncritical loads. When generator is supplying the power breaker is closed and only critical load will get the supply. Generally the interlocking scheme is used to ensure the same. Generally ACB is used as bus coupler.

No of outgoing feeders are taken from the busbar for various loads. Every feeder has its own switching and protection device. ACB/MCCB can be used for outgoing feeders.Purpose of these breakers is to protect the underground cable & give the backup protection to breaker . Panel consisting of incoming breakers, busbar, bus coupler & no of outgoing breakers is called as PCC panel (Power Control Centre Panel).

From outgoing feeder supply is given to loads through MCC panel (Motor Control Centre Panel). Every MCC has its own incoming breaker for switching, protection & isolation purpose generally MCCB/SDF is used . From MCC panel supply is given to various motors. For control & protection purpose every motor has motor feeder combination. Motor requires switching along with overload, single phasing & short circuit protection. In most of the cases switching of motors are frequent, hence switching device should have high life. Generally switching device is contactor, which will give the highest operating life. For overload & single phasing protection of motor Bimetal Relay (BMR) is used.

7

SDF/MCCB is used as SCPD(Short circuit protection device) & isolation purpose.Relevant standards:

IS/IEC: 60947• Part 1 -— General rules for all products• Part 2 —Circuit Breakers• Part 3 — Switch Disconnector Fuses • Part 4 —Contactors/Relays/Starters• Fuses —IS: 13703 / IEC 60269• MCBs —- IEC 60898

Switchgear Distinguishing Features

Type Current Rating Fault Operating Capability

Frequency Of Operation

Life Expectancy (No. Of Operations)ACBS High High . Infrequent Low

MCCBs Medium Medium * Infrequent LowFUSEs Medium/Low . High OnceSDFs Medium/Low High/Medium Frequent MediumCONTACTORS Low Low Very Frequent High

Switchgear Capabilities

Switchgear Switching ProtectionContactor w NRelay N YStarter Y YSwitch/SD Y NHRC Fuse Y YSFU/FSU/SDF Y YACB Y YMCCB Y YMPCB Y Y

CONTACTORS

1. INTRODUCTION : In a domestic load distribution most of the electrical switching is related to switching ON/ OFF lights, fridge, TV, etc. It does not involve handling currents higher than normal rated currents under normal operating conditions. However in an industry various loads like motors, lighting, switching ON/OFF of capacitors, furnaces have to be done. Switchgear is used to cater to the standard requirement of switching various loads under normal & abnormal conditions with inbuilt features of protection against various hazards, one such device used for the above application is "Contactor".

Definition:A contactor is an electromagnetically operated switching device capable of making, carrying and breaking currents under normal circuit conditions including operating overload conditions; which can be operated remotely (not operated by hand).

8

Principle:When a definite voltage is applied across a coil, energizing an electromagnet system comprising of fixed & moving parts, the moving parts get attracted to fixed part, thus the electrical circuit is complete through this contact system. When magnet is de-energized, it gets pushed back by return spring & contacts open. Thus electric circuit is broken,

Contactor is primary switching device. All the regular switching operations are done by contactor. Due to this the contactor need to have long life. Some of the applications require switching on and off at very high frequency. This demands that the contactor should be capable of working at very high frequency of operation.

Advantages & Application of Contactor:High number of make-break operations.High Making & Breaking capacity.Remote ON/OFF.High mechanical & electrical life.No volt protection.High frequency of operations.

Contactors are not designed to break short circuit current. Thus they are said to have limited breaking capacity. Due to this reason back up short circuitprotection device like Fuse or MCCB is required in feeder circuit.

A contactor has two separate electrical circuits.1. Main Circuit2. Control Circuit

1. Main Circuit- The main circuit involves power contacts and handles the power drawn by the load. The main circuit mainly consists of contacts and terminals. Double break contact system ensures long contact life.

2. Control Circuit- The control circuit involves coil and auxiliary contacts. Auxiliary contacts are in built and are used for interlocking, signaling etc.

2. Parts of Contactor & Their Functions:

Rubber padding- It is provided for absorbing shocks during closing and opening of contactor.

Magnets- AC magnets are made up of laminations stacked together to reduce effect of eddy currents. Since in DC frequency is zero, problems of eddy current losses &hysteresis losses do not exist & hence no laminations are required.

There is designed gap between the central limb of Fixed magnet and the moving magnet. When this gap reduces over millions of operation or the magnet starts bulging, it indicates that mechanical life of contactor is over and needs replacement.

If during maintenance or cleaning, emery or sand paper is used, for the magnets, the designed gap between the central limb decreases as surface material reduces, reducing the air gap, hence while opening there remains some residual magnetic flux which does not allow the moving magnet to separate immediately, causing severe pitting of contacts. This results in frequent changing of contacts.

Thus usage of emery / sand paper should be avoided at the magnets, it can be cleaned with CRC 2-26 and a rough cloth.

Contact System- The contacts are made of silver cadmium alloy. These have good anti-welding properties. The gap between the contacts is less than that between magnets which ensures that the contacts are closed before the magnet pole faces touch each other. This is also known as over travelling of contacts

Leaf Spring- The moving contacts have to be held under certain contact pressure which is ensured by

9

providing leaf springs over the contact carrier bridge. It also helps in bounce free closing.

Return Spring- The moving assembly is held against the stored energy of two main springs called as return springs which help the bridge assembly in opening the contacts.

Shading Rings- In AC magnets as the alternating flux (main flux) goes through thenatural zero point force of attraction created become zero. The result in repeated de-energization of the magnets & leads toChattering.

Flux produce byIngmg Due to chattering life

ofcontactor contacts reduces drastically, :since it consumes 100 operations/sec.Some additional force is therefore required to hold the magnet when main flux passes through zero. It does this by generating an auxiliary flux which is out of phase with the main flux; the shading ring is embedded in the pole face of the magnet.

Terminals- Terminals are made ofsilver, plated copper and are provided for both power circuit and control circuittermination.

Auxiliary Contacts- Set of normally open (NO) and normally closed (NC) contacts are available for indication and alarm purpose.

Arc Chutes- While breaking high currents the arc formed between the contacts needto be effectively quenched.

Arc is quenched by following methods:

> Arc is split by providing number of De-ion plates.> Resistance of Arc - Specific Resistivity x Length of arc

Area of arcDue to splitting of arc its length increases. As resistance of arc is directly proportional to length, the resistance of arc increases and thus arc is totally quenched.

Housing- Material having good thermal withstand and mechanical strength are used for housing. Mostly ceramic, SMC (sheet moulded compound) and DMC (dough moulded compound) are used for housing.

3.SPECIFICATI0N OF CONTACTOR & ACCESSORIES:

Rated voltage (Ue)-This is the voltage that would normally be available continuously across the terminals of different phases of the switchgear. Unless otherwise specified, this rating will be 415V for LV Switchgear. This implies that even while 415V @50Hz is present between the terminals, there will not be any flash-over (i.e.) the insulation between phases is sufficient to tolerate a potential difference of 415V continuously.

Insulation voltage (Ui)-Under high voltage condition insulation of switchgear should not fail. This is the max. voltage which the insulation can withstand continuously.

10

Flux produce by coil

Rated Operational Current (le)-The continuous current of a switchgear would vary depending upon,the application and the making & breaking capacities expected in that application. Such a rating is the V rating. Contactor has following main current rating and is selected as per the type of load i.e application/Utilization Category. So as per the utilization category le rating of a same contactor will be different.

Free Air Or EnclosedThermal Current (Ith&Ithe)-This specification implies the maximum current that the contactor can carry continuously for a period of 8 hours, without the temperature rise of any part reaching beyond permissible limits. This test is conducted in open air conditions (for Ith) or in enclosed condition (for Ithe).

Rated Impulse Withstand Voltage (Uimp)-Impulse withstand voltage is required when there are switching or lighting impulses, the contactor should not fail.

Utilization Category- As per the application contactor has to make break current higher than its rated current. So making & breaking capacity greatly depends upon the application.. Standard has specified following utilization categories for different. applications.

1) ACl- NON-INDUCTIVE / SLIGHTLY INDUCTIVE LOADS, RESISTANCE FURNACES:Making Current-1.0 le Breaking Current-1.0 le

2) AC2- SLIP RING MOTORS; STARTING,, SWITCHING OFF:Starting current of Slip Ring Induction Motor is 250% of Irm hence

Making Current: 2.5le, In applications like cranes & hoists, one may have to switch off the motor even as it is accelerating, henceBreaking Current: 2.5le

3) AC3- SQUIRREL CAGE MOTORS; STARTING, SWITCHING OFF DURINGRUNNING:Starting current of S.C. Induction Motor is 600% of Irm hence Making Current: 6.0le

Motor is switched off during running most of application in industry (about 95%) are AC-3, hence Breaking Current: 1.0le4) AC4- SQUIRREL CAGE MOTORS; STARTING, PLUGGING & INCHING:

Starting current of S.C. Induction Motor is 600% of Irm hence Making Current: 6.0le

In applications like cranes &hoists , the motor may have to be inched/jogged hence breaking current :6.0le

VERIFICATION OF NUMBER OF ON LOAD OPERATINAL PERFORMANCE

Utilization Category

Make Break

I/le U/Ue CosO Ic/le Ur/Ue CosO

AC1 1 1 0.95 1 1 0.95

AC2 2.5 1 0.65 2.5 1 0.65

AC3 6 1 0.35 1 0.17 0.35

AC4 6 1 0.35 6 1 0.35

l/le U/Ue L/R (ms) Ic/le Ur/Ue L/R (ms)

DC1 1 1 1 1 1 1

DC3 2.5 1 2 2.5 1 2

DC5 2.5 1 7.5 2.5 1 7.5

11

Life Curve- Two Lives are specified for contactor Mechanical Life & Electrical Life. Mechanical life is constant but the electrical life depends upon utilization category & load current. Electrical life is lesser than mechanical life. But one can increase electrical life by replacing main contacts upto the mechanical life. For a contactor different life curves are given for different utilization categories.The electrical life increases when the same contactor is used for lower current than rated.For mixed duty applications the electrical life can be found out by following formula;

N=[A/{(1+c/ 100(A/B-1))}N= Expected life (No of operations)

A= 100% AC3 Life

B= 100% AC4 Life

C= % of AC4 operations in total no. of operations

Contactor for DC application: For DC application the utilization categories are as follows;

Nature of current

Category Typical applications RelevantIEC productstandard

a.c. AC-1AC-2AC-3AC-4AC-5a '"AC-5bAC-8aAC-6l>AC-7aAC-7b AC-8aAC-&1>

Non-inductive or slightly inductive loads, resistance furnaces.Slip-ring motors: starting, switching off.Squirrel-cage motors: starting, switching off motors during running.Squirrel-cage motors: starting, plugging^, inching2'.Switching of electric discharge lamp control.Switching of incandescent lamps.Switching of transformers.Switching of capacitor banks.Slightly inductive loads in household appliances and similar applications.Motor-loads for household applications.Hermetic refrigerant compressor motor control with manual resetting of overload releases.Hermetic refrigerant compressor motor control with automatic

60947-4

AC-12AC-13 AC-14 AC-1 5

Control of resistive loads and solid-state loads with isolation by optocoupler.Control of solid-state loads with transformer isolation.Control of small electromagnetic loads.Control of a.c. electromagnetic loads.

60947-5

AC-20AC-21AC-22AC-23

Connecting and disconnecting under no-load conditions.• Switching of resistive loads, including moderate overloads.Switching of mixed resistive and inductive loads, including moderate overloads.Switching of motor loads or other highly inductive loads.

60947-3

a.c. and d.c. A

BProtection of circuits, with no rated short-time withstand current. Protection of circuits, with a ratedshort time withstand current

60947-2

For DC applications the contactorcontacts are to be connected in series to quench

12

Utilization Categories as per IS/IEC 60947:

DC1 Non- inductive or slightly inductive loads, resistance furnaces

DC3 SHUNT MOTORS ; STARTING, PLUGGING AND INCHINGDC5 SERIES MOTORS: STARTING, PLUGGING AND INCHING

the severe arc due to DC nature of a current

Control Circuit:

The control circuit consist of two; 1) Coil & 2) Auxiliary Contact. The control circuit voltage has extra significance in defining the circuit voltage in which the contactor will hold positively. Various coils for different voltages can be given for AC and DC. Depending on AC or DC control circuit coils will be different. If DC supply is given to AC coil, coil will be damaged. As in case of DC due to the absence of inductive reactance the current flowing through coil will be high. So different coil is designed for DC voltages inwhich to reduce the current no of turns are increased causing bigger size of a coil.

Limits of operation of contactor: Electromagnetic contactor shall close satisfactorily at any value between 85% and 110% of their rated control supply voltage (for a.c&d.c. coils). The limits between which the contactors shall drop out and open fully are 75%-20% (for a.c) & 75%-10% (for d.c.) of rated control supply voltage.

Frequency: In a.c. magnet system contactor coils are designed for 50Hz frequency. For frequency other titan 50 Hz, V/f ratio shall be kept constant. This is required to maintain the coil performance as specified above.Uf/U50 = f/50 where Uf is voltage at frequency f Hz and U50 is voltage at frequency 50Hz

Pick up & Hold on VA: Pick up VA is a product of Pick up current & rated coil voltageHold on VA is a product of Hold on current & rated coil voltage Foreg. For a 400 Amp contactor

Pick up VA: 2100Hold on VA: 95Coil voltage Calculation of coil current for AC coil-

Pick up Current =PickupVACoil

Voltage

Hold on Current = Hold on VA =95 Coil Voltage 110 = O.86Amp

The pick up VA is more than the hold on VA. When the contactor is open, air gap is presentbetween moving magnet and fixed magnet. The air gap introduces high reluctance in themagnetic circuit. Due to high reluctance, magnetic flux is low. Lower magnetic flux reducesthe reactance of coil. Lower coil reactance leads to higher current during pick up. Oncethe contactor closed, air gap is zero. This gives rise to a high magnetic flux in the magnet. Thismagnetic flux linking to the coil increases the reactance. Increased reactance leads toreduction^ the coil current during hold on. This data is required for the selection of controltransformer. Control transformer should have sufficientcapacity to give supply required

13

=2100110

= 19.09Amp

VAduring pick up &hoid on.

Auxiliarycontacts: There are two types of auxiliary contacts; (NO) Normally Open & (NC) Normally Closed. The normal condition is a de-energised/ open condition of contactor. The aux. contacts are used for indication & interlocking in control circuit. So there are two types of load connected to aux contacts. Based on that two utilization categories are defined for auxiliary contact

AC1- Non- inductive or slightly inductive loads For Indication indicating lamps

AC15- Electromagnetic Loading For Interlocking Contactor coil, Timer coil

Mechanical interlocking kit: Generally electrical interlocking is provided with the help of aux. contacts. But there are chances of failure of aux. contacts. Hence mechanical interlocking kit is provided as a backup.

Surge suppressor:

Surge Suppressors are mainly used to suppress the voltage spikes or surges that occur whenever any inductive load is de-energized. When the coil is de-energized its electromagnetic field collapses and being an inductive load, it opposes this sudden change by producing an emf

This is because the absolute change in current is very high and in a short duration oftime. This coupled with high inductance of the coil produces a voltage spike in thesystem. This voltage spike is of the order 8 to 10 times of the rated coil voltage i.e. for acoil of rated voltage 240Vac the voltage surge can reach a value of around 2kV. Thisvoltagespike generated in the system has the potential to propagate to other components connected to the same supply system. If the surge is not suppressed ontime it will damage any sensitive electronic components connected to the system. Also ifany counters or logic circuits are present in the system then the voltage spike will causethem to change state momentarily, giving erroneous outputs. It may also damage the ICsbeyond repair. Hence It is very much imperative that the voltage spike is effectivelydampened by absorbing the energy associated with it. This is where a surge suppressorcomes into the picture.

A surge suppressor is a device which is connected in parallel with the coil. During normal operation the suppressor does not conduct as the supply voltage is much below its breakdown voltage. When a switching surge is generated the device starts conducting providing a parallel path to the excess current. This prevents the surge from propagating in the system and at the same time the spike is effectively suppressed. The device then automatically resets when the overvoltage goes away,

GUIDELINES FOR SELECTION OF CONTACTOR:

■ Study the Application & Find out Utilization Category■ Select a Contactor Current Rating Equal To or More that Full Load Current for that Particular

Utilization Category.■ Contactor should have Sufficient Breaking Capacity To Break O/L Currents■ Contactor should have Sufficient Making Capacity to Close on Inrush Currents■ No. of Operations Required should; be Fulfilled by Contactor for the Utilization Category■ Coil has to be selected as per Control circuit voltage AC/DC. For DC control a different contactor has

to be used.■ If no. of built in aux. contacts are not sufficient then add-on block has to be selected■ Lesser is the VA rating of coil smaller will be the size of control transformer. So the VA consumption

of a coil should be less.■ For any specific application the special contactor can be selected, as per the need.

Types of contactor-

14

■ Power Contactor- These type of contactors are used to operate actual loads so they consist of main contact system.

■ Control Contactor- These contactors are used only in control circuits. If the auxiliary contacts available in contactor are insufficient then control contactor is used to multiply the aux. contacts. Control contactor consist of aux. contact & coil only.

■ Capacitor duty Contactor- These contactors are specially designed for capacitorduty. Capacitor during starting acts as a short circuit, so very high current flows through the capacitor which has to be made by contactor. Due to high inrush current contactor life gets drastically reduced, so specially designed capacitor duty contactor is used. As shown main terminal 1,2,3,4,5,6 has* additional parallel aux contact block in which there are resistors connected.

15

When the contactor is energized, aux contact closes first so resistors come in series resulting in limiting inrush current. Then after few millisec main contact closes. Once the main contacts are closed the aux contact will be de-latched and open. So resistors will not carry current continuously.

■ AC Contactor- AC contactor has AC control circuit hence AC coil■DC Contactor- DC contactor has DC control circuit hence DC coil

■ Vacuum Contactor- Vacuum contactors have contact system in vacuum. Due to the absence of air the arcing will not be there. So these types of contactors are used where very high electrical life, high making & breaking current is expected. Generally,they6 are available in high ratings,.

■ 3 pole contactor-These contactors are used for switching of loads.■ 4 pole contactor- This contactor is used for changeover between two sources. Where neutral also

has to be disconnected. For these types of contactors the utilization category specified is ACl, since they are not used for switching of any particular load.

■ 1 pole contactor- This has only one main contact. Generally used for single phase motors.

PROTECTION OF LV MOTORSMotors the prime movers of for the industry, consumes 80% of the total generated power. In

industries various types of motors are in use. They can be divided as AC motors & DC motors. Though DC motors have certain advantages over AC motors like precise and easy speed control, AC motors are in use with large population with one single reason as easy availability of AC power. .

AC motors can be further classified as LV & HV motors and further induction & synchronous motors. Induction motors can be further classified into sq. cage induction motor & slip ring induction motor. Primary cause of motor failure is excessive heating, which if sustained over long time period will result in failure of motor. it is understood that O/L & single phasing put together contribute to 44% towards motor failure. Why should the- motor be protected against over load? What will happen, if over load is allowed to persist in a motor? Generally we say that motor gets burnt. A motor would never burn. Winding also would not < .burn, as it is made up of Copper. The metal cannot burn but it melts due to the excessive heating. What then would burn? It is the insulation on the winding that would burn, because/the insulation is made up of combustible material. When the temperature of a combustible material goes beyond its ignition temperature, it would burn. So, insulation materials can be classified based on their tolerance temperatures.

Class of Insulation Maximum Attainable Temperature ( C) (As per IEC 60085)Y 90

A 105E 120B 130F 155H 180C > 180

Motors are insulated to Class 'B' generally and now-a-days motors come with Class Finsulation. Here, even though motor is insulated to Class 'F', the temperature rise is limited to that of Class 'B' only. So, for all practical purposes, motors'canbe considered to be of Class 'B'insulation.

16

Following are the reasons of motor failure;

Overload: When motor is carrying a current higher than its rated full load current, theheat generated inside the motor increases, which causes temperature rise inside the motor. If this condition persists for long duration, it will result in insulation failure due to over temperature.

The heat generated in the motor winding as a result of I²R can be properly dissipated before an abnormal temperature is reached. As long as the motor winding is clean & dry, the air passages are in the motor winding are open, and a supply of relatively cool air is available to dissipate the heat generated, the above conditions will prevail. If twice the normal full-load current is passed through the motor winding, the heat generated in the motor is more than the heat dissipation and therefore the motor temperature will rise. However there is specific time interval duringwhich this may be allowed to happen. The same holds good if 300, 400percentage of themotor full load current is passed through the motor winding. Also it can be seen that as the overload current is increased the withstand time goes on reducing.

Heavy starting. Motor carries 6 times of its full load rated current during starting. So the motor is able to withstand the starting current for specific time. If the motor is starting with heavy load.then the motor starting time increases. So now motor has to withstand the starting current for longer duration which will cause failure of insulation.

Locked rotor: If the rotor of the motor is locked/ stalled then motor continuously carries the starting current of motor for indefinite time.

Supply variation & Unbalance in supply: The variations in voltages or frequency can cause overheating of the motor.

Number of starts: In a particular application like inching if the motor does more no of starts than specified starts per given time, motor can get overheated. During starting motor carries 6-8 times of its rated current. This causes increase in winding temperature. This heat has to be dissipated, but due to frequent starting this heat does not get dissipated so causes temperature of motor going beyond its withstand temperature.Single phasing:

Single phasing is more severe than overload in a motor. In case of overload the balanced overcurrent is carried by all the windings. In case of single phasing there is unbalance in winding since the winding is connected in delta like shown in dia. Is phase B has lost then winding 'a' & 'b' is in series and 'c' is in parallel with it. So winding 'c' carries high current and it will be more overheated as compared with others. So there is chance of failure of winding 'c' only.Also there are -ve phase sequence currents flowing in the motor due to unbalance.These–veph sequence current increases constant losses due to which the core getsoverheated.

Mechanical problem: Any mechanical problem in motor like bearing failure, bending of shaft can cause overheating and further damage to motor.

Anomalies in cooling: For any motor to run smoothly without any damage the heat generated has to be dissipated. If the heat dissipation if affected due to some, reason then the motor can get overheated. If the air passages in winding are not open, cooling fan is broken then the cooling will not be proper.High ambient temp itself is high even under normal condition the heatgenerated will not be dissipated resulting into damage to it.

17

Devices for protect: The various devices that are available todayfor protecting the motors. This list covers only devices for general application and is notexhaustive. The user, depending on importance of application, severity of environment andsupply conditions and expenses that he is prepared incur, has to make a judicious choice.>Temperature sensing device- These devices, to the extent that they are able to sense the exact temperature of winding, offer protection against all the abnormal situations listed above. The temp, sensing devices can beclassified in to two;

1. Thermostat- Thermostat is a temperature operated switch. Here, the trippingtemperature is set and once the set temperature is reached the thermostat trips the supply tothe device/equipment.

Advantage: Most EconomicalLimitation: Low accuracy Applications are limited to the areas where coarse temp sensing can

serve the purpose or accurate protection is not involved like iron-box, geyser, washing machine dryer etc.

2. Thermistor- It is an abbreviation of the words "Thermal Resistor". All materials in theworld have a temperature co-efficient of resistance, either positive or negative. PositiveTemperature Co-efficient of Resistance (PTC) & Negative Temperature Co-efficient ofResistance (NTC) Use of NTC Thermistor are banned by National / International Standards. Incase of open circuit in the controller connection, Resistance detected is very high, Highresistance means low temp hence no protection during fault. >Current Sensing-Device- If one cannot go for Thermistor Protection for Motor Overload Protection, due to-any of the above reasons. If we cannot capture temperature, capture the element which causes the temperature rise or heat (i.e.) capture current. This is the principle of Current Sensing Type Motor Overload Protection. Here, the variants are:

• Electro-mechanical type over current relay• Electronic Motor Protection Relay• Bimetal or Thermal Overload Relay1. Electro-mechanical Type Over current Relay: This is an induction type relay with a disc. In

such a relay, you can set the motor full-load current as the trip level and also the safe trip time can be programmed. Say, if the setting is 100A, and the trip time is programmed such that if 200A flows, it will trip in about 100 seconds. This means that if 200A flow through the motor damage would be done in about 100 seconds. Or the 'At' in about 100 seconds will be about 90°C (40°C to 130°C) for a class 'B' motor. If the 100A motor is started from cold conditions& is overloaded to about 200A, this over current relay would very correctly protect the motor by tripping in 100 seconds. But after this tripping, when would you re-start the motor? Would you wait till the motor cools down which may take about an hour and a half. Nobody would permit you to wait that long. You may have to re-start the motor almost immediately, due to production pressures. Now, if the same 200A overload is still persisting, this O/C Relay would again take about 100 seconds to trip, as has been programmed. But, while re-starting, the motor would be "HOT" & its temperature would be around 120°C. Now, this relay, after a hot start of the motor under over loaded conditions, would still take about 100 secondsto trip. And, in this 100 seconds the 'At' would be 90°C and the ultimate temperature of the motor now would be 120 + 90 = 210°C, sure to damage the motor. By the time the relay trips, the motor would have damaged already. This relay does not bother about the residual heat in the motor during successive starts. Or it doesn't have what is called "THERMAL MEMORY".

So, this type of electromechanical over current relay is not suitable for repeated starts of the motor. The other disadvantagesof this type of relay are:

Size- It occupies a large space in the panel board.High Cost and

18

Accuracy - Typical accuracy is around +/-12.5% That's why this type of relay is not used in motor protection. They are rather used in feeder protection.

19

2. Electronic Motor Protection Relay: The next option would be an Electronic Motor Protection Relay - either analogue type or the digital type. Here, you can do wonders. Whatever type of protection that you may want - including thermal memory feature - can be incorporated in these types of relays. Size is small and also the accuracy is very good at +/-1%. The major disadvantage is its cost, typically around.Rs. 50, 000.00. Hence, not cost-effective for small motors. May be cost-effective from, say, 100HP and above.

3. Bimetal or-Thermal Overload Relay: For smaller motors, we use the/'Thermal Overload Relay" or the "Bi-metal Overload Relay". Two metals with different temperature coefficient of expansion are fused together. With heating one of them expands more & another expands less. This action results in bending of bimetal. This actuates the mechanism in order to trip the motor. The thermal O/L relay has following advantages; •

• Thermal memory- For repetitive faults the relay does not operate as per the curve but due to

residual heat in the bimetal it operates fastcausing faster tripping of the motor.

• Compact- It is compact as compared with electromagnetic relays.

• Accurate protection

• Long life- its life is long and gives consistence performance but that has to be checked

periodically.

• Ambient compensation- A compensating bimetal is used to ensure a consistent trip time in case of variation in ambient temperature.

• Low cost>Voltage Sensing- These devices can give only single phasing protection. They are less costlier than current sensing & are to be connected across the line and suffer a drawback that:

Downstream faults are not protected.

Current unbalance due to inter-turn short is not detected.

Operation & Selection of O/L relay:Selection of OLR is done based on type of relay. There are two types of thermal O/L relay;

1. Single Slide Mechanism Relay- This gives only O/L protection to the motor.2. Double Slide Mechanism Relay- This type of relay gives protection against O/L & Single Phasing

protection

1. Single Slide Mechanism Relay; This relay has only one slide as shown in fig below. This is top view of relay. In case of single phasing also it takes same time tripping as

20

2. Double Slide Mechanism Relay: This relay has two slides as shown in dia.

TRIPPING LEVERSTOPPER

The relay is used for different application motors. Depending on application & motor starting time various. When any relay is selected it has to be selected in such a way that it should not trip during starting. So standard has defined 4 trip classes; 10A, 10, 20, 30 & as per the application the trip class has to be decided.

The relays shall comply requirement given in below table, when tested as per IEC60947 part 4.

Sr.No

Conditions Test current as multiple of set current.

Max non trip current 'A' Max trip current 'B'

1 All poles energized 3P-1.05 3P-1.2

2 When two poles are energizeda. Relays without protection against single phasing

3P-1.0 2P-1.32, 1P-0

b. Relays with protection against single phasing 2P-1.0, 1P-0.9 2P-1.15, 1P-0

Test Procedure:1. With the relay in enclosure & at1.05 times the current setting, tripping shall not occur in less

than 2 hours starting from cold condition applicable for first point2. When thecurrent is subsequently raised to 1.2 times the current setting/tripping shall not

occurin less than 2 hours applicable for first point.3. Staring from thermal equilibrium at the current setting, tripping shall occur at 1.5 times the

current setting as given below.For class 10A relays -less than two minutes ,For class 10, 20 & 30 relays - less than 4,8& 12 minutes respectively.

4. Starting fromthermal equilibrium at the current setting, tripping shall occur at (7.2) timesthe current setting as given below for different trip classes.

Trip class Tripping time in sec. when tested at current 7.2 10A 2<Tr<1010 4<Tr<1020 6 < T r < 2 030 9 <Tr< 30

21

UPPER SLIDE

:' N/CCONTA

ISBIMETALSLOWER SLIDE

Selection of Relay: For all the relays inaccuracies of calibration as well as constraints of manufacture, have a I owe reflect towards maximum setting than minimum setting. For relays operating on Double Slide Mechanism the protection against single phasing is substantially better at max. setting. Following points are to be considered to achieve the best protection

1. Select a relay such that motor current lies towards maximum setting.2. Setting ration of the relay range should be the maximum. Relays available today have a

setting ration of 1.3 to 1.66.3. Generally relay setting is recommended as per actual current drawn by the motor.

Too large or too small cross section of cable terminals may result in excessive temperature rise and nuisance tripping or non -tripping of relay at higher overloads.

Overload relay with manual reset should always be used where continuous contact device are connected (e.g. limit switches, pressure switches) to avoid restarting automatically.

Reset button to be fitted as an external feature in order to make it accessible to all personnel. Overload with auto reset can be used only with impulse contact devices such as push buttons, because on these the cooling of the bimetal strip cannot lead to automatic reconnection.

Backup fuse: Backup fuse for instantaneous trip are needed to protect not only the motor but also the relay against the effect of short circuit. These are clearly marked on the relay as MAX. BACKUP FUSE. However a lower rating of fuse can be selected to minimize the damage to the contactor and other system. Refer fuse selection for further clarification.

Overload protection of high starting time motors: The starting time of a motor depends upon the motor load torque and the total inertia of the system. The starting time can be long when the motor is started on heavy inertia loads. This results in high current to flow through the relay for longer duration. The normal trip time for thermal overload relay at 6 x In is in the range of 5-12 sec. hence, the relay set at normal full load current for a motor trips during heavy starting, resulting in nuisance tripping. To avoid this problem two solutions are available:

1. Use of Higher Trip Class Relay2. Relay bypass during starting.

MOTOR STARTERS

Majority of Industrial load is Induction motor. Amongst them also more percentage of squirrel cage motors than slip ring motors. Generally the percentage of squirrel cage I.M. goes as high as 90 % of total Industrial load. Fig. shows location of starter in a motor feeder circuit.

Starter is required for following functions;

• Switch ON/OFF the motors:• Remote ON/OFF operation• Overload protection• Limit starting current (optional)

Different methods of starting ofsquirrel cage induction motor are;

1. Direct On-Line method (DOL)2. Star Delta starter.-3. Auto Transformer start4. Soft Starter

22

1. DOL Starter: This is the most popular method of starting motors due to the low cost of

control gear involved. DOL starting is used where a high starting torque and rapid acceleration of the motor are required. The starting torque with DOL starting could vary from 100 to 250 percent of full load torque depending on the motor design.

The starting current with DOL starting is about 6 to 8 times the full toad current. One major drawback of this starting method is the sudden voltage drop that occurs in the supply, i.e. which can result in lightsflickeringnearbyV&Ecause of this most power utilities normally permit DOL starter for small motors upto 2,2 to,,5,5!KW, Another disadvantage is the mechanical stress placed on the motor and its load by the high starting torque. This is a big problem in many industrial applications.

Hence DOL starting, especially for large motors, should only be used where source is capable of supplying sufficient power required during starting or where the extent of dip will not affect the performance of other equipment.

Care has to be taken while selecting the short circuit protection device (SCPD) & Relay. The possibility of high current & high starting starting time during the starting must be considered. The relay & SCPD's (i.e. fuse or MCCB) should not trip during starting.

2. Star Delta Starter: The mains supply is first fed to the motor with the winding in star-

connection, which reduces the torque and the current to one third of the normal value needed for a DOL start. Star-Delta starting is preferred to DOL starting because of the following advantages:

• Heavy starling inrush current during DOL starting con create a dip in the system voltage.• Sensitive mechanical drives could get damaged from the high starting torque exerted by a DOL

motor start. This torque therefore has to be limited to a safe value.The induction motor is initially run in star mode for a set time. The starting current and the starting

torque is roughly one third of/that obtained with DOL starting. When the motor speed reaches greater than or equal, to 80% of its rated speed the changeover action is initiated. The changeover is done manually (in the case of manual star delta starters) or through a timer (in the case of fully automatic star delta starters).

There are three types of star delta starter.• Manually Operated star delta starter- the old method was to use manual change over switch

for star to delta change-over once the motor has picked up the speed of about 85% full speed.• Semi-automatic star delta starter- It is similar to manually operated but uses contactors so

remote operation is possible. Change over from star to delta is done manually by start push button.

• Fully automatic star delta starter- in this type of starter the change overisdone by timer. Before starting operator has to set starting time in timer. During operation timer change over its contact to open the star & close the delta

• Care has to be taken while setting overload relay in star delta starter. Since relay is connected in phase circuit the relay will have to set as per the phase current not the line current of the motor,

i.e.SET CURRENT = 0.58 IL s 60% OF IL

At the time of change over from star to delta short time (msec) high current peak of the order of 12 times, same as DOL are experienced. The magnitude of current peak would depend upon the motor speed at the time of change over. This inrush current often produces a sudden jerk. This staring method also requires maintenance.

23

Reason for current peak : During the change over when star contactor drops, the arc between the contacts persists for a brief period. At this time if delta contactor picks up a three phase dead short occurs for a very small moment. This causes a heavy current peak, this may result into contact welding. By delaying the pickup of delta contactor, till the arc is extinguished, we can reduce the risk. The pick up of delta can be delayed by giving pause time between star contact opening and delta contact closing.When wiring a Star-Delta scheme electronic timer, having a built in pause time of 50-80 ms, has a higher safety margin during changeover than the scheme with thermal or Pneumatic timer. Especially electronic timers are recommended to be used for motors of ratings of 37 K W & above.3. Auto Transformer start: Another method 1$ to use an auto transformer with variable secondary voltages which supplies the motor windings with increased voltage after the motor starts running. The torque and current can then be reduced in smaller steps. Compared with star -delta starting, this has drawback that its starting torque is not sufficiently high. Auto-transformer starting is a method that is seldom used with smaller sizes of motors, owing to the rather high cost. This is also a large complex arrangement item requiring maintenance.

4. Soft Start: A modern & effective way of starting a squirrel cage motor is by using an electronic soft starter, often microprocessor based. This unit allows you to control the voltage supplied to a motor without involving mechanically moving parts. This gives you the flexibility of choosing the starting torque/current together with the starring time. The power supply to the motor is gradually increased during the whole starting procedure to achieve a smooth start. This prevents mechanical stress on the motor and the equipment driven. Similar benefits are gained when using a soft starter to soft-stop a motor, especially those that drive pumps.

Fuse In a typical motor feeder Fuse is used as short circuit protection device. The bimetallic thermal

overload relay must protect the motor against O/L upto locked rotor currents. Above this fuse must protect; as starter is not design to operate under S/C fault current. Also in such a condition starter itself can get damaged. In short circuit condition no device can protect the motor, as already the insulation inside the motor has failed, but to avoid further damage to installation & starter we need to protect against the short circuit fault. So we use fuse for S/C protection.

Fuse:A switching device that, by the fusion of one or more of its specially designed and proportioned components, opens the circuit In which it is inserted and break the current when this exceeds a given value for a sufficient time. The fuse comprises all the part that forms the complete switching device.High Rupturing Capacity (HRC) fuse is considered to be the best device for providing protection against short circuit (particularly for contactors with low l2t withstand). No other device with moving parts (CB) can match its breaking speed. Fuses are of various kinds -rewirable, HRC, semiconductor type etc. The most commonly used fuse in industries is HRC type. The fuse gives protection in case of Overloads as well as Short Circuits. For small overloads the fuse has sluggish operating characteristics.

HRC fuse consists of specially designed fuse elements enclosed within an insulated body, usually of ceramic material. The fuse body is fixed with end caps. The fuse body is filled with fine grained quartz

24

sand. Restrictions Known as necks are incorporated in the elements to ensure a greater current density at these points. For high short circuit currents there is little time for heat to dissipateand the element thus melts at these necks. All arc is formed at such point and energy is absorbed from the arc by the fine granules of sand which melt and fuse together to form a glass like substance called fulgurites.

In practice this process is very rapid and may last for 4-5 milli-sec and it is this phenomenon which gives HRC fuse its currentlimiting capabilities.

Many fuse designs employ deposit of low melting point alloy (solder) near to one of its neck. The solder deposit causes M-effect whereby the melting point of the area is reduced. At the time of fuse element breaks at neck portion near to the solder point.

IDENTIFICATION OF FAULT- When the fuse element breaks at the necks of the fuse element, the fault that has taken place is a Short circuit fault.While when the fuse element breaks at the centre neck near solder point it is an Overload fault. The pop-up indication is available with many types of HRC fuses, which shows that fuse has blown.

ADVANTAGES OF FUSE:No maintenance required , Very small/compact sizeRelatively low cost,Extremely high breaking capacity of up to lOOkAVery fast operation in the event of heavy S/C (Can Interrupt even in < 5 msec)Current limiting device- Since fuse operating time is very low, it does not allow current to reach its prospective peak value as shown in fig. so the energy released during operation of fuse is low. This energy is known as let through energy. Let through energy decides the damage to the downstream devices.

P-Prospective peak current of a circuit C-Cut-off current

tl- Pre-arcingtime

t2-Arcing time

T- Total operating time

Pre-arcing time (Melting time): The time betweenthe commencement of a current large enough tocause the fuse element(s) to melt and the instance when an arc is initiated.Arcing-time:The interval of time between the instance of the initiation and the instant of finalarc extension.

Operating time: The sum of the pre-arcingtime arid the arcing-time,

Prospective current: The current that would flow in a circuit if a fuse is situated therein was replaced by a link of negligible impedance without any other change in the circuit or of the supply.

Cut-off current: The maximum instantaneous value reached by the current during the breaking operation of a fuse when the fuse operates in such a manner as to prevent the current from reaching the otherwise attainable maximum.

It can be seen from above dia. that the total operating time for a fuse is very small & the current seen by system is also less than the prospective peak value. So there will be less let through energy

25

TIME

which will limit the damage to the system.

Fuses have four things to do in a circuit:1. The fuse must sense faults.2. The fuse must open quickly and clear itself when a short circuit occurs.3. The fuse must also sense a normal or harmless overload becomes excessive or prolonged.4. The fuse must not change or alter the characteristic of the circuit during normal operation.

Selection of Fuse: For selection of fuse certain terminologies are to be considered. Following specifications are available with fuses.

A. TYPES OF HRC FUSE- There are three types of HRC fuses available based on their connection with the switches.

CYLINDRICAL BOLTED KNIFE TYPETYPE FUSE TYPE FUSE FUSE

B. BREAKING RANGE> 'g' FULL RANGE BREAKING CAPACITY- A current limiting fuse link capable of breaking under

specified conditions all current which cause melting of the fuse - element up to its rated breaking capacity.

. > ‘a’ PARTIAL RANGE BREAKING CAPACITY- A current limiting fuse link capable of breaking under specified conditions all current between the lowest current indicated on its operating l-T characteristics and its Rated Breaking Capacity.

C. UTILIZATION CATEGORY

> 'G'- for the general applications

> 'M'- for the protection of motor circuits ' -

Based on the Breaking range & Utilization category the fuse can be classified into to;

> gG- Full range. - breaking capacity general application fuse> aM- Partial range - breaking capacity motor application fuse

Note: At present gG fuse links are used for motor protection, which is possible if they are suitably selected to withstand the motor starting current

D. LOW POWER-LOSS- One of the most important factors while selecting a HRC fuse-link is its power-loss. The rated power loss of the fuse of the fuse-link is the power-loss value as stated by the

26

manufacturer when the fuse-link is carrying its current under specified conditions. The fuse-link selected should preferably have low power-loss. The advantage of having such fuse-link with low power loss are: 1. Saving in power/ energy 2. Lower temperature rise at fuse-switch terminals and fuse-knives.

RATED POWER DISSIPATION

(AS PER IEC 60269 FOR gG FUSE LINK)

CONVENTIONAL TIME & CURRENT FOR gG FUSE LINKS

E CO-ORDINATION: Co-ordination is the discrimination between the SCPD, Contactorand Overload relay under various conditions during the operation of ^ motor (e.g. starting, full load operation, overload, Short circuit) According |q tbe,<amount of damage acceptable IEC 60947/Part4/Secl defines two types of co-ordinations for starters. •

> Type 1 co - ordination- Requires that, under short-circuit conditions, the contactor or starter shall cause no danger to persons or installation and may not be suitable for further service without repair and replacement of parts.

> Type 2 co - ordination- Requires that, under short short-circuit conditions, the contactor or starter shall cause no danger to persons or installation and shall be suitable for-further use. The risk of contact welding is recognized, in which case the manufacturer shall indicate the measures to be taken as regards the maintenance of the equipment.

SELECTION OF FUSE AS PER TYPE 2 CO-ORDINATION:

27

SIZE In (A) MAXIMUM PERMISSIBLE POWER DISSIPATION (W)

RATED POWER LOSS (W)

00 100/160 7.5/12 7.5

0 160 16 12.7

1 250 23 17.3

2 400 34 24.93 630 48 42.2

RATED CURRENT

IN AMP

CONVENTIONAL

TIMEIN HRS

CONVENTIONAL CURRENT

Inf If

In < 4 1 1.5 In 2.1 In• 4< In <16 1 1.5 In 1.9 In16 < In < 63 1 1.25 In 1.6 In63 < In < 160 2 1.25 In 1.6 In160 < In < 400 3 1.25 In 1.6 In

400 < In 4 1.25 In 16 In

As shown above different fuses will have different cross over points. At cross over point the fuse will take over the relay characteristic i.e. fuse becomes faster than relay after the cross over point. So before cross over point relay is expected to operate & after cross over Fuse is expected to operate. So during starting the fuse is not expected to operate. So first requirement is, Cross over point should come after 6 (starting current of motor) hence we can not use Fuse 'A'.

Upto the Cross over point relay & contactor is supposed to operate. $o the cross overpoint should come before breaking capacity of contactor'i.e. 10. Hence we cannot use fuse'D'. Any Fuse 'B':&-'C' can be used for a motor. Here we are ensuring that O/L currents arehandled by starter& S/C currents are handled by>Fuse only. At the same time when fuseoperates starter should not get damage. To ensure this each combination of contactor , overload relay; and short circuit protective device muse be evaluated and tested. The tests are as follows:Discrimination test: Discrimination testing verifies that the overload relay will protect from over current conditions and that the fuse will protect under fault conditions. This test must be performed to verify overload protection. The currents for the tests shall be:

1. 0.75 Ic + 0%, -5% and2. 1.25 1c+ 5%,-0%.

Ic being the currentcorresponding to the crossover point of the mean curves representing the time-current characteristics of the overload relay and the SCPD respectively.Low level fault test: A discrete low level fault test is performed at rated voltage, related to starter's operating current. This is the more severe test on the starters and the test that will most likely cause contact welding because of the slower operation of the short circuit protective device.

Performance under short circuit:Test at prospective current "r"- The test is performed at appropriate value of prospective test current "r". The values vary with operational current of starter and are given in following table as per IEC 60947 (part 4/Sec 1 The circuit shall be adjusted to the prospective test current.

The contactor or starter and the associated SCPD, or the combination or the protected starter, shall then be connected in the circuit.The following sequence of operations shall be performed:1. One breaking operation of the SCPD with all the switching devices closed prior to the test.2. One breaking operation of SCPD by closing contactor.

Test at rated conditional short circuit current iq- This test is done if the current ''Iq" is higher than the current "r". An overload relay reliability and calibration tests verify tripping characteristics. This test is performed before and after the low and high level fault tests to verify if the overload relays sustained and damage,during above two tests.

Rated operational Current le (AC-3) Amp Prospective current "r" kA

28

0 le < 16 0116le< 63 0363le< 125 05125Ie< 315 10315Ie< 830 18630 le< 1000 301000 le < 1600 421600 le < Subjected to agreement between manufacturer &

user

Dielectric test- Dielectric tests are performed at higher than rated voltage to prove insulation adequacy after the low and high fault current tests have been completed.

Each starter manufacturer must consider a number of factors when performing verification programmes. These include, the withstand rating of the contactors and overload relays, which include both peak let -through current (Ip) related to the mechanical forces imposed on the device, and the ampere squared seconds [I2 t) let through energy, which relates to the thermal heating imposed on the device.

The short circuit device must allow for motor starting inrush currents for a starting time, usually between 2 and 8 seconds. The short circuit device must also be extremely current limiting. This ensures limiting both the duration and magnitude of a fault current to value less than the withstand ratings of the contactors and over load relays.

Selecting a short circuit device: Type 2 protection does not mean a motor starter will completely escape damage. Some device or system component will have already failed causing a fault current to flow in the circuit. It cannot prevent the initial problem from occurring however, does mean that, after the problem device or component has failed, all other components has failed, all other components in the system will be protected from further damage and will be reusable.Application considerations: Most end users over size their contactors, overloads, and fuses to allow for heavy duty start up conditions. However, in type 2 combination starters, the end user must use the manufacturer's tested combinations for that particular motor hp rating and voltage.

Using oversized combinations may cause unsatisfactory motor starter performance. Oversizing or changing any of the components because of the type of applications such as reversing motors, high torque motors, high efficiency motors, and high starting time applications like blower in the starter will drop the guarantee of type 2 co-ordination. Again it must be emphasized that not all motor applications are suitable for type 2*coordination and the user should be aware before problems are created.

SWITCH DISCONNECTOR FUSE

In a typical motor feeder as shown below the Contactor is used for normal switching. ThermalO/L relay is used for O/L and single phasing protection. The Switch Disconnector Fuse (SDF) is for following functions;

• Back up switching in case of contactor fails to operate• Isolation for the maintenance of motor feeder

This switching device is expected to do not only switching but also it has to give isolation in open position for maintenance purpose.

For the isolation the switching device has to be undergone under some tests;1. Uimp should be proper to ensure that when high voltage impulse is appeared the insulation should

not be damaged

29

2. When the 110% of Ue is applied at incoming side, in open position at the outgoing side leakage current should be less than 2mA.

3. Positive ON / OFF indication, this ensures that the handle indicates OFF position only when main contacts are actually open.

4. Clearance &Creepage distance should be sufficient to avoid tracking between two terminals & as per the standard

Following table shows different switch fuse combination units.

SWITCH- Is a device capable of making, carrying and breaking currents under normalconditions Switches are generally available upto 1000 A current rating.

SWITCH FUSE UNIT (SFU) - A switch in which fuse is provided as integral part of it

FUSE SWITCH UNIT (FSU) - A switch in which a fuse or fuse carrier unit with fuse link forms the moving contact system. These devices are quite bulky as compared to SFUs though thefunction is same.

DISCONNECTOR- It is a mechanical switching device. In open condition complies with the requirements specified for isolation functions per* IS/IEC 60947 part III. It can carry normal & certain overload currents,, but cannot make ;& break the currents i.e. it acts as a mechanical switch. It only gives isolation for maintenance. So it is used only as off load isolator but not as switch.

DISCONNECTOR FUSE- It is a combination of disconnector& fuse where fuse is stationary. The function is same as disconnector, also it gives protection since there is inbuilt fuse.

FUSE DISCONNECTOR- It is combination of disconnector& fuse but fuse is moving part like FSUs.

• SWITCH DISCONNECTOR (SD) - As explained above the switch is capable of doing only switching function i.e. making, carrying, breaking on load. Disconnector is only for isolation but not for electrical switching. But in most of the applications we need both switching as well as isolation. So the Switch-Disconnector is used. As it is clear from definition, SDs not onlyperforms switching function but also has isolation (disconnector) properties. The most common application of SD is in motor feeder. Normally, starter is used to switch on and off the motor. In addition the switch disconnector may be used to break the circuit in emergency (contactor contacts get welded)

• SWITCH DISCONNECTOR FUSE (SDF) - A switch disconnector in which one or more poles have a fuse in composite unit.

• FUSE DISCONNECTOR SWITCH (FDS) - A switch disconnector in which a fuse link or a fuse carrier with fuse link forms the moving contact.

Now a day's SDs and SDFs more popular than any other Switch fuse combination units. Depending on the applications severities the performance of switch may change. So following utilization categories are defined for Switches as per the standard. Category 'A' switches are for frequent operation & category 'B' switches are for infrequent operations, i.e category 'A' switch will have more life than the category 'B' switch. Please note that this frequent operation is not for inching duty

.The following tablespecifies the making & breaking capacities for SDF for different utilization

categoriesFrequent ! operation

Infrequent operation

Typical Applications

AC-20A(*) AC-20B(*) Connecting and disconnecting under no-load conditionAC-21 B Switching of resistive loads including moderate overloads

30

MAKE-CARRY-BREAK ISOLATION & CARRY MAKE-CARRY-BREAK & ISOLATIONSWITCH DISCONNECTOR SWITCH-DISCONNECTORSWITCH-FUSE DISCONNECTOR FUSE SWITCH-DISCONNECTOR FUSEFUSE-SWITCH FUSE-DISCONNECTOR FUSE SWITCH DISCONNECTOR

AC-22 BAC-23 B

Switching of mixed resistive, inductive loads, moderate overloadsSwitching of motor loads or other highly inductive loads

> Extended terminal shroud- Terminals are shrouded for protection against phase to phase short circuit through an external conducting path and against accidental human contact with live terminals.

> Mechanical interlock kit- Two Switch-Disconnector-Fuse units can be interlocked by mechanical interlock kit. After interlocking, the following three positions are possible:• SDF 1 in ON position SDF 2 in OFF position• SDF 2 in ON position SDF 1 in OFF position• Both SDFs in OFF position

Hence two mechanically interlocked SDFs can be used as a changeover switch.> Castell interlock- SDF units can be locked in OFF positron with the help of Castell interlock or padlock.

This.is required for the purpose of isolation.> Handle with Padlock & Door interlock- The operating handle is available with two feature Padlock

&Doorinterlock with defeat. The padlock is to lock the switch in OFF position for safety. The door interlock is to ensure, nobody opens the panel door when switch is in ON position. Door interlock feature is given with defeat facility.

> Auxiliary contacts- 1NO + INC auxiliary contact is available as an accessory. This can be suitably wired in the control circuit with: (as per AC -15)• Rated operational current (le) - 4A• Rated operational voltage (Ue)- 415V

> Fuse Puller Handle- A fuse puller is provided to facilitate easy & safe removal of fuses.SDFs are available in two different types; 3 pole& 4 pole. 3 pole SDF is used where the SDFs

31

MOULDED CASE CIRCUIT BREAKER MCCB is one which i$ assembled as an integral inner supporting and enclosed housing of

insulating material for making/carrying and breaking currents between separate contacts under normal circuit condition and also making carrying for a specified time and breaking current under specified normal condition such as those of overload condition and short circuit condition. MCCB are designed to provide protection to low-voltage distribution system.

Using the interruption capacity of an arc in air inside the moulded case,, the MCCB has been designed to interrupt current with range from few amperes to several kilo-amperes. These developments have greatly increased the system flexibility of MCCB and it is now possible to tailor an MCCB to satisfy a wide range of applications.

The three main faults of an electrical systems includes operating overloads, short-circuits and leakage to-earth. These all faults* are handled by separate parts of the MCCB in one unit itself. They are available upto600A, with breaking capacity as high as 70kA. Thermomagnetic and digital releases are the twotypes of releases used in MCCBs. The current limiting MCCB allow low cut-off current, low operating time and hence low let through energy during short-circuits. Some MCCBs also incorporate the mechanical interlocking feature which ensures that under short circuit condition, the contacts get open and gets latched avoiding reclosing.

The MCCB provide additional facilities of auxiliary contact block, trip alarm, shunt release, under voltage release, rotary operating handle, remote operator, pad locking, key locking, box clamping, earth fault release, terminal shrouds, etc. Operating mechanism is quick-make, quick-break and trip free. It is independent of manual operation. The mechanism is so designed that there is no arcing on the current carrying parts of the contacts. The strong wipe action of the contact system keeps the contact surface clear of oxide films. Contacts are of silver alloy which have long electrical life.

Operation:-The circuit breaker comprises of fixed and movable contacts. The contact can be opened and

closed by an operating mechanism. From the release assembly to the moving contacts, the current carrying conductors known as 'braids' that are to be brazed. To the other ends of the braids, heating elements are fixed. Current flows from moving contacts through braids to heating element. During overload current, the bimetal which is fixed to the heating element gets deflected. When short circuit occurs, the magnet is energized due to the magnetic force formed due high rush of current through the breaker due to the short. And the release assembly is energized. Both these operations results in movement of trip plate and consequently tripping of the breaker, opening the contacts (moving and fixed contacts) and breaking of circuit. The breaker has to be reset before switching it on again.

Features of Moulded Case Circuit Breaker :-♦ Operating mechanics is quick-make, quick-break type and trip free. It is independent of manual operation. Contacts cannot be teased into position nor can be held in closed position under fault conditions.♦ All the parts of the circuit breaker are enclosed in the moulded housing which is made of heat resistance insulating material. Only terminals are accessible for external connections. Operating handle gives a clear trip indication .It assumes a position midway between on and the off on tripping. It is provided with magneto-thermal release which has three bimetal to give thermal release for overload protection and electromagnets which offers short circuit protection.♦ Ail releases operate on a common trip-bar, so that all the phases are disconnected even when a fault occurs at only one of them thus eliminatesthe possibilities of single phasing.♦ Contacts are of silver alloy which have long electrical life. The mechanism is so designed that there is no arcing on the current carrying parts of the contacts. The strong wipe action of the contact system keeps the contacts surface clear of oxide films.

32

♦ Terminal have large dimensions to accept links or cable lugs.

33

Applications of MCCB :• Distribution Feeders-ldeally suited for incoming and outgoing feeder circuits on distribution boards• Transformers- Provide-effective protection to distribution transformers as outgoing breakers on the LT

side.• DG Sets- Used for protection and control of diesel generating sets against overloads and short

circuits.• APFC Systems- Suitable for protecting LT capacitors for power factor corrections.• DC Loads- MCCB's are suitable for both AC as well as DC applications. Hence find application in

protecting rectifier panels.• UPS- Used for UPS & Electronic equipment protection.

• Motors- Specially designed MCCBs for Motor back-up protection provide Type-2 co-ordination (as per IEC 60947) in conjunction with suitably rated contactors and relays.

Different types of MCCBs :-The Moulded Case Circuit Breakers (MCCB) are broadly classified into three different categories on the basis their breaking capacity;1. Standard or normal type2. High breaking capacity3. Extra high breaking capacityBased on the fault level the breaking capacity can be decided.Further MCCB can be classified as;1. Conventional Type MCCB; 2, Current Limiting Type MCCB

1. Conventional Type MCCB:The conventional MCCB is a zero crossover instant operating device i.e., in the event of short

circuit, it will clear the fault when it completed the 1st half cycle. The total operating time is about 15-20 m Sec. During this half cycle the entire circuit downstream would undergo tremendous stress, at times resulting in damage to the equipment.

2. Current Limiting type MCCB. As per IS/IEC 60947'*' (Partll): current limiting breaker should have break time short enough to prevent the short circuit current reaching its prospective peak value. To meet this requirement, the current limiting MCCB must respond quickly in case of fault. To achieve high speed contact separation, repulsion principle of contacts is used. Two closely spaced contact fingers, carrying current in opposite direction create a strong magnetic repulsion between the conductors at the time of S/C. High speed contact separation is produced by electromagnetic repulsion forces generated by the fault current itself. The higher the current, the greater is the force pushing the contact apart. Although rapid contact opening is important, just opening the contact quickly is not enough, the next concern is to control the arc voltage across the contacts to ensure proper arc extinction.This is accomplished by forcing the arc into the arc chute and is cooled and broken into segment in the arc chute until it is de-ionized and ceases to conduct current, thus being extinguished.

After quenching of arc when current comes to zero due to spring again fixed contact will come to original position & there can be reclosing on fault. To avoid this Magnetic release is provided which senses the fault & it opens moving contact before fixed comes to original position.

Now days we can also find two versions of MCCBs, one is with thermal-magnetic & other is with microprocessor release.

Thermal-magnetic is working on base of bimetal and electro-magnetic trigger. Thermal protection is for O/L electromagnetic principle is used for S/C. but electro magnet is peak sensing device, so even momentary transient peak can cause nuisance tripping of CB. This CB gives following protections

• Overload Protection-Through Bimetal• Short Circuit Protection- Through Electromagnet• Under Voltage Protection-Through U/V Release

34

Microprocessor or electronic release is working on monitoring of current true, R.M.S value. It is simulated and calculated from peak values, which installed microprocessor, can detect. The microprocessor based release can give following additional protections;

• Earth Fault• Neutral protection• Instantaneous over ride• Earth Leakage• Current Unbalance• Overvoltage

Specifications: MCCB has specifications similar to ACB since both follow same standard IS/IEC60947 except lcw

rating & utilization category. MCCB is instantaneously operated circuit breaker at the event of S/C. No lcw is specified with MCCB so it is not meant for the intentional S/C time delay. Hence it is not suitable for time based discrimination. So it has utilization category A• Rated Operational Voltage- Ue• Rated Impulse Withstand Voltage- Uimp

• Nominal Rated Current- ln• Rated Ultimate S/C Breaking Capacity- lcu

• Rated Service S/C Breaking Capacity- lcs

• Rated S/C Making Capacity- lcm

• Utilization Category- A

Selection of MCCB for Motor application Introduction:

MCCBs are used for overload, short circuit, earth fault and neutral protections. However selection parameters change with respect to the loads & applications. Typical Selection parameters:• Current rating• Breaking capacity• Type of protection- Thermal or Microprocessor based• System voltage

Motor characteristics being unique, needs to be considered while selecting switchgear for motors.One constraint against MCCB is the initial cost when compared to the cost of SDF.

However with MCCB as SCPD following are the factors which are superior to fuse links.1. Ease of Maintenance 2. Reduced down time3. Simplicity of operation4. No recurring cost.

Current limiting MCCB’s do have a number of practical advantages over fuses if one considers these to be worth the extra cost. These advantages will have to be weighed against the advantages offeredby fuses/like low initial cost, very high breaking capacity, very low peak cut-off current and let-through energy, etc.

A fuse is better than a current limiting MCCB in providing protection against the effects of short circuits at very high fault levels. But in most of the cases, the short circuit current are limited by the contactor, relay, cable and source impedance, generally upto 30 times the ratedcurrent of the motor. In such events, the MCCB acts reasonably.

35

Advantages of MCCB:1. Practically ho maintenance is required ,2. No stock of fuse hence reduced cost of inventory3. It avoids single' phasing when fault in one phase only4. They can be tested at sight

5. They give separate indication in case it trips on fault (centre position), on position (top) & reset position (bottom).Motor Starting Current:Motor starting current is one of the most important electrical parameter of motor to understand its electrical characteristics. The current drawn by the motor in different phases is shown in the diagram below.• Sub transient phase• Transient phase• Steady state operation• Transient phase: We are aware of the locked rotor current (LRC) encountered while starting any induction motor. The magnitude of LRC (generally published by Manufacturer) varies from about 6 to 8 times of the rated full load current of the motor (RMS value); variation is mainly due to motor design & construction.• The duration of this current is dependent on starting methods as well as connected loads. The starting times can vary from 30-40 seconds for e. g. centrifugal pump, to tens of seconds depends upon loads & applications. Careful consideration must be given to both the magnitude and the duration of the starting current.• Sub-transient phase: During the initial phase of motor starting, there exists a sub transient current, which is generally known as inrush current or peak current. The duration of this current is generally in milliseconds.

• This current is due to magnetizinginrush component of the motor starting current. Motor circuits are highly inductive; Motorcar] be started at any point of time i. e. at any point on voltage wave of the circuit. Depending on the initiation of the circuit i.e. point on the voltage wave, the LRC is offset or shifted by a DC component, this offset is called an asymmetrical current (peak current). Please refer below diagram. The magnitude of the asymmetry is directly related to X/R ratio of the circuit. By theoretical and empirical means it is established that the maximum ratio between peak and LRC can go up to 2.2.

• Device co-ordination:The devices which are commonly used in a motor feeder are1. Short-Circuit Protection Device (SCPD)- typically an MCCB, Fuses or a Switch Fuse unit2. Switching device, such as a contactor3. Protection device against overload, such as a thermal or electronic relay

With reference to the behavior of the above devices against short circuit, IS/IEC 60947-4standard has defined two types of co-ordination typologies-Type 1 & Type 2 co-ordination.

"Type 1" co-ordination permits use of devices with lower sizes hence there is an initial costsaving. This leads to quite unsafe situations because devices get damaged on fault.-Replacement of devices increases the production downtime as well.

"Type 2" coordinationmeets higher safety requirements and in case of fault, the switching and protectionequipment could start operating again without being replaced hence minimize risk ofproduction

36

downtime.It is alwaysrecommended to follow the Type 2 coordination charts provided by the switchgear

manufacturers

MCCBs for Motors:While selecting an MCCB for motor application, it is recommended that the instantaneous release setting in the MCCB is set to a value higher than the highest anticipated magnetizing inrush current, white switching-on the motor.The values for Magnetising inrush current (sub transient phase) are higher in case of high efficiency motors as compared to standard efficiency motors. Basically for energy efficient motors, X/R ratio of the system is higher hence the asymmetry component is also higher. By theoretical and empirical means it is established that the maximum ratio between peak and LRC can go upto 2.5 for high efficiency motors. Standard efficiency motors: Typical Magnitude of LRC (It): 6 times the motor rated current (Ir)Transient inrush current (peak): 2.2 x 6 Ir = 13.2lrPeak value of non trip of MCCB= 1.414*0.8* lmag

Where,lmag (Inst setting of MCCB) = X InIn: Currentrating of MCCB

Hence to avoid nuisance trip, 1.414*0.8* lmag>13.2lr Hence, lmag> 11.67lnHigh efficiency motors:Transient inrush current (peak): 2.5 x 6lr= 15lrPeak value of non trip of MCCB- 1,414*0.8* lmag

Where, lmag (Inst setting of MCCB) « X InIn: Current rating of MCCBHence to avoid; nuisance trip,1.414*0.8* lmag>15lrHence, lmag> 13.26ln

Hence MCCB used for motor back up protection should have instantaneous settings (lmag) higher than the

transient inrush current (peak) of the motor.

Contactor &Relay selection with Motor Protection Range MCCBs:It is observed many times, that whenever a customer wants to replace Switch-Fuse Combination in a motor starter feeder with an MCCB, he simply removes the Switch-Fuse Combinations and replaces the same with an equally rated MCCB. Unfortunately, the user does not pay any attention to the adequacy of the short time withstand capabilities of the contactor & the overload relay, with the revised SCPD (i.e.) MCCB.It is to be noted that when a particular frame of contactor and relay we're suggested for use with a switch-fusecombination unit, they were sized based on the I2t let through energy of the Fuse, which would clear a short circuit. As HRC fuses are fast acting (typically operating within very few milli-seconds for a heavy short circuit fault), the energyjet through would also be less during a short circuitand thus the contactor and relay would be subjected to a lesser degree of short circuit stresses. Hence, they would be sized accordingly. However, when the switch-fuse combination is replaced with an MCCB, now during a short circuit, the MCCB would clear the fault. Even with the most advanced current limiting type of MCCBs, the fault clearing time would be around 10 milli-seconds and thus the energy let through would be higher during a short circuit, than that with a HRC Fuse. Now, the smaller contactor and the relay - which were actually selected for the shorter let-through with HRC fuses - could not handle the increased let- through, with MCCBs and thus would damage.

37

Thus, what was Type-2 Coordination with Switch-Fuse combination gets converted into Type-1 Coordination with MCCB now. Thus, it leads to damage to starter components and increased production downtimes.

To avoid this, it is always recommended to use the respective 'type 2' charts provided by the switchgear manufacturers.

Effect of selecting the wrong MCCB:For motor applications, manufacturers recommend to use MCCBs designed especially for motor

protection. Selection of MCCB is recommended as per type "2" co-ordination charts published by respective manufacturers.

In case if MCCB used for motor back up protection also needs to be used for overload protection of motor, care has to be taken to match the overload curve of MCCB with that of motor. This will ensure no damage to the motor in case of overloads. However life of contactor by design is much higher than that of MCCB. Hence above solution will give much lesser life and therefore not preferred. Also if MCCB is used then there is no true protection against the single phasing of motor which can be achieved in case of overload relay. Hence this solution shall not be preferred and it is recommended to abide by the type "2" charts. In case VFDs or soft starters are used for motor protection, then High speed fuses (generally termed as semi conductor protection fuses) should be used as SCPD and not MCCB because electronic circuit (Thyristor) will get damaged by the time MCCB will clear the short circuit fault (less than 10ms).Installation care for MCCB:

MCCB should be mounted vertically, with name plate readable. It should never be mounted inverted i.e. upside down. Otherwise the arc & the heat produced at the time of breaking cannot escape. It will go into the mechanism & will lead to damage to MCCB or even flashover since arc always move in upward direction. Whenever mentioned the line should always be connected to the top & load at bottom side. This care should be taken for MCCB with line load bias. Now a day's most of the MCCB do not have line load bias. '

38

ACCESSORIES OF MCCB:Internal:

• Shunt Release: The shunt release provides remote tripping of the circuit breaker. The coil of shunt release is designed for short time. The pick up voltage is 65% to 130% of the rated coil voltage in AC& 75% to 120% of the rated voltage in DC.

As it is short time rated, microswitch is provided in the release to cut off the supply. Once the MCCB trips, this prevents burning of coil even if the supply to shunt coil is continuously fed

• Under Voltage Release: The under voltage release is used for protecting theinstallation against low voltage situation. The under voltage release is designed to operatewhen the voltage drops below a tripping threshold. This can also be used for remote tripping& electrical interlocking purpose.

Operating Voltages: Pick up Voltage- 85% to 110% of rated coil voltage Drop off voltage- 35% to 65% of rated coil voltage

• Auxiliary contact block: The auxiliary contact block is used for remote signaling and control purposes. This consists of one or more than one potential free change-over contacts. It indicates the circuit breaker status whether open or closed.

• Trip Alarm Contact: The trip alarm contact gives tripping indication once the MCCB trips. The potential free change over contacts can be used for indication or for control circuit.

Note: MCCB is very compact device so there is accessory fitting limitation. There is space to fix only two internal accessories inside the MCCB.External:

• Adaptor Terminal with Shroud/Spreader Links with Barriers (Shrouds): Adaptor terminals/ Spreader links can be connected to basic MCCBs to facilitate termination of higher size of cables / links (eg/ Alluminium conductor). The barrier is made of material with high dielectric strength. It is used to ensure the electrical clearance between breaker terminals. It also prevents troubles caused by contact of conductive material to the terminals.

• Rotary Operating Mechanism: The circuit breaker has a toggle handle operating mechanism; which also serves as switching position indicator - ON, OFF, TRIP But when the circuit breaker is installed in switchgear cubicles or distribution boards, it requires to be operated outside of the switchgear cubicles or distribution boards. The/mechanism is designed for, direct attachment to the MCCB and transforms the toggle handle movement into a rotation, ft 'facilitates operation of MCCB when-the panel door is closed. It facilitates following features; -, * ,il

> Easy operation> Door interlock 'mechanism- It does not allow the circuit breaker to be switched "ON" when

the door of switchgear, cubicles or distribution boards is open thus prevents the operate from an unsafe condition.

> Door interlock defeat facility- This enables only authorized personnel to open the door with MCCB in ON condition for the purpose of inspection.

External Earth Fault Module: It gives Protection against earth faults. The module has in built CBCT. It has solid state design. The adjustable current setting range is available is from 10 to 50% of rated current. Also there is 100/200 msec selectable trip time delay. It is suitable for both 3 phase 3 wire & 3 phase 4 wiresystems. The earth fault module has built in test facility. There is potential free NO contact to trip MCCB through shunt trip. Manual reset provision is given for positive fault acknowledgment

39

.• Electrical Operating Mechanism (EOM): EOM is used to operate the MCCB from any remote

location like an ACB. It has motorized operated mechanism which is used to close the MCCB from any remote location. Electrical tripping is done through shunt release.

• Mechanical Interlocking Kit: Two MCCBs can be interlocked using base plate mechanism, in side by side configuration. This accessory can be used where there is need to operate any one CB at a time. Like, interlocking between Transformer & DG breakers.

• Mechanical Interlocking using key locks: For mechanical interlocking through extended rotary operating mechanism, a panel mounted key lock is available. With the key lock arrangement interlocking can be done for 3 l/C& 2 Bus coupler scheme.

MOTOR PROTECTION CIRCUIT BREAKERS

MPCB's combine, for a motor feeder, the functions of switch, short circuit protection deviceand an adjustable range overload relay. While the short circuit threshold levels in MCCBs arein range of 5-10 times the rated current for distribution application purpose, in MPCBs. It ismore than 11 times considering the typical motor starting phenomenon.They are available from0.16 A to 63 A and are suitable for switching motors upto a rating of45HP.Need for MPCB ?The MCCB has some limitations while used for motor switching:

1. It cannot provide effective protection in overload range (100%-200%)2. Ambient temperature compensation may not available3. The Mechanical life is not very high4. TYPE 2 Co-ordination requires special attention5. Nuisance tripping during starting.

Hence motor protection MCCB can only provide Back-up protection for motor and starter equipment. An MPCB has a switching capacity of AC-3 rating, having an adjustable thermal overload release and also suitable short circuit threshold. It provides Short circuit protection through an instantaneous release set at 12 In.

ADVANTAGES:' '

1. Very high short circuit breaking capacity- upto 100 KA2. Equipped with wide range of current adjustment, so that actual motor current can be set in

MPCB.3. Panel Size- Considerable space; js required to install a starter & SCPD (i.e. either fuse or MCCB) so

the panel size has to be increased, Due to compact size of MPCB & elimination of O/L relay the overall size gets reduced.

4. Stand alone motor protection- MPCB can be used for switching, control, O/L & S/C protection of motor in machine with single motor and not requiring remote switching.

5. Increased life of motor feeder- In order to have compact MCC, fuseless motor protection can be offered using MPCB with contactor. Due to a contactor life of motor feeder increases; it also provides remote switching.

APPLICATIONS OF MPCB's : • They can be used for switching, control and protection of motor in machine with single

motor and not requiring remote switching.• MPCB's ore also suitable for DC switching.• Fuse Monitoring.

40

MPCB can be fitted with following accessories :• Auxiliary contact block (1 NO + INC)• U/V Release• Shunt Release• Alarm Contact Block• Combined alarm & Aux contact• Shunt trip device• Under voltage trip device• External operating device

41

MCB

MCB is used for overload & short circuit protection of feeders having lower current rating e.g. lighting & distribution feeders. It is available from 0.5 Amp to 100 Amp.

MCBs are primarily designed to protect wiring installation from over current faults

Thermal characteristics : MCB has following non trip current and trip current

1.13 In - Must not trip in under hour

1.45 In - Must trip within 1 hr<63 A and 2 hr>63 A

2.55 In - must trip between 1 & 60s < 32 A and 1& 120s > 32 A

Right selection of MCB and wire :

In order to achieve perfect overload protection the normal current of the MCB (Im) should not be less than the design current of the circuit (Ic) and that (Im) should not exceed the current-carrying capacity of the conductors (Iw), and that the current causing effective operation of the protective device Im does not exceed 1.45 times the current-carrying capacity of the conductor Ic, expressed as

lc<lm<Iw

Example:

42

Cable fails drastically30A

If. For load a of 800w, 220V; Load current is 4A, then wire capacity should be minimum 11A (i.e. lsqmm) and MCB rating should be 6A.

11A

8.7A 6A

4A

0

As shown in fig the tripping current of MCB is 8.7A i.e. 1.45 times the In rating (as per standard) which is lower than continuous current rating of cable, hence MCB will trip before the wire damages. While the wire damage current is 16A i.e JU45 times wire capacity.

So the MCB will trip when the current is higher than load current but less than the short duration capacity of wire,

The selection of MCB also depends No of poles. For single phase circuit 1 pole, 2 pole or SPN MCB's are used, For three phase circuit 3 pole, 4 pole orTPN MCB's are used.

MCB is available in 3 tripping characteristics : B, C, D

The thermal characteristic is common for all, but the magnetic thresholds are set at differentlevels.

Type Magnetic setting(Non-adjustable

Application

B 3 - 5 In For sensitive circuitsC 5-10 In For general lighting fan, socket outlet circuitD 10-20 In Switching of motors, capacitors, control transformers, etc.

MCB con be fitted with accessories like• U/V Release.• Shunt Release.• Auxiliary Contact Block.• Trip Alarm Contact(Not all accessories can be used together.)

Advantages of MCB: -1. It can be used by skilled/unskilled workmen.2. The overload and short circuit settings cannot be tampered since they are non adjustable3. Mechanism of MCB is trip free.4. It can be used as a functional switch.5. MCB can be used as an isolator.6. Its a fully enclosed unit and hence no ageing problems.7. MCB is a cost effective device.

Selection Parameters for MCB:43

Short duration capacity of cable (1,45 times FL)

16A

Continuous current "rating of Cable

Tripping current of MCBContinuous current rating ofMCB

Maximum design current incircuit

• Rated Voltage• Rated Current• Rated S/C Breaking Capacity• Type of Curve• Number of Poles• Energy Limiting Class

RCCB:

A device for making and breaking a circuit manually under normal conditions and for breaking a circuit automatically when the residual current exceeds a predetermined amount.

A Residual Current Circuit Breaker gives protections against direct or indirect contact. It is available in;2 P]ole (for single phase loads) & 4 Pole (for 3 phase loads). RCCB cannot give O/L & S/C protection^

During earth leakage a small leakage current returning back to earth through earthingconductor or human body (not flowing bock through RCCB) causes tripping of RCCB.

RCCB is available in following sensitivities30 mA- Forpersonal protection (e.g. Domestic application)100 mA- Installation protection in industries.300 mA- Installation protection in industrieshaving inherently high leakage currents.

44

45