SPE-DiSPR25.07.2003

Type-Z Co-ordination

It is well known that induction motor is most widely used drive in the industry. The termdrive stands for a motor and its control apparatus i.e. starter. The power is fed to themotor through starter. The stafler consists of a Shon Circuit Protective Device SCPD(Fuse or MCCB), Switching Device (Contactor) and Overload relay. Co-ordination is thediscnmination between the SCPD, Contactor and Overload relay under vanousconditions during the operation of a motor (e.g. starting, full load operation, overload,Short circuit)

Ttpes of co-ordination

The flow of current in the starter during the breaking time may cause damage to thestarter itself. According to the amount of damage acceptable ISl3947/Part4/Secl definestwo tvDes of co-ordinations for starters as.

Type-1 Co-ordinationDunng type-l co-ordination test: -

o Damage to Contactor and O/L Relay is acceptable.It means that starter may need maintenance of contactor or overload relay or theirreplacement as a whole.

r The Starter may be inoperative after each operation.This means that even if starter becomes inoperative i.e. needs maintenance after eachoperation during type-l test, it can pass the test.

o No discharge of parts beyond the enclosure is allowed.The discharge of material or parts is acceptable upto the extend that it may not burstout of the starter enclosure which may result into a causality or an accident.

. Contactor and/or O/L Relay and release of CB shall be reset if necessary. In caseof fuse protection all fuse links shall be replaced.

Type-2 Co-ordinationDuring tlpe-2 co-ordination test: -

o No damage to O/L Relay and other parts of starter is acceptable.

r Welding of contactor contacts is permitted if they are easily separated (e.g. by ascrewdriver) w/o significant deformation.

lt means that if due to the excessive heat produced in case of an overcurrent or shortcircuit the contacts of the contactor get welded and it is possible to separate them byputting a screw driver between them and it is seen that contacts does not lost theirshape r.e. there is no significant dcformation of contacts. The starter can pass thetvDe-2 t€st.

. No replacement of pal1s is permitted dunng test except replacement of fuse links.Any condition should not anse that may result into replacement of any part of thestarler like O/L relay or contactor. Only replacement of fuse links is allowed in casethey get melt down due to short circuit.

Criteria for type-2 Co-ordination.

o Fuse should not blow dunng stafting of motor.Generally the starting current of the motor is several times the full load current of themotor (depending upon the tlpe of staner e.g. 6-8 times for DOL, 2.5-3 times forStar-Delta, 2.5-4 times for auto-transformer Starter,2.6 times for rotor resistancestarter). The fuse characteristic should be selected such that fuse should not blow atthe time of starting otherwise it will not be possible to start the motor.

o Cross-over of relay and fuse characteristics must be before the breaking capacity oIcontacIor.

I y p e - 2 C o - o . d i . o t i o . C u r v e

F L r s e C h d r o c t e r i s 1 : i c s

O L R C h o r o c t e . i s r i . s

, - - , / C a . ra . l . r B . e o l , . i . E

! o p a c r r y

r O/L relay should withstand the cuffent till the fuse clears the fault.

How to achieve type-Z Co-ordination.

r Selection of Fuse.For selection of fuse time- current ('I' vs. 't') characteristic of motor for starting period

and fuse characteristic should be plotted on same curve. To avoid blowing of fuse at thetime of startins the characteristic of fuse should lie above the motor curve.

r!

ti

There should be appropnate time margin to ensure that fuse does not operate undulyduring staning. For this the pre-arcing time (melting time) of the fuse should be highetthan starting time of the motol.The pre-arcing time current of a fuse is the time between commencement of fault currentlarge enough to cause the fuse elements to melt and instant when arc is initiated.

o Cross-over of fuse and O/L relay characteristicCrossover of fuse and O/L relay characteristic should take place before breaking capacityof contactor is reached. To ensure this we must select a fuse whose characteristics wouldcross relay characteristic in the zone of more than 6 times, the rated current of the motor

but less than the rated breaking capacity of contactor.

Overload Relay Trip Class

According to IS13947 (Part4/Secl):1993, Table II(Page 29) the Trip Class of an OILRelay designates the value of the maximum tripping time(in seconds) under theconditions specified in sub-clause 7.2.1.5.l,Table III(Page 40), Column D.

TABLE IITrip Classes of thermal, time-tlelay mdgnetic or solid state overload relays

time (Tp) in rxlconds lhe cq,lditions specified in Sub-

'?<Tpcl0

44Tpcl06<lp<209{Tp<30

TABLE IIILimits of operation of time-delay overload relay when energiTed on all poles

Reference ambientair temperature

+40 DegreeCentigrade

+20 DegreeCentigrade

As per Clause 7,2.1.5.1(IS 13947'Part 4/Sec 1)

The o/L relay shall comply wirh the requirements of table III when tested as follows:

a) with the b/L relay or starter in its enclosure, if normally fitted, and at A times the

cument serting, tnpping shall not occur in less than 2h starting from the cold state, at

the value of reference ambient air temperature stated in table IIL However, when the

O/L relay terminals have reached thermal equilibrium at test current in less than 2h,

the test duration can be the time needed to reach such equilibrium'

b) when the cunent is subsequently raised to B times the current setting, tripping shall

occur in less than 2h.c) For class 10A O/L relays energized at c times the curent setting, tripping shall

occur in less than 2 min staning from thermal equ'ilibrium at the cufrent setting, in

accordance with IEC Publication 34-1 Sub-clause l8'2'

d) For class 10,20 and 30 o/L relays enetgized at c times the current setting, tripping

shall occur in less than 4,8 or 12 min respectively, starting from thermal equilibrium

at the current setting.e) At D times the curent setting, tripping shall occur within the limits given in table II

for the appropriate trip class starting from the cold state'

ANNEXURE.I

The need for co-ordination of Shorl Circuit Protective device (SCPD) and Overloadprotective device is very important because, without which motor and its feeder cableswill get damaged.When a short circuit occurs, the SCPD takes a finite time to intenupt the fault. Duringthis time, the current rises rapidly and a huge amount of energy is let through the circuit,If the let through energy is high, it can cause damage to the downsffeam devices e.g.change in O/L relay characteristics.Fuses were traditionally used as SCPDs because of low initial costs and because they arecapable of interrupting faults faster. Fuses have excellent current limiting characteristicsand can be applied safely on systems having high fault levels. However the cost ofreplacing fuses and downtime is very high and hence use of MCCB, as SCPD isbecoming more prominent. The use of MCCB will not expose the motors and associatedcables connected to the system to heavy flow of currents and heaVy dtssipation of energyfor every fault on the system.One constraint against acceptance of MCCR is the initial cost when compared to theswitch fuse units. For detailed comparison on cost basis, one must consider also theadvantages listed below & the associated cost.1. The size and weight of MCCB is much lesser.2. MCCBs are practically mainienance free.3. The down time of the systeni is much smaller.4. No fuse inventory is required.5. MCCB does not create intentional single phasing.6. MCCB gives positive trip ildication.7. MCCB can be set for a typical current.8. MCCB can give indication status either locally or remotely and can accept

accessories like auxiliary contacts, shunt trip, etc.

To sum up, MCCBs do have a number of practical advantages over fuses, if one considerthese to be worth the extra cost.

Let through energy (I2t)rWhen a shorl circuit occurs, the S.C.P.D. takes a finite time to interrupt the fault. Duringthis time, the current rises rapidly and a huge amount of energy is let through the circuit.If the let through energy is high, it can cause damage to the O/L telay, thus changing itsch aracteristi c s.The Operating time of a MCCB is greater than that of a fuse because the operalingmechanism of MCCB takes some time to open the contacts of MCCB but fuse meltsimmediately. So, the let through energy in case of MCCB is higher than fuse.The time-cuffent curves for various fuses and MCCB's are studied and it is found that theOperating time for DM range of L&T MCCB's is around 6ms. The operating time i.e.pre-arcing time of fuses is as follows.2-63A fuses 0.01-10000 seconds for a fault current of 1kA.63-630Afuses 0.04-10000 seconds for a fault cunent of25 kA.So for low value of faults MCCB is faster than t'use. As fuse is having inverse timecharactenstics and MCCB is having definite time characteristics so for high value of faultcurrents fuse will be faster because the operating mechanism of MCCB delays theinterruption of current.

Because of this the downstfeam devices are subjected to a greater amount of let through

energy in case of MCCB's. So the contactor rating in case of MCCB fbr a parttcular

rating of motor is higher than the fuse.A Study is done for L&T's MNX contactors used to achieve type-2 co-ordination for

DOL Starters with Fuse-switch and MCCB for various ratings of motors and the data is

compiled in tabular folm.

Motor Rating(kw)

MNX Contactor forDOLwith F-Sw

MNX Contactor forDOL with MCCB

) . I MNXI8 MNXI84.5-5.5 MNXI8 MNX327.5 MNXI8 MNX8O9.3-11 MNX25 MNXSO

l3 -15 MNX32 MNXSOt8.6-22.5 MNX45 MNX8O26-33.5 MNXTO MNX8O37 MNXSO MNXSO45 MNXg5 MNX95l l MNXIIO MNX1IO6'7 .5-7 5 MNX140 MNXl4O80-90 MNX185 MNX2251 1 0 MNX225 MNX265130-147 MNX265 MNX265150-180 MNX325 MNX325200 MNX400 MNX4OO

-Sangh Priya RahulSPE-Design