mccb fundamentals

Superior Series MCCBcurrent limitingmoulded case circuit breaker

LK-Electric Company

1

The acronym LKE stands for Lauritz Knudsen Electric. In the late 1970s, the LKElectric Company was established in Singapore by its parent company, LKE(Europe) of Denmark. It is to manufacture LK’s range of products, namely, theDomino, the Tabular of low tension switchboards, the ELC-24 medium voltagepanel and the Ring Main Unit (RMU).

By the mid-80s, with an influx of technology from Denmark, a ComponentsDivision was set up. This was also to cater to the growing demands in the lowvoltage sector. Popular products such as the Switch Fuse of the QSA series,Miniature Circuit Breakers (MCB), Moulded Case Circuit Breakers (MCCB),Load Break Switches (LK’s ELC-24) and Vacuum Circuit Breakers (LK’s VB-1)were all produced in the Singapore factory.

In 1992, the LKE Electric Company was established in Malaysia and by1994, has offices in Zhuhai, Shanghai and Beijing in China. At the same time,LKE Electric entered into a partnership with the Cubic Modular System A/Scompany of Denmark to produce the Cubic Modular Switchboard. And sincethen, there is no looking back for the company.

The company is always striving to benefit its customers. Efforts in R&D areconstantly focused, especially with the current era of modern technology, toenable its products to be of a higher quality and safer, yet at the same time,aesthetically pleasing and affordable. The company also prides itself withupgrading of its production facilities, in keeping up with technology, to fulfillstringent process and quality control requirements. Building a relationship withthe customers and understanding their needs with a zero-defect andunbeatable range of products are the main objectives of LKE Electric.

Focusing on these objectives, LKE Electric has become an industry leaderwith its MCCB Superior Series, 6 & 10kA MCB series, 12kV Load Break Switch(LBS) and Vacuum Load Break Switch (VLBS) and SF6 Breakers.

The History of LKE Electric

2

LKE Electric’s Superior Series Current Limiting MCCB was developed with thelatest technology for heavy duty usage: a magnetic trip unit for reliable qualitytripping when short circuit occurs, repulse force for moving and fix contactconstruction, zero arc distance for the arc chute moulded in with thread nut for thecase and cover and a long lasting BMC material for mounting, super mechanicaland electrical strength.

ApplicationThe current limiting MCCB Superior series is suitable for circuit protection in

individual enclosures, switchboards, lighting and power panels as well as motor-control centers. The MCCB is designed to protect systems against overload andshort circuits up to 65kA with the full range of accessories.

MechanismThe MCCB Superior series is designed to be trip-free. This applies when the

breaker contacts open under overload and short circuit conditions and even if thebreaker handle is held at the ON position. To eliminate single phasing, should anoverload or short circuit occur on any one phase, a common trip mechanism willdisconnect all phase contacts of a multipole breaker.

MaterialThe Superior series circuit breakers’ housing is made of BMC material, which

is unbreakable and has a very high dielectric strength, to ensure the highest levelof insulation. The same material is also used to segregate the live parts in betweenthe phases.

AccessoriesTo enhance the Superior series MCCB, internal and external modules can be

fitted onto the breaker. They are as follows:• shunt trip coil • undervoltage release• auxiliary switch • alarm switch• motorized switch • rotary handle• plug-in kit (draw-out unit) • auxiliary & alarm switch

International StandardsThe MCCB Superior series conform and meet the requirements of these

international governing bodies:• IEC 60947-2 from the International Electrotechnical Committee• BS EN60947-2 from British Standards• BT/T14048-2 from China• NEMA AB-1 from American Standards• VDE 0660 from Germany.

Superior Series Current Limiting MCCB

NEMA

Having undergone rigid testings and achievingaccreditation from SIRIM QAS of Malaysia andTILVA from China, these test reports affirm thesuperior quality of LKE Electric Company’sCurrent Limiting Superior Series MCCB.

Accreditation of the SuperiorSeries MCCB

3

4

The MCCB Superior series has exceptional performance characteristics at the

rated breaking current of 50KA. This includes:

• Limiting short-circuit current, lp, to 106KA (maximum peak let-through

current)

• Interrupting fault current, Ic, 50.7KA at 436V

• Breaking time of approximately 0.00949 seconds

• Arc-quenching time at approximately 0.0066 seconds.

As a result, the peak short circuit current (lp) is limited to the cut-

off current (Ic). This leads to a substantial reduction in electrodynamic

stresses in the overall system. l2 let-through (proportional to the

shaded area) is considerably reduced, resulting in lower thermal

stresses in down-stream equipment and connecting cables.

Exceptional Current LimitingQuick-Breaking Performance

Testing Current Wave Curve

5

Features1 BMC material for

base and cover

2 Arc chute

3 Mounting for ST or

UVT connection block

4 Trip-free mechanism

5 Moving contacts

6 Clear and IEC-

compliant markings

7 Magnetic trip unit

8 Thermal trip unit

9 Compact size

Featuresa Arching chamber

b BMC

c Handle

d Magnetic trip unit

e BMC

f Tripping mechanism

g Moving contact

h Fixed contact

i Thread nut

2

1

6

8

7

43

5

9

cdba

h g f e

i

The Superior Series MCCBan in-depth look

MCCB Arc Chamber (diagram 1)The MCCB arc chamber is specially designed with an arc channel as a

flow guide to improve the capability of extinguishing the arc and reducing thearc distance.

MCCB Base (diagram 2)Mounting screws are used to insert thread nuts in the MCCB base. The

cover can withstand high electromagnetic force during a short-circuit; thisprevents the MCCB cover from tearing off. This is an improvement over self-taping screw of other models.

Fixed Contact (diagram 3)The MCCB fixed contact does not have any mounting screws near the

contact points. A steel screw can generate heat and the magnetic fluxsurrounding the conductor carrying the current can create a very hightemperature. If a short-circuit occurs, it will cause the contact points to bewelded or melted.

Materials (diagram 4)The base and cover of the MCCB are made of a specially formulated

material, i.e. bold moulded compound (BMC). It has a high-impact thermalstrength, fire resistant and capable of withstanding high electromagneticforces that occur during a short-circuit. Majority MCCB manufacturers in themarket use pheonolic compounds with less electrical and mechanicalstrength.

Repulsive Force (diagram 5)An electromagnetic repulsive force is where the force works between a

current of the movable conductor and a current (I) in the reversed directionof the fixed conductor. This is an improvement of the electromagnetic forceduring breaking over other models.

The Technology of Tripping Devices

Diagram 1

Diagram 2

Diagram 5 Diagram 4 Diagram 3

6

7

Thermal Magnetic Type (Solenoid)

MCCB Superior Series, all models

Time-Delay Operation

Time-delay operation occurs when an overcurrent heats and warps the bimetal

to actuate the trip bar.

Instantaneous Operation

If the overcurrent is excessive and the magnetization of the solenoid coil

strong enough to attract the armature, an instantaneous operation will

actuate the trip bar.

Hydraulic Magnetic Type

MCCB Superior Series, selected models only

Time-Delay Operation

In an overcurrent flow situation, the magnetic force of the coil overcomes the

spring and closes to the pole piece, thereby attracting the armature and

actuates the trip bar. The delay is obtained by the viscosity of silicon oil.

Instantaneous Operation

If the overcurrent is excessive, the armature is instantly attracted without the

influence of the moving core.

The Technology of Tripping Devices

Thermal magnetic tripping

(available for all models)

Hydraulic magnetic tripping

(available for LKS-63 C and S

and LKS-100 C models only )

1.30 In (heated state) Operative Time (hr)1.05 In (cold state) Operative Time (hr)

Thermodynamic Release Ambient Temperature; land +40ºC, marine +45ºC

63 < In ≤ 100

Operating Current forMagnetic Release (A)

Table

A

Thermodynamic Release Ambient Temperature; land +40ºC, marine +45ºC

1.05 In (cold state) Operative Time (h) 1.30 In (heated state) Operative Time (h)

100 ≤ In ≤ 800

630 ≤ In ≤ 800

10 ≤ In ≤ 63 1

2

2

2

1

2

2

2

10In + 20%

10In + 20%

5In + 20%10In + 20%

8In + 20%

Rated Current (A)

Operating Current forMagnetic Release (A)

Table

B

10 ≤ In ≤ 63 2 2 12In + 20%

Rated Current (A)

Bi-metal Overload Tripping

Bi-metal overloads are designed to protect the motor against overheating due

to excessive current loading and at the same time, allow full utilization of its rating.

To date, LKE designs MCCB according to the international standards (see below).

These thermomagnetic overcurrent releases (bi-metal) are non-interchangeable

thermomagnetic devises. They incorporate heat sensitive elements for protection

against overcurrent and the rated current of the releases (Ith) must be equal to or

greater than the operating current of the circuit breaker.

Inverse Time Delay Tripping

The thermodynamic release of LKE’s circuit breaker affects the inverse time

delay tripping, while the magnetic release affects an instantaneous tripping. It is

shown in Table A (distribution circuit breaker) and Table B (motor protection circuit

breakers).

Tripping Characteristics

Multiple of Set Current (A)

1.05 (In > 63)

1.05 (In < 63)

> 2h

> 1hour

cold

cold

< 1hour warm

< 2hour warm

1.30 (In < 63)

1.30 (In > 63)

Tripping Time Operating Condition

8

9

Further adjustments are unnecessary or allowed for the circuit breaker or its

accessories during service as their settings have been fine tuned by LKE Electric.

The handle of the circuit breaker has three positions which will indicate when

the breaker is closed, opened or tripped respectively. When the handle is at the trip

position, it must be pulled backward first so as to reset the breaker and be ready

for closure.

If the security seal of the circuit breaker is kept intact for 24 months from the

delivery date, and instructions are followed for its storage and maintenance, any

inherently defective product will be repaired and/or replaced at no further expense

to the customer.

Recommended Tightening Torque of the MCCB Terminal Screws

Installation and Fittings

Terminal Screw

Pan head screw M8

Pan head screw M5

4.9 - 6.9

2.3 - 3.4

7.8 - 12.7

13.7 - 22.5

18.6 - 31.4

22.5 - 37.2

40.2 - 65.7

Hex. socket head screw M8



Hex. socket head screw M10 c/w terminal bar

Hex. socket head screw M12 c/w terminal bar

Tightening Torque (Nm)

Preferred Conductor Sizewith Preference to Current Rating

Current Range (A) Conductor Size (sq. mm)

8

12

20

25

32

50

65

85

100

130

150

175

200

225

250

275

300

350

400

1

1.5

2.5

4

6

10

16

25

35

50

50

70

95

95

120

150

185

185

240

Copper Bar Dimensions forCurrents above 400A

Rated Current (A)

Copper Bars

Dimension (mm)Number

400

500

630

800

1000

1250

2 30 x 5

40 x 5

50 x 5

60 x 5

80 x 5

100 x 5

2

2

2

2

2

Arc Quenching Distance A & B (mm)Model Code

LKS-100S LKS-100H LKS-225CLKS-225N

LKS-225SLKS-225H LKS-400C

LKS-400SLKS-400HLKS-600SLKS-600HLKS-800SLKS-800H

LKS-63C 15

20

50

60

100

LKS-63SLKS-100CLKS-100N

Side view Front view

measurements are in millimeter (mm)

Arc Quenching Distance

Due to the unique design of the arc chute with an Arc Top Plate, the arc quenching

level is very low compared to other conventional models.

10

11

It is very important to select and apply the right MCCB for a long lasting and

trouble-free operation in a power system. The right selection requires a detailed

understanding of the complete system and other influencing factors. The factors

for selecting a MCCB are as follows:

1 ) nominal current rating of the MCCB 2 ) fault current Icu, Ics

3 ) other accessories required 4 ) number of poles

Nominal Current

To determine the nominal current of a MCCB, it is dependent on the full load

current rating of the load and the scope of load enhancement in future.

Fault Current Icu, Ics

It is essential to calculate precisely the fault current that the MCCB will have to

clear for a healthy and trouble-free life of the system down stream. The level of fault

current at a specific point in a power system depends on following factors:

a ) transformer size in KVA and the impedance

b ) type of supply system

c ) the distance between the transformer and the fault location

d ) size and material of conductors and devices in between the transformer

and the fault location

e ) the impedance up to the fault junction.

One can safely use an empirical formula, assuming a 5% impedance of the

transformer, to arrive at the projected fault level at transformer terminals of the

secondary side. This means that the projected fault current will be approximately

20 times the full load current of the transformer. The impedance of the cables and

devices up to MCCB further reduce the fault current.

Icu: ultimate short circuit breaking capacity whereby the prescribed conditions

according to a specified test sequence does not include the capability of

the circuit breaker to carry its rated current continuously.

Ics: service short circuit breaking capacity whereby the prescribed conditions

according to a specified test sequence includes the capability of the circuit

breaker to carry its rated current continuously.

Other Accessories Required

The selection of other accessories required will depend on the control and

indications as required. The range available are as follows:

a ) Under voltage release b ) Shunt-trip release

c ) Auxiliary contact d ) Trip alarm contact

e ) Rotary operating mechanism f ) Motor operating mechanism

g ) Insulation barrier h ) Plug-in kit

How to select a proper MCCB for protection

LKS-100C

LKS-63S

60

12

LKS-100N

10

Symmetrical 5 15 18 35 50 65

Interrupting Capacity (kA)415V AC

Bre

aker

Rate

d C

urr

ent

(A)

Quick & Wide Selection Guide

LKS-800H

LKS-600H

LKS-400H

LKS-225H

LKS-100H

LKS-800S

LKS-600S

LKS-400S

LKS-225S

LKS-100S

LKS-225C LKS-225N

15

20

30

40

50

75

100

125

150

160

180

200

225

250

300

315

400

500

600

700

800

The Superior series current limiting MCCB is available in 8 frame sizes, with ratings from 10A to 800A. Each

frame size offers several interrupting capacities (Icu), up to 65kA, at AC 415V. Available in C, N, S and H

configurations for various breaking capacity, the space-saving current limiting MCCB Superior series provides

greater design flexibility than before. The C and N configurations are for general use in a general circuit. A best-

seller worldwide, the C and N ranges from 60A to 800A in frame sizes. Also for general usage, the S and H

configurations have a higher interrupting capacity, from 15A to 800A in frame sizes, is actually an upgrade from

the C and N range.

LKS-63C

13

68

Model Code

Ele

ctr

ical

Chara

cte

rist

ics

AF Frame Size

Ui

P

n

Icu

Rated Insulation Voltage(V), 50 Hz

Poles

OperationalPerformance

Capability

Rated Ultimate Short Circuit

Breaking Capacity(kA)

415V

240V

MC

CB

– E

lect

rical

& M

echa

nica

l Fea

ture

sO

verc

urr

ent

Rele

ase

s

Rated Service Short Circuit

Breaking Capacity(kA)

Adjustable Thermal & Magnetic Trip Unit

Test Trip Button

Weight (3 pole)

Mechanic

al

Chara

cte

rist

ics

kg

mm

a

b

Ics

a

bc

C N S H

100

690

415

3

1500

8500

15, 20, 30, 40, 50, 60, 75, 100

18 35 50 65

35 70 100 130

9 18 25 33

18 35 50 65

available

–

available

1.6

90

155

LKS-100

C S

63

690

415

3

1000

8500

10,15, 20, 30, 40, 50, 63

5 15

10 30

3 8

6 15

available

available

–

0.9

75

130

68

LKS-63

A Rated Current at 40˚ C

withcurrent

w/ocurrent

415V

240V

Ue Rated Voltage (V), 50 Hz

c

c

Thermal & FixedMagnetic Trip Unit

14

86

S H

800

690

415

3

500

2500

700, 800

50 65

100 130

25 33

50 65

available

available

available

10.5

210

275

103

LKS-800

S H

600

690

415

3

1000

4000

500, 600

50 65

100 130

25 33

50 65

available

available

available

9.5

210

275

103

LKS-600

S H

400

690

415

3

1000

4000

250, 315, 350, 400

50 65

100 130

25 33

50 65

available

–

available

6

140

257

103

LKS-400

C N S H

225

690

415

3

1000

7000

125, 160, 180, 200, 225

18 35 50 65

35 70 100 130

9 18 25 33

18 35 50 65

available

–

available

3.5

105

165

LKS-225

15

LKS-600 S, H

LKS-400 S, H

LKS-800 S, H

LKS-225 C, N, S, H

LKS-100 C, N, S, HLKS-63 C, S

Outline Dimensions of the MCCB

LKS-63 C, S LKS-100 C, N, S, H

Operating Characteristics & Ambient Compensation

LKS-225 C, N, S, H

LKS-600 S, H

LKS-400 S, H

LKS-800 S, H

16

17

Auxiliary Contact (AUX)

The auxiliary contact is used for remote signalling and control purposes. This

consists of one or more than one potential free change-over contacts. It also acts

as an indicator whether the circuit breaker’s status is opened or closed.

Configurations: 1NO + 1NC

2NO + 2NC

Undervoltage Release (UVT)

The undervoltage release is used to trip the MCCB when there is a drop in

voltage. The UVT can also be used for remote tripping and electrical interlocking

purposes. The tripping threshold is 35% to 70% of the rated voltage. Pick-up

voltage is ≥ 85% of the rated coil voltage. The operating voltage is AC 220V or

380V at 50/60Hz.

Shunt Trip (ST)

The shunt release is used for remote tripping of the MCCB under abnormal

conditions. The operating voltage is 70% to 110% of the rated voltage.

Alarm Switch (AS)

When a tripping occurs in the MCCB, it is indicated by the alarm switch. The

potential free change-over contacts can be utilized for indicative and circuit control

purposes.

Configurations: 1 NO + 1 NC

2 NO + 2 NC

Internal Accessories

Auxiliary contact

Undervoltage release

Shunt release

Alarm switch

Insulation Barrier

The insulation barrier should be utilized on the MCCB to facilitate

termination of cable links. Used on the incoming side of the MCCB, it

provides additional safety as it is made of superior insulating materials

that have good mechanical and electrical properties. The insulation

barrier prevents accidental contacts and flash-over between each

phase and is highly recommended for the breakers especially during

installation of a switchboard.

Plug-in Kit (PIK)

The MCCB plug-in kit is designed to replace the standard

terminal with a rear connection to improve the opening capability.

Suitable for isolation, the plug-in kit has a better contact performance

in the MCCB when there is less force and a low temperature. It is also

important to note that the MCCB can be drawn out without

disconnecting the incoming live cable.

Rotary Handle (RH)

The MCCB toggle handle operating mechanism is used to

facilitate the ON/OFF operation when the MCCB is installed in the

cubicles of distribution boards. It is designed to be attached directly

onto the MCCB and transform the toggle handle movement into a

rotation switch to serve as a position indicator switch.

Motor Operating Mechanism (MOD)

The motor-operated mechanism enables the MCCB to be

switched ON or OFF automatically. The MCCB should also be

equipped with an alarm switch for automatic resetting purposes.

External Accessories

Plug-in kit

Rotary handle

Motor operating mechanism

18

19

Auxiliary Contact

Frame Size

Conventional Current (Ith)

Durability Make & Break Capacity

100A < In < 630A

3

0.3A

10

1

0.3

1

1

0.3

6050

360

> 0.05

10

1

0.3

1

1

0.3

10

120

> 0.05

100A < In < 630A

3

0.3ARated Operational Current at AC 380V

I/Ic

U/Uc

Cos ø

Make

I/Ic

U/Uc

Cos ø

Number of cycles

Frequency (t/s)

Time (s)

BreakCategoryAC-15

Shunt Trip

Internal Accessories Specification

MCCB model

Cut-off Switch

OperatingVoltage

Operating Time

63 C, S equipped

equipped

equipped

equipped

equipped

equipped

240V 5 - 15 mins

7 - 15 mins

7 - 15 mins

7 - 15 mins

7 - 15 mins

7 - 15 mins

240V

240V

240V

240V

240V

100 C, N, S & H

225 C, N, S & H

400 S & H

600 S & H

800 S & H

Auxiliary Contact and Alarm Switch

“Open” Position “Close” Position

Circuit Breaker > 400A2 NO + 2 NC




External Accessories

Installation and Fittings

20

21

Shortest Distance between Hinge & Handle Center and available Shaft SpaceOutline & Mounting Drawing

HandleDoor Hole for Handle

Outline Dimensions of Rotary Handle & Door Hole

Features

• can be pad-locked in both ON and OFF positions.

• when door is locked in ON position, can be opened in OFF position.

• protective class (based on IEC529 standards) at IP54.

Selection Table & Installation Guide for Accessories

UVT* Undervoltage Release

Shunt Trip* Remote Trip Unit

Auxiliary Switch* On & Off Indication

Name of Accessory LKS-63 LKS-100 LKS-225 LKS-400 LKS-630LKS-800

Alarm Switch* Trip Indication

Shunt Trip + Auxiliary Switch

Shunt Trip + UVT

2 Auxiliary Switch

Auxiliary + UVT

Alarm + Shunt Trip

Alarm + Auxiliary Switch

Alarm + UVT

Alarm + Auxiliary + Shunt Trip

Alarm + 2 Sets of Auxiliary

Alarm + Auxiliary + UVT

* 1. Only lead wire type is available

* 2. For Alarm, Auxiliary, Switch and UVT,a module ismounted externally on the cover.

item

Alarm

Auxiliary switch

Shunt trip

UVT

symbolLeft Side Right Side

MCCB On/Off Toggle

22

23

ACMSAL - 225

35.5

Outline Drawings of AccessoriesDCBAModel CodeShunt Trip Release

DCBAModel CodeUndervoltage Release

DCBAModel CodeAlarm Switch

ACMSVT - 100

ACMSVT - 63 39 30.5 37.5 23.1

30 29.5 23.4

ACMSVT - 225 39.5 34.5 31.2 30.3

ACMSVT - 400 58.5 35 63.4 28.3

ACMSVT - 630 58.5 50.8 97 27.9

ACMSST - 63 39 31 42 23.5

ACMSST - 100 29 32.7 38.5 22

ACMSST - 225 29 34.5 43 30

ACMSST - 400 62.5 60 37.5 28

ACMSST - 630 63.5 60 37.5 28

ACMSAL - 100 29.5 30.6 37.5 23.6

37.5 30.6 40 28.6

ACMSAL - 400 55 63 28 29.5

ACMSAL - 630 55 63 28 39


24

Outline Drawings of Accessories

ACMSAA - 630

ACMSAA - 400

ACMSAA - 225

Auxiliary Switch

DCBAModel CodeAuxiliary + Alarm Switch

ACMSAA - 100 29.5 27 37.5 23.6

37.5 30.6 40 38.6

55 63 28 29.5

55 63 28 39

DCBAModel Code

ACMSAX - 100

ACMSAX - 225

ACMSAX - 400

ACMSAX - 630

29.5

37.5

55

55

27

30.6

63

63

37.5

40

28

28

23.6

28.6

29.5

39


25


ACMRH - 400

ACMRH - 225

ACMRH - 630

ACMRH - 100

CBAModel CodeMotor Operating Mechanism

DCBAModel CodeRotary Handle

ACMRH - 50 100 25 49 68

104 30 49 69

143 35 55 72

195 129 83 110

195 129 83 110

ACMSMOD - 400 226 132 143

ACMSMOD - 630 226 207 143

ACMSMOD - 100 117.5 90 91

ACMSMOD - 225 156 105 101


Plug-in Kitmodel code MZ1-100/30 MZ1-225/30 MZ1-400/30 MZ1-630/30

A

A1

B

B1

C

D

D1

E

E1

F

F1

G

H

H1

J

K

L

L1

M

m

m1

m2

N

92

60

30

70

104

6

0

134

0

60

M10

13

26

16

M10

14

90

60

M5

0

62

122

0

108

70

38

73

106

6

10

144

26

70

25

13

34

15

6

17.5

105.5

70

M5

108

79

134

18

136

44 140

50 58

135 143

175 184

10 10

13 13

225 243

32 0

87 140

28 44

18 17

40 53

24 20

8 11

27 27

144 210

87 140

M8 M8

120 0

79 146

0 0

15 15

213



26

27

Short-circuit in a Network

When a short-circuit in a network occurs, it will create a highly damaged and

abnormal condition to the system, whereby the normal insulation of the system, be

it the cables or equipment and load, are damaged.

The function of the MCCB as a protection device, is to protect overloads and

bring the effect of this faulty condition under control at a fast speed in order to

reduce the damages.

The LKE Superior series MCCB, with the right combination of accessories and

proper selection to coordinate between the down-stream and up-stream of the

rated current and fault level, is one of the more reliable circuit breaker protection

device available.

It is important to understand the full load current and fault level to determine

the rated current and short-circuit kA of the MCCB before selecting the right

MCCB to protect the down-stream cable, equipment and load.

The value of the short-circuit current at a fault-junction depends mainly on:

• the kVA of the supply source, (either a transformer or generator).

• the type of supply system.

• the length and cross section of the cable and device lying in between the

source of supply and fault-junction.

Types of Short-circuit

Before calculating the short-circuit current at any point of the network, one

must be able to differentiate the various types of short-circuit. In a three-phase

network, short-circuits are generally classified as below, depending on the number

of conductor affected and with or without fault-to-earth.

Definition of Short-circuit and Short-circuit Current

Three-phase fault

Isc = Uo

∑ z

Two-phase fault

Isc = Uo

∑ z

One-phase shorted to Neutral

Isc = Uo

√3 z

Cross-country – three-phaseshorted to Neutral

Isc = Uo

z = 1/z1 + 1/z2 + 1/z2

28

The Peak Value of the Short-circuit Current

When an R-L series circuit is closed with an A/C source, the current

component results in:

1 ) an A/C component with a phase shift with respect to the voltage

2 ) a D/C decaying component.

The arc component is superimposed on the D/C component. The initial peak

value of the short-circuit current depends on the voltage at the instance of the

breaker closing. The two extreme cases are:

a ) when the breaker is closed at peak voltage, the D/C component is zero and

the fault current is symmetrical or balanced.

b ) when the breaker is closed at zero voltage, the D/C component is

asymmetrical or unbalanced.

29

The initial peak value depends on the instance of the breaker closing and on the

factor “K = R/X” [Refer Fig.1]. In practical applications, the value of “K” lies mostly

between 1.1 to 1.5. The electro-dynamic stress on the current carrying parts

depends on this peak value “Ip”.

Calculation of the Short-circuit Current close to the Transformer

If the MCCB is used as a main switch, whether as a transfer switch or a

distribution breaker close to the transformer, a rough estimate of the short-circuit

current is sufficient. The percentage impedance of the transformer Z can be read

out from the name plate. Otherwise, it is generally assumed as 5%. The short-

circuit current can be calculated with the help of the following simple rule:

Isc = In x 100/Z

where,

Isc - short-circuit current ( A )

In - rated current of the transformer (Full load current)

Z - percentage impedance of the transformer

The rated current of the transformer is calculated as follows:

In = S x 1000 /√3 x Ue

S = rating of transformer in kVA

Ue = rated voltage at the low tension side in Volts

e.g. :

A transformer with S = 1000 kVA, Z = 5% and Ue = 415 V

In = 1000 kVA x 1000 / √3 x 415 V = 1393 A

Isc = 1393 A x 100 / 5 = 27860 A

In this example, the short-circuit current close to the transformer is ~28 kA. The

breaking capacity of the MCCB installed at this point must be higher than this

value. This is applicable if a high breaking capacity MCCB with an ultimate short-

circuit breaking capacity Icu = 35 kA or 50 kA is used here. It is immaterial whether

the simple formula used above is sufficiently accurate or not. The selected circuit

breaker will have enough capacity in reserve.

The short-circuit current calculated above can also be read out from the table

“Rated and short-circuit currents of 3-phase standard transformers” (refer to

page 30).

Determination of the Fault Current

Determination of the Fault Current at Transformer Terminal

50

100

160

200

250

315

400

500

600

700

800

900

1000

1250

1500

2000

2500

3000

70

139

223

278

348

448

556

696

836

975

1115

1254

1393

1741

2089

2786

3482

4179

1391

2782

4452

5565

6956

8765

11130

13912

16714

19500

22286

25072

27860

34820

41780

55720

69640

83580

Transformer Rating (kVA)Rated Current (A)

at full load currentShort-circuit Current (A)at secondary terminal

Rated and Short-circuit Currents of 3-phase Standard Transformers at Secondary Terminal.

Secondary rated voltage = 415V AC; percentage impedance of transformer “Z” = 5%

30

31

Table

D

5

5

5

5

4.95

4.9

4.85

4.75

4.47

5.5

5.5

5.5

5.5

5.45

5.4

5.35

5.2

4.85

8.3

8.3

8.25

8.2

8.0

7.9

7.7

7.3

6.4

10

10

9.9

9.8

9.5

9.3

9.0

9

7

12.5

12.3

12.2

12

11.5

11.1

10

9.6

7.8

16.4

16.2

16

15.8

14.6

13.8

12.8

11

8.6

20

20

19.6

19.2

17

16

14.4

12

9

24

23.5

23

22

19.2

17.6

15.6

13

9.3

32

30

29

27.7

22

20

17

13.7

9.6

38

36

34

32

24

21.4

18

14

9.7

53

47

43

39

27

2.2

19

14.6

9.9

Short-circuit Current (415V)

100

70

60

50

30

25

20

15

10

Upstream Fault Current (kA)

Calculation of the Short-circuit Current in a Supply System

In a supply system, the further away from the transformer, the higher the

impedance. As such, the lower the value is for the short-circuit current. Each length

of conductor or device in the circuit provides an impedance which reduces the

short-circuit current. To calculate the maximum level possibility of the short-circuit

current, all the impedances lying between the transformer and the MCCB must be

considered, be it with formula or simple diagram.

Rapid Determination of Fault Currents

The following monogram provides a simple method of determining the fault

current at any distance of cable from a transformer. To determine the fault current

at the end of a line through monogram for a cable with a cross section of 3 x 95

mm2 and at a length of 60 m is as follows:

The upstream ( source ) fault current, e.g. 50 kA,

e.g. If, length of cable = 60 m

Cable cross section = 3 x 95 mm2

Fault current at source = 50 kA

Then, from the 80 m column in Table C, follow towards the cable size, and then

down to Table D to the upstream fault current, at the intersection reads the current

value, that is 12 kA.

source

fault current atfault junction

It may be noted that a 100kA

fault at upstream side can be

reduced to a mere 5kA level

at the end of a 150m long 70

sq.mm cable.

Table

C

70

50

35

10

6

95

70

50

35

25

16

120

95

70

50

35

25

16

10

120

70

50

35

95

25

16

16

120

95

70

50

35

25

150

120

95

25

185

150

120

70

50

35

150

95

70

50

35

185

120

95

70

50

120

95

70

461016253550

4610162535

2.546101625

1.52.5461016

1.52.54610

1.52.546

1.52.546

Copper Cable Cross-section (mm2)

150

120

80

60

45

30

20

15

12

8

6

4

3

2

1.5

1.2

Length of Cable (m)

32

Protection for Generators

Frequency 50Hz - Voltage 400V

Rated Power ofAlternator (kVA)

630

710

800

900

1000

1120

1250

1400

1600

1800

2000

2250

2500

2800

3150

3500

909

1025

1155

1299

1443

1617

1804

2021

2309

2598

2887

3248

3608

4041

4547

5052

1250

1250

1250

1600

1600

2000

2000

2500

2500

3200

3200

4000

4000

5000

5000

6300

Rated Current ofAlternator (A)

Rated Current ofCircuit Breaker (A)

Frequency 60Hz - Voltage 450V

Rated Power ofAlternator (kVA)

760

850

960

1080

1200

1344 - 1350

1500

1650 - 1680 - 1700

1920 - 1900

2160 - 2150

2400

2700

3000

3360

3780

4200

975

1091

1232

1386

1540

1724 - 1732

1925

2117 - 2155 - 2181

2463 - 2438

2771 - 2758

3079

3464

3849

4311

4850

5389

1250

1250

1250

1600

1600

2000

2000

2500

2500

3200

3200

4000

4000

5000

5000

6300

Rated Current ofAlternator (A)

Rated Current ofCircuit Breaker (A)

33

The IEC standard classifies the coordination of the breaker and contactor into

the following 3 categories for damages on the contactor when a fault occurs on

the load side:

Category A – coordination is when the magnetic contactor is damaged to the

extent that it will require replacement. Other major components

may also require replacement or complete assembly.

Category B – coordination is when repair requirements are only to the

component parts, due to welding of contacts or melting of the

thermal relay heater.

Category C – a perfect coordination is achieved when no damages are

sustained by the contactor.

Coordination with Wiring

The wiring leading to the motor should be installed in accordance with

international standards requirements.

Coordination with Thermal Overload Relay

In a system arrangement with a MCCB, contactor and thermal overload relay,

the MCCB long time delay must exceed that of the thermal overload relay’s curve.

This is important when any overload on the motor occurs, the thermal overload

relay is able to operate instead of the MCCB.

In case of a short-circuit or heavy overload such as a locked rotor, where the

current may reach 5 to 7 times the motor rated current, the protection is then taken

over by the MCCB.

Coordination with Motor Starting Current

Motors with starting times of 15s or less are generally considered safe, while

those with starting times of longer than 15s are considered undesirable for any

standard motors. Motors with starting times longer than 30s are considered

dangerous and should be avoided altogether.

Selection Principle

1. The MCCB current rating should be higher than the motor full load current.

2. The motor starting current and starting time should be below the minimum

time/current curve of the MCCB. A margin of about 50% should be allowed for

the starting time to allow for the voltage drop or increase of a mechanical load

friction.

3. The MCCB magnetic trip current should be 1.4 to 1.7 times the motor rated

starting current ( lock-rotor current).

4. For star- delta or auto-transformer starters, the MCCB magnetic trip should be

at least 2 to 2.4 times the motor rated starting current (or lock-rotor current).

Protection of Motor by Breakers

34

Capacitance Load

The capacitors must be able to withstand a continuous overload of 30% due

to the harmonic currents. As a result, the circuit breaker must be derated b 30%.

where

Zs = Impedance of Power Source

Za = (Zs + Zt) • Zm

Zs + Zt + Zm

Selection Guide

Capacitor

Capacity (kVAr)

12.5

20

30

50

75

90

120

150

190

225

300

18

29

44

72

110

132

173

216

274

324

433

25

40

63

100

160

200

250

320

400

500

630

Current at Capacity (A)

Circuit Breaker

Rating (A)

Impedance in 3-phase Capacity(converted to 1000kVA standard capacity)

Trans.Cap(kVA)

% Impedanceof Trans.

Zt (%)

% Impedanceof MotorZm (%)

Total % ifImpedance

of Power SourceZA (%)

50 33.4 + j37.8 82.2 + j493.2 28.98 + j36.33

18.28 + j29.39

13.46 + j23.03

8.341 + j16.57

6.161 + j12.64

3.914 + j9.773

2.064 + j6.696

1.327 + j5.266

0.957 + j4.372

0.607 + j3.278

0.449 + j25

54.8 + j328.8

41.1 + j24.8

27.4 + j164.4

20.55 + j123.3

13.7 + j82.2

8.22 + j49.32

5.48 + j32.88

4.11 + j24.66

2.74 + j16.44

2.055 + j12.33

21.6 + j31.47

16.0 + j24.8

10.0 + j18.07

7.4 + j13.8

4.8 + j10.9

2.56 + j7.62

1.68 + j6.16

1.22 + j5.21

0.773 + j3.99

0.57 + j3.035

75

100

150

200

300

500

750

1000

1500

2000

Average Impedance in 3-phase Transformer

TransformerCapacity

(kVA)

Impedance (%)

% X% R

50

75

100

150

200

300

500

750

1000

1500

2000

1.67

1.62

1.60

1.50

1.48

1.44

1.28

1.26

1.22

1.16

1.14

1.89

2.36

2.48

2.71

2.76

3.27

3.81

4.62

5.21

5.99

6.07

35

Selection Guide

ø1.6 mm 8.92 0.103 0.143 0.287 0.123 0.182 0.344

0.134 0.273 0.116 0.161 0.327

0.127 0.256 0.115 0.152 0.308

0.138 0.279 0.020 0.167 0.335

0.126 0.261 0.110 0.152 0.314

0.120 0.247 0.110 0.145 0.297

0.116 0.236 0.110 0.140 0.283

0.111 0.218 0.106 0.134 0.261

0.105 0.204

0.195

0.187

0.178

0.172

0.173

0.155

0.148

0.142

0.134

0.126

0.118

0.112

0.105

0.104

0.100

0.100

0.097

0.095

0.094

0.092

0.091

0.090

0.089

0.087

0.086

–

–

0.127

0.122

0.118

0.115

0.111

0.107

0.104

0.106

0.101

0.101

0.099

0.097

0.095

0.094

0.245

0.234

0.225

0.214

0.206

0.196

0.186

0.178

0.170

0.161

0.151

0.142

0.134

0.216

0.091

0.098

0.095

0.092

0.087

0.086

0.087

0.084

0.084

0.082

0.080

0.079

0.078

0.097

0.096

0.100

0.092

0.092

0.092

0.088

0.086

0.083

0.083

0.081

0.079

0.078

0.076

0.076

0.075

0.073

0.073

0.072

–

–

5.65

3.35

9.24

5.20

3.33

2.31

1.30

0.824

0.623

0.487

0.378

0.303

0.230

0.180

0.144

0.118

0.092

0.072

0.057

0.045

0.037

ø2.0 mm

ø2.6 mm

2

3.5

5.5

8

14

22

30

38

50

60

80

100

125

150

200

250

325

400

500

Resistance

Cable(mm2)

Rw(mΩ/m)

Reactance Xw (mΩ/m)

50Hz 60Hz

single core6cm distance

single core,closed

2-core,3-core

single core6cm distance

single core,closed

2-core,3-core

Impedance of Electric Cable

NOTE: The resistance values are based on JIS C3307 660V grade polyvinyl chloride insulated and vinyl sheathed cable (w).The reactance value L = 0.05 + 0.4605 log10 D/r (m/H/km)

(D = core center to center distance, then Xw = 2.π fl x 10 -3 (mΩ/m), f = frequency was calculated).

36

Selection Guide

Impedance of Bus Duct (Zb)

Rated Current(A)

Resistance(mΩ/m)

Reactance (mΩ/m)

60Hz50Hz

400 0.158

0.127

0.085

0.065

0.053

0.041

0.025

0.020

0.017

600

800

1000

1200

1500

2000

2500

3000

0.039

0.033

0.024

0.018

0.014

0.012

0.014

0.013

0.011

0.046

0.039

0.028

0.022

0.017

0.014

0.017

0.016

0.013

Comparison of Different Methods of Starting

Method ofStarting

Ist/Idol

Current (I) Torque (T)

Ist/In Tst/Tdol Tst/Tn

Direct-on-line 1 4 - 8 1 1 - 1.15

0.33 1.32 - 2.64 0.33 0.33 - 0.49

0.28 1.12 - 2.24 0.25 0.25 - 0.37

0.39 1.56 - 3.12 0.36 0.36 - 0.54

0.59 2.36 - 4.72 0.56 0.56 - 0.84

0.7 0.7 0.5 0.5

1.4 1.4 1 1

2 2 1.4 1.4

Star-delta

Auto transformer 50%



Rheostat, severity 0.7



37

What is Selectivity?

Selectivity between 2 protective devices in series, such as the MCCB1

& MCCB2, is also called discrimination. The purpose of selectivity is to

coordinate the 2 circuit breakers in cascade, eg. A and B (see diagram). This

means only the B breaker trips in case of fault occurring at C and a

continuous supply of power to the remaining loads through the A breaker.

Total and Partial Selectivity (Diagram 1 & 2)

• Total selectivity between A & B breakers is when fault occurs at C, up to

the prospective short circuit current of the B breaker, and only when the B

breaker is tripped while the A breaker remains untrip.

• Partial selectivity between A & B breakers is when the B breaker trips but

the A breaker does not, but only for fault currents lower than the maximum

prospective short circuit currents that may occur in the line connected to

the B breaker. For a higher fault current, up to the maximum prospective

short circuit current of the breaker B, both B & A breakers may trip

together.

Selectivity Techniques (Diagram 3)

There are two techniques for ensuring selectivity:

1. Current selectivity

2. Time selectivity

These 2 techniques are effected intervening in the operation of the

breaker of setting the tripping current (Im) & the tripping time delay (Tm).

Current Selectivity

This technique is commonly used in low tension switchboards,

achievable by adjusting the tripping unit current setting. For 2 breakers in

series, the pick-up current on the upstream breaker is set to a value higher

than the prospective short circuit current at the point of the fault junction of

the down stream breaker.

This selectivity technique is used particularly for links between main

boards and secondary boards.

Time Selectivity (Diagram 4)

This time selectivity technique requires the “selectivity” circuit breaker –

a breaker with an adjustable time trip device:

• Time delay with adjustable unit in the breaker tripping system

• The breaker must be able to withstand the thermal & electrodynamic

effect of the short circuit current for the period of the time delay.

Selectivity

Diagram 1

Diagram 2

Diagram 3

Diagram 4

LKE’s Low & Medium VoltageRange of Products

capacity of products range from 380VAC – 36kV and rated current from 5A – 6300A

LK-LBSCompressed Air

Load BreakSwitch

LK-VLBSVacuum

Load Break

Switch

LK-GLBS SF6 Load Break Switch

LK-VB1Vacuum Circuit Breaker

LK-LTPMSF6 Ring Main Unit

LK-LTHOSF6 Pole-MountedSwitch Disconnector

LK-ATSAuto TransferSwitch

LK-ACBAir Circuit Breaker

LK-MCCB, LH-MX, LK-SF, LK-RCCBLow Voltage Circuit Breakers

LK-LCASF6 Ring Main Unit

LK-LCACompressed Air

Ring Main Unit

LK-CUBICLow VoltageSwitchboard

As standards, applications and designs may change from time to time, please contact our nearest agent for the latest information. For further technical references,

please refer to the respective product catalogue.

LK-Electric Co Pte LtdBlk 219 Henderson Industrial Park

#06-03 Henderson Rd, 159546 SINGAPORE telephone 65 271 5388facsimile 65 271 5088

LKE Electric (M) Sdn Bhd1 & 3 Jalan SS13/3C, Subang Jaya Industrial Estate

47500 Petaling Jaya, Selangor D.E., MALAYSIAtelephone 603 5633 7010/7011

facsimile 603 5633 8368, 5632 3014

LK-Electric (Zhuhai) Co LtdNo 4, North of Industrial Area Xiangzhou

Zhuhai, 519000 P.R. of CHINAtelephone 86 756 226 7005facsimile 86 756 226 7007

CUBIC Electric (Shanghai) Co Ltd18th Floor, No 159 Handan RoadShanghai, 200437 P.R. of CHINA

telephone 86 21 6555 7237facsimile 86 21 6555 7119

LKE Electric Europe A/SEgestubben 16-26

DK 5270, Odense, DENMARKtelephone 45 63 18 1560facsimile 45 63 18 1590

[email protected]

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