Post on 07-Apr-2015
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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
Hex. socket head screw M11
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
Circuit Breaker > 225A1 NO + 1 NC
Circuit Breaker > 400A2 NO + 2 NC
Circuit Breaker > 225A1 NO + 1 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
measurements are in millimeter (mm)
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
measurements are in millimeter (mm)
25
Outline Drawings of Accessories
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
measurements are in millimeter (mm)
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
measurements are in millimeter (mm)
Outline Drawings of Accessories
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%
Auto transformer 60%
Auto transformer 75%
Rheostat, severity 0.7
Rheostat, severity 1.4
Rheostat, severity 2.0
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
info@lke-electric.comwww.lke-electric.com
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