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TECHN I CAL DATA
MCB AP P L I CAT I ON
218
Crabtree circuit breakers are generally designed for use on ac systems, but
can also be used on dc supplies. The selection of the most suitable circuit
breaker depends on the following conditions:
•
The type of circuit
•
The circuit time constant
•
The short circuit current
•
The circuit voltage
TYPICAL CIRCUITS
l
POLESTAR MCB APPLICATIONS – DC SUPPLIES
l
NORMAL CIRCUIT CURRENTS
l
The rating and normal running temperature of the MCB are unaffected by dc.
The MCB can be selected using the thermal section of the standard time/
current curves in the normal manner.
Magnetic tripping on dc is different from the equivalent ac by a factor of
√
2.
ie type B ac magnetic range = 3–5I
n
type B dc magnetic range =
√
2(3–5)
I
n
= 4–7I
n
type C ac magnetic range = 5–10I
n
type C dc magnetic range =
√
2(5–10)
I
n
= 7–14I
n
type D ac magnetic range = 10–20I
n
type D dc magnetic range =
√
2(10–20)
I
n
= 14–28I
n
SHORT CIRCUIT CURRENTS
l
The maximum short circuit current possible on a dc system is determined by
the voltage of the battery and the total internal resistance of the cells. It is
given by Ohm’s law:
I
SC
=
V
b
where V b is the voltage of the battery (with the battery 100% charged)
R b is the internal resistance of the battery cells
R b
(
this is usually quoted by the manufacturer)
CIRCUIT TIME CONSTANT
l
The time constant is given by:
L
where L is the inductance of the circuit
R is the resistance of the circuit
The time constant is usually given in milliseconds (ms). Ideally, dc circuits
would be mainly resistive (ie a low number), as inductive circuits produce a
back emf when the current suddenly falls. This in turn tends to prolong arcing
during switching operations, and so reduces contact life.
CIRCUIT VOLTAGE
l
The voltage of the circuit is dependent upon the power supply. The lower the
voltage the easier switching operations will be, but the voltage makes no
difference to the normal running of the MCBs.
Contact life can be significantly increased by reducing the voltage drop across
each pole. This can be achieved by wiring poles in series. It is also
recommended that for frequent switching the voltage across each pole
should not exceed 110V dc.
Crabtree Polestar MCBs have been successfully tested on dc and can
be used under the following conditions:
Circuit Time Constant L/R 15ms max
Voltage
24–220
V
Short circuit breaking capacity 9kA
C50 MCBs are not suitable for dc circuits and must not be used.
U
F1
F2
F3
Load
E
NOT EARTHED
FAULT F1
Will produce the maximum short circuit current with half the supply
voltage dropped across each pole
FAULTS F2 & F3
No effect
U
F1
F2
F3
Load
E
EARTHED AT THE SUPPLY CENTRE POINT
FAULT F1
Will produce the maximum short circuit current with half the supply
voltage dropped across each pole
FAULTS F2 & F3
Will produce a short circuit current of less than the maximum.
Half the supply voltage will appear across the affected MCB
U
F1
F2
F3
Load
E
EARTHED AT ONE OF THE SUPPLY TERMINALS
FAULT F1
Will produce the maximum short circuit current with half the supply
voltage dropped across each pole
FAULT F2
Will produce the maximum short circuit current with all of the supply
voltage appearing across one pole
FAULT F3
No effect
R
b
R
POLESTAR & LOADSTAR MCB APPLICATIONS – MOTOR STARTERS
l
In general miniature circuit breakers by themselves can only provide short
circuit protection for motor loads.
Motor start-up currents can be as high as 12 times the normal running
current. MCBs in general cannot accommodate this and provide the close
thermal protection required by motors.
They can, however, be used to protect lightly loaded motors or motors
started off load, or they can be used in conjunction with thermal overload
relays. In this case the MCB will protect the cable to the motor against short
circuit faults, and the motor will be protected by a second thermal device.
RECOMMENDED TYPE D MCB RATINGS
for single phase 230/240V ac motors
Motor power
Normal running Start-up
MCB current
HP kW
current (A)
current (A)
rating (A)
0.25 0.18
1.5
18
6
0.50 0.37
3.0
36
6
0.75 0.55
4.5
54
6
1.00 0.75
5.5
66
10
1.50 1.1
8.5
102
10
2.00 1.5
10.5
126
16
3.00 2.2
15.5
186
20
4.00 3.0
20.0
240
32
5.00 3.75
24.0
288
32
7.50 5.5
34.0
408
40
10.00 7.5
45.0
540
63
RECOMMENDED TYPE D MCB RATINGS
for 3 phase 400/415V ac motors
Motor power
Normal running Start-up
MCB current
HP kW
current (A)
current (A)
rating (A)
0.25 0.18
0.7
8.4
6
0.5 0.37
1.35
16.2
6
0.75 0.55
1.55
18.6
6
1.0 0.75
1.93
23.2
6
1.5 1.1
2.5
30.0
6
2.0 1.5
3.5
42.0
6
3.0 2.2
4.8
57.6
6
4.0 3.0
6.4
76.8
10
5.0 3.75
7.8
93.6
10
7.5 5.5
11.0
132.0
16
10.0 7.5
14.4
172.8
20
12.5 9.33
17.3
207.6
32
15.0 11.0
21.0
252.0
32
20.0 15.0
28.0
336.0
40
25.0 18.5
35.0
420.0
50
30.0 22.0
40.0
480.0
50
40.0 30.0
54.0
648.0
63