0
10,000.1
1.
Therefore the earth fault loop impedance low enough allow least 30A flow the
circuit under fault conditions, the protective device will operate within the time required by
Regulation 411. The mul-
tiple depends upon the characteristics the particular devices.0
1000.0
10.4
.01
0.0
100 1000
Prospective current (rmsA)
10,000 100,000
200
0.0
10.
Example
A Type MCB 60898 which used protect domestic lighting circuit will trip
within 0.
FIGURE 12.5(a) shows the time/current characteristics for Type MCB to
BS 60898.0
Time
(s)
100.3 Appendix the IEE Regulations deal with fuses
and Figs 3.
It can seen that the prospective fault current required trip MCB in
the required time multiple the current rating the device. This graph shows that fault current 4000A will trip the
protective device 20ms.4, 3.0
Time
(s)
6
A
10A
16A
20A
32A
50A
100.1, 3.0
20ms
100 1000
4000
Prospective current (rmsA)
(a)
10,000 100,000
5
A
15
A
30
A
60
A
(b)
1 10
0. Thus:
● Type MCB 60898 has multiple 5
● Type MCB 60898 has multiple 10
● Type MCB 60898 has multiple 20.
1 10
0.
Appendix the IEE Regulations gives the time/current characteristics
and specific values prospective short-circuit current for number of
protective devices.
The characteristics shown Appendix the IEE Regulations give the specific values of
prospective short-circuit current for all standard sizes protective device.
Figures 3.
These indicate the value fault current which will cause the protective
device operate the times indicated IEE Regulation 411.5 and 3.0
1000.0
10,000.5
Time/current characteristics (a) Type MCB 60898 and (b) semi-enclosed fuse to
BS 3036.66ms, the 50A Type
B MCB suitable and will clear the fault current before the temperature of
the cable raised dangerous level.01
0. Since this quicker than 82.6 with MCBs.1
1.2 and 3.Basic Electrical Installation Work
246
Figure 12.4s when times multiple that 30A, flows under fault conditions
