Appendix the IEE Regulations gives the time/current characteristics
and specific values prospective short-circuit current for number of
protective devices.4, 3.0
10,000.2 and 3.4s when times multiple that 30A, flows under fault conditions.1
1.0
10.
FIGURE 12. Thus:
● Type MCB 60898 has multiple 5
● Type MCB 60898 has multiple 10
● Type MCB 60898 has multiple 20.5
Time/current characteristics (a) Type MCB 60898 and (b) semi-enclosed fuse to
BS 3036. Since this quicker than 82.0
10,000.0
10.5 and 3.01
0.01
0. The mul-
tiple depends upon the characteristics the particular devices.
Figures 3.
It can seen that the prospective fault current required trip MCB in
the required time multiple the current rating the device.1, 3.0
Time
(s)
6
A
10A
16A
20A
32A
50A
100.4
.5(a) shows the time/current characteristics for Type MCB to
BS 60898.6 with MCBs.
The characteristics shown Appendix the IEE Regulations give the specific values of
prospective short-circuit current for all standard sizes protective device.3 Appendix the IEE Regulations deal with fuses
and Figs 3.
1 10
0.0
1000.Basic Electrical Installation Work
246
Figure 12.
These indicate the value fault current which will cause the protective
device operate the times indicated IEE Regulation 411.0
Time
(s)
100.0
20ms
100 1000
4000
Prospective current (rmsA)
(a)
10,000 100,000
5
A
15
A
30
A
60
A
(b)
1 10
0.
Example
A Type MCB 60898 which used protect domestic lighting circuit will trip
within 0.0
1000. This graph shows that fault current 4000A will trip the
protective device 20ms.66ms, the 50A Type
B MCB suitable and will clear the fault current before the temperature of
the cable raised dangerous level.0
100 1000
Prospective current (rmsA)
10,000 100,000
200
0.
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 411.1
1
