12.0
1000.66ms.0s
1.66ms under short-circuit conditions.0
100.Overcurrent, short circuit and earth fault protection
245
Example
A PVC sheathed mineral insulated (MI) copper cable short circuited when
connected 400V supply. The impedance the short-circuit path 0.
From Fig.0
0.08 clear fault current 1000 A.
Time/current characteristics protective devices
Disconnection times for various overcurrent devices are given the form
of logarithmic graph.4
Time/current characteristic overcurrent protective device. Therefore, the protective device
must disconnect the supply the cable less than 82.
The maximum time that 4000A fault current can applied this 10mm2
cable without
dangerously raising the conductor temperature 82.4 can seen that the particular protective device repre-
sented this characteristic will take disconnect fault current of
50 and 0.5.1 gives value for 115.1
0.66ms.1Ω.4 and 12.
I
V
Z
I
(A)
V
0. So,
t
k S
I
t
2
2
2 2
3
115 10
4000
82 10
(s)
mm
A
s
∴ .
.0
10.
These logarithmic scales are shown the graphs Figs 12.
10,000.
Let now back the problem and see the Type MCB will discon-
nect the supply less than 82.08s
0. This means that each successive graduation the
axis represents times change over the previous graduation.0
8.
Manufacturers’information and Appendix the IEE Regulations give the operating times of
protective devices various short-circuit currents the form graphs.1
A
Fault current A
400
4000
4000
Ω
∴
For PVC Sheathed copper cables, Table 43. Calculate the
maximum permissible disconnection time and show that 50A Type MCB 60898
will meet this requirement. Let come back to
this problem few moments.01
1 100 1000 10,000
100,000
50A
Fault current (rmsA)
Time
(s)
Time
FIGURE 12
