Let now back the problem and see the Type MCB will discon-
nect the supply less than 82.4 can seen that the particular protective device repre-
sented this characteristic will take disconnect fault current of
50 and 0. Therefore, the protective device
must disconnect the supply the cable less than 82.01
1 100 1000 10,000
100,000
50A
Fault current (rmsA)
Time
(s)
Time
FIGURE 12.0
100.
I
V
Z
I
(A)
V
0.
These logarithmic scales are shown the graphs Figs 12. Calculate the
maximum permissible disconnection time and show that 50A Type MCB 60898
will meet this requirement.4
Time/current characteristic overcurrent protective device. The impedance the short-circuit path 0.1 gives value for 115.0
10.08s
0.66ms.1
0.5.0
1000.08 clear fault current 1000 A.
From Fig. 12.
Time/current characteristics protective devices
Disconnection times for various overcurrent devices are given the form
of logarithmic graph.Overcurrent, short circuit and earth fault protection
245
Example
A PVC sheathed mineral insulated (MI) copper cable short circuited when
connected 400V supply. So,
t
k S
I
t
2
2
2 2
3
115 10
4000
82 10
(s)
mm
A
s
∴ .66ms.1Ω.
Manufacturers’information and Appendix the IEE Regulations give the operating times of
protective devices various short-circuit currents the form graphs.
10,000.
The maximum time that 4000A fault current can applied this 10mm2
cable without
dangerously raising the conductor temperature 82.66ms under short-circuit conditions.
. Let come back to
this problem few moments.4 and 12.0s
1.1
A
Fault current A
400
4000
4000
Ω
∴
For PVC Sheathed copper cables, Table 43.0
8.0
0. This means that each successive graduation the
axis represents times change over the previous graduation
