en05000132_ansi.vsd
51 51
A1 B1
Feeder
Time axis
t=0 t=t
1
t=t
2
t=t
3
ANSI05000132 V1 EN
Figure 566: Selectivity steps for a fault on feeder B1
where:
t=0 is The fault occurs
t=t
1
is Protection B1 trips
t=t
2
is Breaker at B1 opens
t=t
3
is Protection A1 resets
In the case protection B1 shall operate without any intentional delay (instantaneous).
When the fault occurs the protections pickup to detect the fault current. After the time
t
1
the protection B1 send a trip signal to the circuit breaker. The protection A1 starts its
delay timer at the same time, with some deviation in time due to differences between
the two protections. There is a possibility that A1 will start before the trip is sent to the
B1 circuit breaker. At the time t
2
the circuit breaker B1 has opened its primary contacts
and thus the fault current is interrupted. The breaker time (t2 - t1) can differ between
different faults. The maximum opening time can be given from manuals and test
protocols. Still at t
2
the timer of protection A1 is active. At time t
3
the protection A1 is
reset, that is the timer is stopped.
In most applications it is required that the times shall reset as fast as possible when the
current fed to the protection drops below the set current level, the reset time shall be
minimized. In some applications it is however beneficial to have some type of delayed
reset time of the overcurrent function. This can be the case in the following applications:
1MRK505222-UUS C Section 22
Inverse time characteristics
1181
Technical reference manual
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