I> I>
A1 B1
Feeder
Time axis
t=0 t=t
1
t=t
2
t=t
3
The fault
occurs
Protection
B1 trips
Breaker at
B1 opens
Protection
A1 resets
en05000205.vsd
IEC05000205 V1 EN
Figure 50: Sequence of events during fault
where:
t=0 is when the fault occurs
t=t
1
is when the trip signal from the overcurrent protection at IED B1 is sent to the circuit breaker. The
operation time of this protection is t
1
t=t
2
is when the circuit breaker at IED B1 opens. The circuit breaker opening time is t
2
- t
1
t=t
3
is when the overcurrent protection at IED A1 resets. The protection resetting time is t
3
- t
2
.
To ensure that the overcurrent protection at IED A1, is selective to the overcurrent
protection at IED B1, the minimum time difference must be larger than the time t
3
.
There are uncertainties in the values of protection operation time, breaker opening
time and protection resetting time. Therefore a safety margin has to be included. With
normal values the needed time difference can be calculated according to equation 19.
40 100 40 40 220t ms ms ms ms msD ³ + + + =
EQUATION1266 V1 EN (Equation 19)
where it is considered that:
the operate time of overcurrent protection B1 is 40 ms
the breaker open time is 100 ms
the resetting time of protection A1 is 40 ms and
the additional margin is 40 ms
1MRK 505 291-UEN A Section 6
Current protection
103
Application manual
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