Setting Guidelines (Undercurrent); Figure 97: Cb Fail Timing - GE P4A Technical Manual

Chapter 8 - CB Fail Protection
Fault occurs
Normal
operation
Breaker failure
operation
V00693

Figure 97: CB Fail timing

The following examples consider direct tripping of a 2-cycle circuit breaker. Typical timer settings to use are as
follows:
CB Fail Reset Mechanism
Initiating element reset
CB open
Undercurrent elements
Note:
All CB Fail resetting involves the operation of the undercurrent elements. Where element resetting or CB open resetting is
used, the undercurrent time setting should still be used if this proves to be the worst case.
Where auxiliary tripping relays are used, an additional 10-15 ms must be added to allow for trip relay operation.
5.3

SETTING GUIDELINES (UNDERCURRENT)

The phase undercurrent settings (I<) must be set less than load current to ensure that I< operation correctly
indicates that the circuit breaker pole is open. A typical setting for overhead line or cable circuits is 20%In. Settings
of 5% of In are common for generator CB Fail.
The earth fault undercurrent elements must be set less than the respective trip. For example:
IN< = (IN> trip)/2
192
Protection Maximum breaker
operating time clearing time
Protection
CBF back-up trip time delay
operating time
tBF Time Delay
CB interrupting time + element reset time (max.) + error in tBF
timer + safety margin
CB auxiliary contacts opening/ closing time (max.) + error in tBF
timer + safety margin
CB interrupting time + undercurrent element (max.) + safety
margin operating time
CBF resets:
1. Undercurrent element asserts
2. Undercurrent element asserts and the
breaker status indicates an open position
3. Protection resets and the undercurrent
element asserts
CBF Safety
reset margin
time time
Local 86
operating
time
Maximum fault clearing time
Local Bks clearing
time
Remote CB
clearing time
Fault occurs
Typical Delay For 2 Cycle Circuit
Breaker
50 + 50 + 10 + 50 = 160 ms
50 + 10 + 50 = 110 ms
50 + 25 + 50 = 125 ms
P54xMED-TM-EN-1
P54A/B/C/E
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