5.154
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Circuit Breaker Failure Protection
The recommended setting for LCPU1 is the sum of the following:
➤ Nominal circuit breaker operate time
➤ 50LP1 dropout time
➤ Safety margin
Calculate the safety margin by subtracting all conditions required to isolate the
fault during a circuit breaker failure condition from the maximum acceptable
fault clearing time. The safety margin will be longer in this case than for the fault
current logic because the total acceptable time to clear the fault at these lower
fault duties is longer.
Load Current Circuit Breaker Failure Initiation: BFILC1
Program SELOGIC control equation BFILC1 (Load Current Breaker Failure Initi-
ation) to initiate this scheme. For example, use the auxiliary contacts from the
circuit breaker to detect when the circuit breaker is open. Relay Word bit LCBF1
asserts if Relay Word bit BFILC1 remains asserted for time LCPU1 and the relay
detects load current.
Circuit Breaker Flashover Protection
Circuit breaker failure protection during flashover conditions is independent of
the other circuit breaker protection functions. Use this protection either alone or
in addition to the other protection.
Use current flow to detect when an open circuit breaker pole flashes over. Set
BLKFOA1 to TPA or CLS1 to block flashover protection for 6 cycles if an
A-phase single-pole trip occurs, or when circuit breaker BK1 closes.
Figure 5.116 shows the flashover circuit breaker failure logic. Flashover timer
FOPU1 (Flashover Time Delay—BK1) starts timing if the circuit breaker is open
and current exceeds setting 50FO1 (Flashover Current Pickup—BK1). The relay
uses pole-open logic BnOPH to determine whether the circuit breaker is open.
The output of the flashover protection is Relay Word bit FOBF1. Use this output
to activate an external alarm, retrip the circuit breaker, or energize a lockout relay.
Circuit Breaker Failure Trip Logic
The SEL-421 has dedicated circuit breaker failure trip logic (see Figure 5.117).
Set SEL
OGIC control equation BFTR1 (Breaker Failure Trip—BK1) to assert for
circuit breaker failure trips from Relay Word bits FBF1, NBF1, LCBF1, and
FOBF1.
When this SEL
OGIC control equation asserts, the relay sets Relay Word bit
BFTRIP1 (Breaker Failure Trip for Circuit Breaker BK1) to logical 1 until
BFTR1 deasserts, timer TDUR3D times out, and an unlatch or reset condition is
active.
Unlatch Circuit Breaker Failure Trip Equation
Use SELOGIC control equation BFULTR1 (Breaker Failure Unlatch Trip—BK1)
to define the conditions that unlatch the control outputs that assert during a circuit
breaker failure trip. BFULTR1 unlatches the circuit breaker trip condition
BFTRIP1.
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