5.92
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Zone Time Delay
Independent Zone Timing
Use Relay Word bits ZnPT (Time-Delayed Zone Phase-Distance Protection) and
ZnGT (Time-Delayed Zone Ground-Distance Protection) to select independent
zone timing in SEL
OGIC control equation TR (Trip) (n = 1–5).
The example below uses independent timing for Zone 2 phase and ground-dis-
tance protection:
TR := Z1P OR Z1G OR Z2PT OR Z2GT
Common Zone Timing
Use Relay Word bits ZnT (Zone n Distance Protection) to select common zone
timing in SEL
OGIC control equation TR (Trip) (n = 1–5).
The next example uses common timing for Zone 2 distance protection:
TR := Z1P OR Z1G OR Z2T
If the timer input drops out while timing, the relay suspends the common zone
timer for two cycles. This feature prevents resetting the timer when a fault
evolves (e.g., the fault changes from a single phase-to-ground to phase-to-phase-
to-ground). If the timer expires, the relay blocks the suspend-timing logic. When
the zone timer is set to OFF, the output from the timer is blocked.
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