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Breaker failure / reclosing / sync check (50BF / 79 / 25)

Trip-loop completers: 50BF detects breaker failure → trips adjacent breakers / DTT. 79 reclosing handles 80-90% transient faults (single-shot transmission, 3-shot distribution). 25 sync-check verifies Δθ / Δf / ΔV before close.

Senior ~12 min

Step 1 — 50BF + 79 + 25: the trip-loop completers

Animation speed 0.55×

ANSI — state — dead-t —

Reference notes

Three ANSI functions complete the trip loop after a primary protection operates: 50BF breaker failure, 79 automatic reclosing, and 25 synchronizing check. Use Next → to walk through each function's logic, timing, and coordination with primary protection.

ANSI 50BF — Breaker Failure

Initiation: any primary protection trip command (87T, 87B, 21, 67, 51, etc.) also starts the 50BF timer.
Monitoring: 50BF watches breaker current via its CT.
Detection: if I > 0.05-0.10 pu after 8-12 cycle delay, breaker failed.
Escalation: trip ADJACENT breakers on same bus, or send DTT to remote line ends.
Total clearing time for failed breaker: ~12-19 cycles (3-4× normal). Stability margin shrinks.
Standard on all transmission breakers per NERC PRC.

ANSI 79 — Automatic Reclosing

80-90 % of transmission-line faults are TRANSIENT: lightning, debris, vegetation, animal contact. Self-clear when the arc is extinguished by the breaker opening.

Transmission single-shot: one reclose attempt at 30-60 cycle dead time. If unsuccessful, lockout — no further attempts to protect stability.
Distribution multi-shot: 3 attempts at 30 / 120 / 600 cycle dead times → lockout. Catches transient feeder faults without crew dispatch.
Critical lines (765 kV, key ties): often NO automatic reclose — manual restoration only.

Dead-time selection

Must allow: (1) arc deionization (~40-80 ms minimum), (2) mechanical reset of breaker. Longer dead times handle slower-clearing faults but extend outage.

Single-pole vs three-pole reclosing

Three-pole: all three phases trip and reclose together. Standard for most installations.
Single-pole: only the faulted phase trips and recloses; other two phases stay in service during dead time. Maintains partial power transfer for stability on weak / radial systems. Used on critical 500 kV+ lines. Requires phase-selective relays + individually-tripping breaker mechanism (~20-30 % cost premium).

ANSI 25 — Synchronizing Check

Before allowing a breaker to close between two power systems with independent voltage sources, verify matching across the breaker contacts:

Parameter	Typical tolerance
Phase angle Δθ	±20-40 electrical degrees
Frequency Δf (slip frequency)	< 0.1-0.2 Hz
Voltage magnitude ΔV	< 5-10 %

Plus optional dead-line / live-line / live-bus / dead-bus combinations for restoration scenarios. Without 25, out-of-step close at 90° phase difference could create currents ~2× fault levels with severe mechanical shock — potentially destroying breaker, bus, or generator.

Coordination diagram (trip-loop timing)

t = 0: fault occurs.
t = 1-2 cycles: primary protection (87/21) trips.
t = 4-5 cycles: healthy breaker opens.
t = 8-12 cycles: 50BF check. If failed, escalate to adjacent breakers / DTT.
t = 30 cycles: 79 reclose attempt begins.
t = 31 cycles: 25 sync check verifies matching.
t = 32 cycles: breaker closes (success) — service restored.

Distribution recloser sequence (industry standard)

Initial trip (51 picks up).
First reclose at 30 cycles.
If still faulted: trip → 120 cycles → second reclose.
If still faulted: trip → 600 cycles → third reclose.
If still faulted: LOCKOUT. Manual reset required.

Total cycle time ~13 seconds. Pole-mounted reclosers (S&C, ABB, G&W, Cooper Power) or substation microprocessor relays (SEL-451, GE F60, ABB REF615) implement this.

Modern integrated relays

SEL-411L / 421, GE D60, ABB REL670, Siemens 7SA8 all include 50BF + 79 + 25 alongside primary protection (87L / 21 / 67 / 50/51), sequenced in series via programmable scheme logic that interlocks the functions automatically:

On primary trip → 50BF starts → if 50BF picks up, send adjacent-breaker trips.
On healthy clearance → 79 dead-time timer.
Before close → 25 sync-check; if 25 fails, hold off close.
If reclose successful → reset all timers.
If reclose unsuccessful → second-shot or lockout per setting.

Take-away. 50BF detects failed breakers and trips adjacent ones to clear the fault through a different path (8-12 cycle delay, escalation adds 3-5 more cycles). 79 automatic reclosing handles the 80-90 % of transient faults that self-clear after de-energization — single-shot on transmission, multi-shot on distribution. 25 sync-check prevents out-of-step closure (Δθ, Δf, ΔV tolerances). Modern integrated relays implement all three plus primary protection in one device.