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

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:

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)

1. Initial trip (51 picks up).
2. First reclose at 30 cycles.
3. If still faulted: trip → 120 cycles → second reclose.
4. If still faulted: trip → 600 cycles → third reclose.
5. 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.