Operation of auto reclosure

Auto reclosure is the automatic re-closing of a circuit breaker a short time after it has tripped on a fault, so that the line is restored without an operator if the fault was temporary.

Why it is used

About 80–90% of faults on overhead lines are transient (lightning flashover, birds, tree branches touching in wind). Once the arc is cleared and the air deionises, the line can be safely re-energised.

Operation

1. A fault occurs; the protective relay trips the breaker.
2. The breaker stays open for a dead time (typically 0.2–0.5 s for high-speed reclosing on EHV lines; a few seconds on distribution lines) so the fault arc path deionises.
3. The auto-reclose relay sends a close signal to the breaker.
4. If the fault has cleared (transient fault), the breaker stays closed and the relay resets after a reclaim time.
5. If the fault is still present (permanent fault), the protection trips the breaker again.
6. After a preset number of attempts (usually 1–3 shots), the breaker locks out and stays open until it is reset manually.

 Fault  Trip   dead time   Reclose
   │     │◄────────────►│
 ──┴─────┘               └─────── (stays closed: transient)
                         └─ trip again → lockout (permanent)

Types

• Single-shot / multi-shot (number of reclosures).
• Three-pole (all phases) or single-pole (only the faulted phase, for single line-to-ground faults; keeps power transfer and stability).
• High-speed (below 0.5 s) and delayed auto reclosing.

Advantages

• Improves continuity of supply and reduces outage time.
• Improves transient stability of the system.
• Reduces operating staff and patrolling.