Automatic reclosing restores supply after transient faults by closing a tripped breaker after a defined dead time. It is valuable on overhead feeders but can be dangerous or pointless on cables, transformers, busbars and persistent equipment faults.

Learning objectives

Define when reclosing is permitted, coordinate breaker sequence and protection logic and block the function after faults that require inspection.

Core engineering principles

Application decides whether reclosing is appropriate

Overhead-line flashovers can self-clear when de-energised. Internal transformer, cable and bus faults usually do not, so automatic re-energisation can increase damage.

Dead time allows deionisation and system recovery

The interval from opening to reclose should cover arc deionisation, breaker mechanism capability and system stability requirements.

Shot count and reclaim time limit repeated duty

A scheme can use one or more shots with different delays. Reclaim logic defines when a successful close restores the sequence; otherwise the relay proceeds to lockout.

Protection functions create selective blocks

Breaker failure, differential, busbar, lockout, manual trip and VT or synchronism problems may block 79, while selected feeder overcurrent trips may initiate it.

Synchronism and voltage supervision may be required

Networks with multiple sources or distributed generation must not close out of phase. Dead-line/live-bus and synchronism-check logic become part of the sequence.

Engineering application method

  1. Step 1: Classify faults and trip functions that may initiate reclose.
  2. Step 2: Define dead time, shots, reclaim and final lockout.
  3. Step 3: Verify breaker rated operating sequence and stored energy.
  4. Step 4: Add voltage, synchronism and interlock supervision.
  5. Step 5: Test successful reclose, persistent fault, blocked trip and manual reset.

Practical example

A feeder 51 trip may start one reclose shot, but a transformer 87T or busbar 87B trip should operate lockout and block 79. Using one general trip signal for all functions loses this necessary distinction.

Common mistakes

  • Enabling 79 on every feeder by default.
  • Ignoring breaker operating sequence.
  • Reclosing after differential or breaker-failure trip.
  • Leaving distributed generation connected without synchronism logic.
  • Resetting lockout automatically without policy.

Design and review checklist

  • Which trip functions initiate or block 79?
  • What dead time and shots are justified?
  • Can the breaker perform the sequence?
  • Are voltage and synchronism supervised?
  • Is final lockout and reset clear?

Standards basis and official sources

Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.

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