Air Circuit Breaker Maintenance Checklist

Manufacturers publish maintenance schedules for their breakers. They are written to cover every application, so they tend to say "inspect" without saying what a fail looks like. This list says what we check, what we measure, and what result sends a breaker to the bench. Frequency is covered separately in ACB service intervals.

Before touching the breaker

Isolation and proving dead. Upstream isolated, locked off, proved dead at the breaker terminals. Drawout units racked to the test or disconnected position; springs discharged (open, then close-and-open with the manual buttons until the indicator shows discharged).
Records. Nameplate, serial, operation counter reading, trip unit type and current settings, date and result of the last test. Photograph the trip unit settings before anything else.
History. Any trips since the last service, any fault clearances, any change of duty.

Visual inspection

Arc chutes. Remove and inspect. Fail: cracked plates, splitter erosion, heavy carbon tracking, loose plates.
Main and arcing contacts. Measure wear against the manufacturer's limit (most ranges have a wear indicator or a dimension). Fail: below the limit, pitting through the contact face, misaligned contacts, signs of overheating on the carrier.
Insulation. Fail: tracking marks, cracked mouldings, discoloration from heat.
Terminations and clusters. Fail: discoloration, pitting on cluster fingers, loose or annealed spring washers, evidence of previous overheating.
Mechanism. Fail: hardened or discoloured grease, corrosion on pivots and springs, debris.
Secondary wiring and connector. Fail: chafed insulation, damaged pins on the secondary disconnect, loose crimps.

Mechanical checks

Manual charge, close and open. Pass: clean, complete action with a definite latch at each stage. Fail: slow, grinding, incomplete, or needs a second attempt.
Electrical charge. Pass: motor runs and stops at charged within the manufacturer's charge time. Fail: over time, runs continuously, or trips its supply.
Electrical close and open. Pass: operates from the coil at rated and at minimum voltage (typically 85% for close, and the UV release drops out and holds in within its band).
Anti-pumping. Pass: a maintained close command produces one closure only.
Racking (drawout). Pass: smooth travel through disconnected, test and connected, with the position switches operating at the right points. Fail: stiff, jumps, indicator disagrees with the switches.
Interlocks and auxiliaries. Every auxiliary contact, ready-to-close and position switch proved in both states.

Measurements

Contact resistance. Micro-ohmmeter across each pole at 100 A or the instrument's maximum. Pass: within the manufacturer's figure and balanced across poles — an outlier pole is a fail even if inside the absolute limit.
Insulation resistance. Pole to pole, pole to earth, across open contacts, at the manufacturer's test voltage. Recorded, and compared with last time — a downward trend is a warning.
Operating times. Open and close time where the site's protection scheme depends on it.

Trip unit

Settings verified against the protection study or the previous record. Any difference explained and signed off.
Secondary injection across every enabled element — long-time, short-time, instantaneous, ground fault — with pick-up and trip time recorded and compared against the published curve. Pass: within the manufacturer's tolerance. Injection testing, explained.
Battery and self-test where fitted; firmware and comms where the unit has them.
Trip-unit-to-breaker trip. The injection test must actually trip the breaker, not just light an LED — this proves the trip actuator.

Service

Clean throughout; remove old lubricant from the mechanism and re-lubricate with the manufacturer's specified grade — general-purpose grease hardens.
Dress or replace contacts as measured; replace arc chutes as inspected.
Torque all main and secondary terminations to specification.
Replace any coil, motor or auxiliary that failed a check; do not "clean and retry".

The report

The record should carry every measurement above with the instrument used, the settings during test, the findings, the parts fitted, and a plain statement of the breaker's condition: serviceable, serviceable with a defect noted for next time, or removed for repair. A maintenance visit that returns a tick-list without numbers has not maintained anything you can prove.

We carry out this inspection on site or in the workshop, and the report is written to be filed with your insurer and your protection study. Book a maintenance visit.