Air Circuit Breaker (ACB) Maintenance: A Step-by-Step Procedure

Air circuit breakers (ACBs) protect the main incomers and large feeders in most commercial and industrial LV switchboards. When an ACB fails to trip on a fault, the damage spreads. When it trips for no reason, the whole facility goes dark. Regular servicing keeps it doing exactly what it should.

Why ACB maintenance matters

• ACBs often carry the full load of a building or production line
• Mechanisms that are not operated for long periods can stick and fail to open on a fault
• Dust, heat and humidity, common in Nigerian switch rooms, wear contacts and mechanisms
• Loose connections on the busbar or cable terminals overheat and can start fires

Safety first

ACB servicing must be done by qualified personnel with the breaker isolated, racked out where it is a withdrawable type, and the supply locked off and proved dead under a permit-to-work. Use proper PPE and follow the manufacturer’s instructions for the specific breaker.

The servicing procedure, step by step

1. Record the as-found condition

Note the breaker’s make, rating, protection settings, operation counter and any alarm or trip history before touching anything.

2. Visual inspection

Check the arc chutes, main and arcing contacts, terminals and secondary wiring for burning, pitting, corrosion, cracks and signs of overheating.

3. Cleaning

Remove dust and contamination from the breaker and its cradle using suitable cleaning materials. Do not use products that leave residue on insulating parts.

4. Lubrication

Lubricate the mechanism and racking parts only where, and with what, the manufacturer specifies. Over-lubrication attracts dust.

5. Mechanical operation

Operate the breaker several times manually and electrically: close, open, charge and rack in and out. Check that the closing and tripping coils, motor and interlocks all work correctly.

6. Tighten connections

Check terminal and busbar connections and torque them to the manufacturer’s values.

7. Testing

• Contact resistance: a high reading points to worn or dirty main contacts.
• Insulation resistance: between phases and to earth.
• Protection trip test: secondary injection of the trip unit to confirm it trips at the set current and time.
• Undervoltage and shunt trip: where fitted.

8. Restore and report

Return settings to their recorded values, rack the breaker in, and record all results in a service report.

How often?

Follow the manufacturer’s guidance. As a general rule, service ACBs at least once a year, and more often in dusty or hot switch rooms, or after the breaker has cleared a heavy fault.

Common problems we find

• Breaker fails to close because the spring charging motor or closing coil has failed
• Nuisance tripping caused by wrong protection settings
• Burnt main contacts after repeated heavy switching
• Hot terminals caused by loose connections

ACB servicing is part of a wider plan. See our yearly electrical maintenance calendar.

Talk to Corustar

Corustar Technological Services is an electrical engineering contractor based in Ikeja, Lagos, working across Nigeria. See our HV/LV Switchgear Maintenance & Commissioning service, request a quote, or call +234 816 976 9672. For outages and faults, call our 24/7 emergency line on 0705 739 9297.