Why do ACBs trip and how is maintenance done?

ACBs are equipped with an intelligent electronic trip unit (also called a release or controller) that continuously monitors the circuit. They are designed to trip for several specific reasons:

Reasons for Tripping:

• Overload (Long Time Delay): This occurs when a circuit draws more current than its rated capacity for an extended period. The trip unit initiates a delayed trip to avoid nuisance tripping from temporary in-rushes (like a motor starting) but will trip to prevent wires from overheating.
• Short Circuit (Short Time & Instantaneous Delay): This is a massive, near-instantaneous surge of current caused by a direct fault between phases or between a phase and neutral. The ACB trips instantly (within milliseconds) to prevent catastrophic damage and arcing.
• Earth Fault: This occurs when current leaks from a live conductor to the earth, often due to insulation failure. This is a critical safety trip to prevent electric shock and fires.
• Under-Voltage: The ACB can be set to trip if the supply voltage drops below a safe operating level, protecting sensitive equipment like motors from damage.
• Shunt Trip: An external signal (e.g., from a fire alarm panel, emergency stop button, or remote command) can intentionally trip the breaker for safety or operational reasons.

Maintenance Procedures:

Maintenance is crucial for ensuring the reliability of an ACB. Key steps include:

1. Cleaning: The breaker is racked out and thoroughly cleaned with a vacuum cleaner and lint-free cloths to remove dust, as dust can compromise insulation and jam mechanical parts. Special attention is paid to the arc chutes, which must be clear of carbon deposits.
2. Visual Inspection: Check for any signs of overheating (discoloration of terminals), wear on moving parts, and damage to the breaker's body or arc chutes.
3. Mechanical Operation: Manually operate the breaker's opening, closing, and charging mechanisms several times to ensure they move smoothly without jamming.
4. Lubrication: Apply manufacturer-recommended lubricants to pivots, sliding surfaces, and other moving mechanical parts.
5. Electrical Testing:
   • Contact Resistance Test: Measures the resistance of the main contacts to ensure a low-resistance path for the current. High resistance indicates worn or dirty contacts that can cause overheating.
   • Insulation Resistance Test (Megger Test): Checks the integrity of the insulation between phases and from phases to earth to prevent short circuits.
   • Secondary Injection Test: A specialized test set is connected to the electronic trip unit to simulate fault conditions (overload, short circuit, etc.). This verifies that the "brain" of the breaker is working correctly and will trip at its set points without needing to pass actual high current through the breaker.