The circuit breaker used for an A.C. generator is typically an Air Break Circuit Breaker. It is frame-mounted and of the draw-out type, allowing it to be isolated from the busbar and alternator input contacts by moving it horizontally. An interlock ensures the breaker is turned off before being drawn out.

Main Components:

• Contacts: High-conductivity, silver-coated copper contacts ensure efficient current flow.
• Arcing Contacts: Separate arcing contacts protect the main contacts by taking the arc during the operation. These arcing contacts open slightly after the main contacts and are replaceable.
• Arc Chutes and Splitter Plates: Electromagnetic forces guide the arc to the arcing horn, where the arc is elongated and quenched. The arc is divided into sections by splitter plates, effectively extinguishing it.
• Anti-Bouncing Devices: These prevent rebound of the contacts, ensuring a clean break.

• The circuit breaker's protection system (e.g., overcurrent relay) detects an excessive current flow indicating a short circuit.
• The breaker opens, interrupting the flow of current and preventing further damage. If only one generator is operating, a complete blackout occurs. If generators are in parallel, the load is transferred to the other generator(s), potentially causing an overload trip.
• In a parallel system, the remaining generator(s) assume the load, which may overload them and trigger a trip.
• This can lead to a complete system shutdown (blackout).

Checks following the event:

• Open the backside of the switchboard associated with the tripped generator and inspect for short circuits.
• Check all outgoing feeders individually to locate and clear the fault.
• Inspect the affected generator’s armature, field circuit, AVR, and connections for insulation resistance, physical damage, and overheating.

• Immediately operate the emergency manual trip mechanism to isolate the affected generator.
• Completely isolate the generator from the system by switching off its supply and load. This is the most important step to prevent further damage.
• Open the generator's field circuit supply to cease voltage generation.
• Once the generator is fully isolated, proceed to locate and clear the fault.
• After the fault is resolved, perform insulation resistance tests on the armature, field, and AVR circuits.
• Thoroughly inspect all components of the generator for any damage or overheating.
• Once the fault is rectified and all checks are satisfactory, the alternator can be safely brought back into service.