What is the purpose of a mechanical interlock between a circuit breaker and a disconnect switch?

• A. Prevents unsafe simultaneous operation and ensures a safe isolation sequence.
• B. Enables the disconnect to operate automatically without control.
• C. Increases the fault current capacity of the breaker.
• D. Improves the transformer efficiency during switching.

Answer: (A)

A mechanical interlock between a circuit breaker and a disconnect switch is all about safe isolation and proper operating sequence. In a setup where both devices can control the same circuit, you want to prevent any situation where energizing and isolation parts are in conflict. The interlock physically links the two handles so they cannot be operated in a way that would leave a live path uncovered or allow both devices to be closed at once. This means you can’t close the disconnect while the breaker is closed, and you can’t re-energize a circuit without first ensuring the path to the supply is safely opened. In practice, this enforces the safe sequence: de-energize via the breaker, then isolate via the disconnect, reducing arc flash risk and exposure to live parts for maintenance. The other options don’t fit because the interlock isn’t about automatic operation, increasing fault current, or changing transformer efficiency.