Why Interlocks Exist
Medium-voltage switchgear can cause serious harm if operated in the wrong sequence. Interlocks are built-in checks that make certain unsafe actions physically or electrically impossible, protecting both people and equipment.
Common Mechanical Interlocks
- Racking interlock - prevents racking the breaker in or out while it is closed. See our racking mechanism guide.
- Door interlock - prevents opening the compartment door while the breaker is in the service position.
- Earth switch interlock - prevents closing the earth switch onto a live circuit, and prevents racking to service with the earth switch closed.
- Shutter interlock - keeps shutters over the live contacts whenever the breaker is withdrawn.
Electrical and Key Interlocks
Electrical interlocks use auxiliary contacts and relays to block closing unless conditions are met. Key interlocks add a procedural layer, where a captive key must be moved between locks in a defined sequence before an operation is permitted.
When an Interlock Blocks Normal Operation
If a breaker will not rack or close, do not force it. First check that every condition the interlock is checking is actually satisfied, then look for worn, bent, or misadjusted interlock parts. Forcing an interlock is a common cause of expensive mechanism damage.
For mechanism parts, browse VCB spares or send us your breaker details.