Transfer Switch Interlock
A transfer switch interlock is a mechanical and/or electrical interlocking arrangement that ensures a load can be connected to only one power source at a time, such as normal utility versus generator, and that the switch between sources is break-before-make, in compliance with CSA/UL/ANSI standards and NEC/WorkSafeBC requirements. It prevents simultaneous closure of normal and alternate supply switches, preventing backfeeding and hazardous cross-connection.
On a shop floor, a transfer switch interlock is applied in emergency/standby generator systems, critical production equipment with dual feeds, and integration with lockout/CSID concepts. In manual systems, an operator opens the main breaker, an interlock plate slides to block it, then the generator breaker can close, ensuring only one source energizes the panel. For automatic transfer switches, internal mechanical/electrical interlocks enforce break-before-make transfer. Key-transfer interlock systems may enforce proper isolation sequences. Under WorkSafeBC, a safety-rated interlock switch can be acceptable as a control system isolating device when safety-rated and used in positive mode, but does not replace lockout.
How does a transfer switch interlock differ from a general interlock switch in WorkSafeBC/CSA terminology?
A transfer switch interlock is specific to source-selection, preventing parallel connection of power sources and enforcing break-before-make. A general safety interlock switch is used for machine guarding, such as door interlocks, and is recognized by WorkSafeBC as a control system isolating device when safety-rated. The key difference is that transfer switch interlocks focus on source isolation and electrical system integrity, while safety interlock switches prevent hazardous motion or access.
Under WorkSafeBC, can worker safety rely on a transfer switch interlock without lockout?
BC Hydro Safety Practice Regulations allow reliance on CSA-approved transfer switches or key interlock switches as exceptions to the rule that worker safety must not depend on interlocking. However, for shop-floor lockout under WorkSafeBC Part 10, lockout of energy-isolating devices remains the primary method. A transfer switch interlock may provide additional assurance but does not replace lockout and verification of zero energy.
How is break-before-make implemented in an interlocked transfer switch mechanism?
Mechanically, a cam or linkage design forces normal contacts to open fully before alternate contacts can close, with a physical detent ensuring a zero-connection state. Electrically, control logic for automatic transfer switches opens the normal source contactor, waits a programmed time delay, then closes the generator contactor. Standards like UL 1008 and CSA C22.2 No. 29 require that interlocked transfer equipment prevents simultaneous closure and ensures break-before-make for open-transition designs.