Principles, Standards and Implementation

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Structure of Safety Related Control Systems

Interlock Switch

The limit switch is a mechanical device. The task it performs is a simple one — open the contacts when the guard opens. Years ago, limit switches were used in this fashion. But their design has limitations that do not provide enhanced safety performance.

Electrical standards require short circuit protection devices (e.g., fuses or circuit breakers) for branch circuits. This protection may not be enough to prevent a welded contact in the limit switch. The contacts in the limit switch are designed to open by the force of a spring. Unfortunately, the spring force is not always strong enough to overcome the force of a welded contact.

A second consideration is the spring itself. Repeated flexing may eventually lead to breakage and the force exerted on the contacts may not be enough to open the circuit. Other internal faults in the operator head or the linkage may also result in the contacts remaining closed when the guard is opened. Another important consideration is defeatability. When the guard is open, the limit switch is easily defeated by pushing the lever into the actuated position and holding it in place with tape, wire, or simple tools.