machine safety guard switches

Hi Bob,

As far as I know there is no standard that covers this. Even EN 1088 gives no guidance. It comes down to your risk assessment and the potential loss of the safety function ( and risk reduction loss) if the switch, or its circuit has a fault.

You must consider what faults could occur and how they could be detected. Single and dual channel circuits behave differently under certain single faults, but the problems become more dangerous if there is a build up of faults or the monitoring of the circuit cannot detect all faults.

The one thing that I come across regularly is that people never check each switch individually ( if they check them at all!). Even if you have a dual channel redundant circuit going back to a safety relay, you can only check the correct operation of the first switch that you open and the last that you close. Wiring a number of switches in series can hide faults. Even daily testing of swicthes done wrongly could prove nothing, so consider the process for testing as much as the frequency. Apologies if I'm teaching you to suck eggs.

Jim