SIL3 Category 3 application

brendan.buchan said:

What type of gate switch are they? Manufacturer / part no.?

If they are mechanical style (eg. tongue and groove) then generally they are not suitable for category 3 and higher applications unless there are TWO gate switches used on the same gate for redundancy.

This is due to the possibility of a single fault preventing the correct operation of the safety system which could be where the screws holding the actuator to the gate fall out and the actuator stays latched into the gate switch.

You can avoid this requirement by using an electronic non-contact style gate switch. Personally I use these as standard in all safety related guarding and haven't used mechanical style switches in a few years. I've can recommend both SICK and Pilz non-contact switches.

As for your question regarding wiring in series, again I can't give an accurate answer without knowing more details regarding the specific type of switch but it is possible to wire certain types in series and still meeting category 3 and higher requirements. For example the two switches I mentioned above have been certified by the manufacturer to be wired in series.

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brendan.buchan said:

Ahh, yes they are a good style of guard switch to use.

I don't believe there is any limit to how many you can connect in series.

Theoretically it may be possible that you may reach some type of voltage drop or input impedance limit after wiring a HEAP of these in series and depending on total circuit length, but that would be far more than 7.

In my opinion you should be fine.

P.S. Your schematic needs a little work though. Anyone looking at that would assume they are mechanical style guard switches.

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