Pilz PNOZ 4 safety relay reliability

[Neale]

The only info I have is from the sketchy diagram in the data sheet. I think the idea is that you can see how it works (5 relays, apparently, with all sorts of interconnections) but without much detail - there's a lot more in there than they show! However, the diode is shown as being in series with the supply, together with a resistor (no value given). I'm guessing protection against a short to another supply in the external e-stop circuit (diode) or short to earth (current-limiting resistor). I would guess that in my case the diode has gone pop (never trust a semiconductor) but it might just be a broken track. No back-emf diodes shown, although they might be there somewhere. I have another 24V relay in the box, driven from the 24V SMPS, with no diode involved. I don't know what kind of back emf could be generated but these aren't high-current relays anyway.

If this hadn't worked, I was considering making my own poor-man's version. Take the usual latching relay type of setup, with a momentary n/o switch to enable. Put a second relay in parallel, and wire NC contacts in parallel and NO contacts in series. That should cover you against either relay failing to operate or sticking on.

I run e-stop switches plus fault outputs from the stepper drivers through the e-stop input on the safety relay although limit switches just go to the motion controller. The safety relay will enable/disable the motion controller, the stepper drivers via the enable inputs, and the stepper PSU via another relay.