Thread: How reliable are safety relays?
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24-11-2020 #1
Strange question, maybe, but...
I have a Pilz PNOZ safety relay in my control box, bought "new but old stock" from eBay. Unused, in rather tatty box. Been working fine for some time, but recently something has been a bit flaky about the machine. I can turn it on, but the "reset" button that should energise the safety relay has no effect. I spend some time checking connections, power to the relay, switches, etc, but nothing is wrong. The pilot LEDs on the relay do not light and there is no "clicking" from inside it. It just doesn't work! I have even had the safety relay out and tried to dismantle it, but it's pretty difficult to get into it far enough for real fault-finding. I wander away and look at its data sheet, which has a diagram of its internal connections, to try to get more ideas about testing. I go back to the machine and it works perfectly, even though it's been switched off during that time and I have not changed anything. I can't absolutely tie the problem to the safety relay but it looks that way. At one time I thought that the problem might be associated with the CSMIO controller but now that's been replaced with a PV Automation ET6 and the problem still occurs. Very unpredictable about when, and maybe once or twice every couple of months.
This afternoon's fault was slightly different. I turned the box on, hit the "reset" button, and the safety relay energised but the relay switching mains to the driver PSU did not come on. I could trip the mains relay and the machine worked, indicating that there was a problem with the safety relay contacts feeding the mains relay, confirmed by voltage measurements. In fact, I swapped the mains relay feed to an unused set of contacts on the safety relay and all worked as it should. Looks as if one of the safety relay internal contacts had failed (although I'm only ever switching 24V to another relay plus front panel LED so hardly stressing it).
The internal diagram of these safety relays shows a bunch of interconnected relays with all sorts of latching/set/reset contacts and external connections. However, a simple interpretation of its function is that it consists of two relays wired in parallel, with suitable latching contacts, but with the relays' N/O contacts wired in series. So both relays have to latch on, and the latching circuit is arranged so that if one relay fails to operate, the other will also be disabled. In principle, either a relay coil failure or the failure of any relay's contacts will prevent system operation. Even if one relay's contacts weld shut, the other relay will cause the external circuit to disconnect if the estop button is hit. The safety relay actually has 5 or 6 relays internally to allow for more sophisticated operation.
I think that the effects I'm seeing are consistent with some kind of internal safety relay fault, albeit intermittent, and while it fails safe, it's a pain in the b****y neck! Has anyone else seen anything similar? I would expect these relays to be built from fairly reliable and conservatively rated components and contacts.
Although it's working at the moment, having swapped relay connections, I'm now scouting eBay for a replacement. Any sensibly-priced more modern alternatives that anyone can suggest as what's available all seems to be ex-equipment? As one possible solution, I'm thinking that I could pick up a 4pole c/o relay plus base from somewhere like Rapid for a tenner or so, use one contact set for latching in series with estop buttons, and still have the equivalent of 2N/O plus 1N/C that I need for my machine. Sound like a good way to go?
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