Periodic trip of safety relay

boneless

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Hi all!

Very strange issue, I have a customer that reports a trip of our safety relay (Schneider Preventa) and subsequently main breaker trip (cause by safety relay).

The failure comes periodically every 7days, 6 hours and 52 minutes. Short enough to trip our safety relay, but not long enough for any of our other safety signals (Siemens 3RH2131-1JB40) to trip.

We replaced our Preventa unit, and error consists at the expected times.

We now know there is another safety unit in place, which I suspect causes the issue. Also a Schneider unit, XPSMCMCO000EM + XPSMCMCP0802 + 2 XPSMCMMX0802. I am not familiar with this particular device.

Has anyone run into where the above system, periodically opens the outputs for a very very short amount of time?

Sorry, hard to explain what I am seeing, as I am unsure what the heck is going on!
 
Can you provide us with a wiring diagram? I think that would help.

As for the XPSMCMCP0802 that is a Safety CPU. The MX0802 is a Expansion module with 8 in and 2 out. I couldn't find anything on the CO000EM so not sure what that is. Since my knowledge is very limited with Schneider equipment, I am going assume that this package is a safety PLC type of device. I couldn't find anything on the programming of the unit so I am a little confused on how it works.
 
I don't have much experience on those specfic units, but here's some general advice:

I know many safety devices I have seen do periodic diagnostics checks on solid state outputs. I've seen these called bit-pattern tests and light/dark tests. Basically, if the output is on for a certain amount of time, it has to turn it off to verify that it can. Similarly, if it is off for a certain amount of time, it may have to turn it on to verify that it can. These are very brief actions (sub millisecond). Historically these weren't detectable by most safety relays, but as systems have gotten faster/more intelligent, these brief blips can get noticed as faults.

Sometimes I've seen devices with configurable safety inputs that require a minimum duration to trigger, or I've also seen adding relays in the middle to mask the problem.
 
Start with the simple things. We had a palletizer that they said shuts down on pattern X on the fifth layer. Turned out to be a prox cable had worn out from the layer pusher going by when it got to the 5th layer. Was just enough of a drop on the 24vdc to trip the safety relay and shut the machine down.
 
Is the timing on that consistent from alarm to alarm? 7 days, 6 hours, and 52 minutes between alarms? It looks like you said so, but I just want to confirm.

The XPSMCMCO000EM...is there a missing zero in there? I find a XPSMCMCO0000EM, with four(4) zeros. That's a Modbus TCP module.


A shot in the dark, but I'm wondering if there is an overflow somewhere on a timing/counting routine. That's a very specific time between alarms, and some converting yields an interesting coincidence: 2^20(Bit 20 flipped) is 1048576, and converting that time span(7 days, 6 hours, and 52 minutes)to hundredths of a second(62952000) is right at 60*2^20. It's not exact(60.036---), but I'll consider 3 significant figures close...Exact time would be 7 days, 6 hours, 45 minutes, 45.6 seconds.


Pi
 
Hi all,


Thank you all so much!


@Bullzi, I have one screen grab but it is very low res. All I see is two relays connected to the module mentioned, which are connected to my system.


@mk42, makes sense! Thank you!


@JaxGTO, I doubt that is the case. The errors are very consistently timed.



@Pireciter, client provided me with the part numbers, but I think you may be right. The first unit is an ethernet module. Thanks for doing the math :).



@BryanG, thank you!
 

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