safety relay faults

Forgive me if this question has been covered previously (i am a newbie)
I have a problem on an Oven, the process as old as me
i occasionally get the safety circuit "Tripping" and shutting down the gas burners on "lock-out", this could be any one,of a number of faults,on a number of zones??
unfortunately the cicuits are all in series,so fault finding can take hours etc etc
Has anyone seen/or designed a simple plc programme that would detect when an input has dropped out (from one of the switches)and latch to tell me which one has dropped out ??
any help would be appreciated

The only safe and legal way to do what you want is to monitor the auxillary contacts of your safety switches with PLC inputs.

The drawback is that it is possible with some switches that contacts in the safety strings may not always change states in sync with their respective monitoring contacts. And even if they do work correctly, the contact must change states long enough to be "caught" by the PLC input which can be scan time dependent.

Unfortunately, isolating the problem switch(es) can be very time consuming, especially if the fault is intermittent.

Ideally, all of the switches will be brought back to a terminal strip to help minimize the amount of time it takes to check them all.

If it's a dual channel system, it might help to use the safety relay indicators to determine which channel is having the problem, thereby cutting the troubleshooting in half.

In most cases, it is not acceptable to connect the safety circuit directly to a monitoring device. In reality, though I suppose it could be done on a temporary basis. Often, I have seen people use a jumper wire to bypass sections of the safety circuit TEMPORARILY for troubleshooting. This may violate the laws of your country and your company, so don't do anything like this without plenty of CYA.

If you post more details about the circuit and components, we may be able to offer more insight.
(Drawings, part numbers, safety category...etc...)

Paul

Thanks for your time in replying
The circuit (Approx 20 years old)is made up of pressure switches,relays and controllers all in series etc
if the air or gas pressures drop on any switch,if the exhaust fans go out on overload or do not start,if the oven temps go outside the set values it drops the main control contactor out
Obviously you cannot link any suspect switch out because you are working with gas,therefore by the time you arrive on the scene you have to try and predict the fault or go through the whole circuit searching for the faulty component
so it can be a pain in the *** especially at 4 oclock in the morning
i just thought that without rewiring the whole oven which incidentally is about 100yds long there must be an easier way

Perhaps something in this thread could be of use:

http://www.plctalk.net/qanda/showthread.php?t=35449&highlight=tattle+tale+relay

Paul

ABSOLUTELY PERFECT
you are a star well done