Testing devices connected in series

Anyone got any good ideas for testing long circuits of input devices connected in series? A customer has a problem with something in a long string of devices tripping their plant, but doesn’t know which one is causing it - they only feed the final connection into the PLC, not each individual device. It’s an intermittent problem which makes testing really difficult.

I thought at first of testing continuity at the mid-point of the line to see which half contained the tripped device, but I realised this would effectively short out half of the devices, so I couldn’t do this when running - this is a safety circuit. I then thought of using spare terminals on each relay (if available) to wire a second loop and do the testing on that, but that would be very time consuming to install.

Any other approaches I’m missing?

If you have a common available at each point you could take a little phoenix contact relay or something similar and a push button and make a latching circuit at the exit of each safety. Then the first one that is no longer latched after an event would be the problem.


Alternatively, if you have a bunch of multi meters with a min/max function you can stuff them in the panels and put them on MIN. Then the first one in line that shows 0 Volts would be where the problem started.

Be aware that many modern safety relays expect their own signal to pass through all the contacts, so that it can be pulsed off briefly (~1ms) every so often to check for short circuits. Dual-channel devices often have the polarity reversed on the second channel to catch cross-wiring errors. Hanging extra circuits off the chain might not work as expected, and raises the possibility of disturbing the safety function.
If you are going spend for extra monitoring relays, consider using a miniature safety PLC instead that can accept each input on its own pin, and echo their status for your monitoring to auxiliary outputs. I’ve found the Jokab Safety (now ABB) Pluto products ideal for small tasks like this.