Hello all,

I have a coater machine that is throwing an intermittent dc safety power fault. Everytime it happens it throws that fault along with a handful of faults for safety devices. the machine can immediately be reset and ran again. Sounds like either a loose wire somewhere or a voltage drop causing this. I noticed that every alarm or fault we get with the dc power fault is tied to one slot on the plc. Everytime i test the circuit i see good resistance and voltage reads across everything. I have done tug tests on everything as well. all the wires that looked corroded or slightly loose have been tightened but were still getting the fault randomly. I have even tried to increase the voltage a few volts on the supplies to try to fix any potential voltage drop that may be occuring. This happens inconsistently as well and isnt tripped by any certain action. You can be standing there while it runs and with no intervention it will happen. Sometimes it will run for 2 days without happening and sometimes it happens a few times a day. for the life of me i cannot isolate what is causing this but i am positive it is somewhere between the power supply and the inputs for that specific slot on the safety plc. I am at a dead end with this. Any help or pointers for narrowing this down would be extremely helpful. Thanks guys.

 

@cotybrooks,

Please be more specific about the manufacturer and model number of the PLC and the specific card. Tell us if the card is made by the PLC manufacturer or another company. Tell us what the card/slot function is for in the process.

Are you sure no field work has occurred on any of the devices or the field wiring connected to the card?

Is it possible to swap the card from the slot with another card in a different slot? If the problem follows the card it’s likely the card has issues. If the problem stays with the slot it’s likely the PLC or the PLC backplane. Or the power supply connected to that slot or card.

I’m not a fan of taking a card from stores and just putting it in the slot/rack to see if that solves the problem; if the problem is the slot/rack or field wiring or field devices connected to that card it’s possible that the new card can be damaged. But sometimes—if there isn’t another card in the rack that can be swapped to see if the problem is the card or the slot—then sometimes pulling a card from stores is all one can do. I do Iike to be as positive as I can be that the problem isn’t the field wiring or devices before exchanging any card either from stores or a different slot in the rack.

Also, if you could be more specific about the alarm text messages that might help, too.

 

If you have electrical schematics for the machine, post the page showing the PLC safety card you are having issues with along with card part number. Does the card have test pulses wired as a supply to inputs?

 

So I ended up finding a bad ib8s safety module. Never would’ve thought that was the cause. Swapped the module with another ib8s next to it and started seeing new faults associated with that terminal.