We faced a problem that on 24VDC digital input termination unit. a power of 110V is given by mistake to the -ve side besides its original floating -12VDC.

how this can affect the TU and the sD/I modules.

Is this mistake can cause some D/I channels to be failed on short circuit?? As we know that the D/I channels fail on open circuit and this is first time that the D/I channels fail in short circuit.

I know there is optoisolator on the D/I channel, but when this can fail on short circuit??

 

I've had opto-isolators "re-program" so that they don't switch off. This happens when the signal is kept on for 99.9999% of the time and I believe the solid state components actually re-bias themselves such that they don't switch at the same levels anymore. I would recommend that the opto-isolators are replaced after 10 years, more or less depending on observed failure rates.

Chris Jennings