Hi,

I have S7 - 400 PLC on one of our asset for drilling system. 2 Months before, there was a intermittent malfunctioning in our system functionality, so we were troubleshooting on that & found that 2-3 PLC 24 V DC Digital inputs are getting 60-70 V AC mixing signal (interference) at the DI module terminals. Then we troubleshooted that issue & system was back to normal.

But, I have one doubt. If DI module is getting 60-70 V AC signal, but still it is partially working & no damaged to the DI module. Is there any isolation protection circuit in modules for preventing damage from unwanted AC supply?

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Split from PLC programming.