plc output card blowing out

We have a Schneider OTB module with several TM3 I/O cards. How do you diagnose the reason why they do not last very long? We have a relay output card that the same output goes bad way too soon.


One thing is that there is a selector switch that parallels this output so that they can operate the contactor manually as well. So in this case when they operate the switch, 120V would get back fed to this relay output. Is this hard on outputs to have them paralleled like that?

They have checked for shorts in the wiring to make sure that is 10-4. The other day they said they installed a new card and it lasted about a minute till it cracked like a 22. But before this, it had at least lasted several months.

The contactor coil is an inductive load. It would be wise to put a snubber (surge suppressor) on it or to use an interposing relay (both actually)...

It could be that there is already a snubber on the coil and it has failed, so now your relay is suffering from the surges that are no longer snubbed.

With the same output getting destroyed, the strongest possibility is the load which you said was a contactor.

Is your contactor old? I've seen the tracks worn to the extent that the moving element of the contactor jams before the coil core is complete. This causes excessive current to flow in the coil and could easily wreck your output relay.