PLC General error fault

Hi all,

We have two Production Lines one controlled with a PLC 3 and the other with a PLC 5 AB. When starting up the second line,(two main compressors powered by 6000 volts with 2600KW and 2350 KW respectively) the first line shuts down with a PLC general fault. Take into account that this does not always happen, and that we can operate for months without problems. any sugestions ?? We are investigating our earthing.. (mixed power and instruments earths).
Any Ideas out there ??

I would need to know a great deal more about the wiring and the proximity of the PLC to the compressor starters & power lines etc...

DC relays making & breaking are murder on A-B PLCs, (induction & radio freq. from contact arc.)
Inductive kick from a large relay coil can cause problems.
Communication cables in close proximity to heavy currents.

You need to just start ruling out items, without knowing your system layout the first thing I would try is to power the PLC from a seperate clean source or UPS. Secondly, I would put suppressors across the coils of the compressor starters. Experiment, try to duplicate the problem when the facility is down.
With the problems so far apart it may take you a long time, I hate those kind of problems.

Good luck