Last year I had a different story with a contactor pulling in crow-baring the PSU, cycling the 24V system long enough for the PLC to shut down, and then restart.

That contactor had an in-rush of 28A, with steady state / holding current of just under 0.5A.  It switches up to 400A circuits.

My 10A P/S failed miserably.

I paralleled a 2nd Siemens 10A supply and they held against the 28A in-rush just fine.  The next version of that system had a 20A supply and it worked just fine even with that large in-rush.

It was quite fun watching it (knowing it would not hurt anything), but not fun for my customer. 

#dP

Okay.
Please allow me the attempt to reply to the 'inrush' question.

Modern PSUs (power supply units) are designed to handle even short-circuits. The power output from the PSU is reduced when the demand is too high for too long.

The theoretical inrush specified for the PLC would be controlled / throttled by PSU. The PLC can still switch on even if the current to the PLC does not reach the inrush current value. The important thing is that the switch on current is controlled.

The specification of the PSU needs to keep in mind the requirements of the whole circuit. The devices pulling from the PSU could easily be more than the PLC; the mere numbers can contribute strongly.

Having a too big PSU is also dangerous. A 24V/20A PSU could easily push enough power into a short circuit that cables may be damaged. It is best design to break up the design into smaller portions where each is protected. A 24V/2A fuse would work for example. Here the potential inrush plays a role; best to not have a too quick reaction fuse.

@Ste39, please amend the above as from your own knowledge. It should be most welcome for the topic starter.

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