Control panels wired closest to Switch Gear, Grounding problem?


Ontario, Canada
Controls Designer
The Problem: I have two (2) panels at two (2) different locations that are giving me roughly the same problems.
In both panels, the Processors have had major faults of various kinds. Most of them recoverable.
However, in one case, the processor was "bricked" and PLC power supply needed replaced.

The Information:
One panel is in Canada using 600V and the other is US using 480V.
They both use RA Compactlogix 1769-L33ER processors with PB4 power supplies.
The basic setup is the 480, or 600, steps down (ahead of disconnect) via TX to a UPS at 120. The UPS powers the 24VDC supply.
The 24VDC also powers the 1769-PB4 for the processor/PLC backplane.
Each power level is used for other things, but this is the basic structure.

During a recent storm, the Canadian panel blew a fast acting glass fuse on the 24V supplying Contactor Coils (Rockwell EJ Electronic coils) to a bunch of fans.
The processor also had a "backplane timout error". But it was recoverable.
The contactors shouldn't have changed state until the processor de-energized the outputs on Fault.
But the processor shouldn't have faulted because it is behind the UPS, 24V supply, and PB4 supply.

The Aha Moment:
What we just figured out is that both panels, at their locations, are wired closest to the switch gear. Meaning the shortest cable.
So, my only thought is the Grounding.
We do ground the TX, UPS, and 24V common.
Because these panels are closest, electrically, to the switch gear room, maybe these panels are absorbing grounding disruptions?

Does anyone have advice on how to mitigate this problem?
Or maybe i'm looking in the wrong place?


Staff member
San Francisco Bay Area, CA, USA
Electrical Engineer
People who make what's called "paralleling switchgear" for generators regularly put PLCs IN the switchgear. That's not the issue in and of itself. It sounds more like a Common Mode noise issue, i.e. something not grounded properly that is leaking CM noise onto the ground plane of everything.


Senior Member
Chicago, IL
Make sure the 24V common is grounded at only one place and before it splits to feed different loads. Oherwise ground loops may cause coupling of noise and spikes in series with the 24V supply common.
Also, is the UPS the online double-conversion type? Other types may have short dropouts and allow voltage spikes and transients to be conducted.


Senior Member
Northern illinois
AC UPS systems often make things worse. use a DC UPS if you feel the need for a UPS.

Is the common on the DC power supply grounded?

have you carefully followed the instructions for how far from the PB4 power supply I/O modules can be located?

Richard DB

New User
If your 24VDC either the + or - line got grounded most probably the glass fuse will blow but for PLC to get damaged is not good. There are some tools used to detect fault current (+ or -) for VDC supply (either stationary or portable type).

Try to use DC-DC power conditioner after the 24vdc PSU converter.