UPS Fault

[jackhammerJIM]

Good evening all . I am hoping someone can shed some light on a reoccurring problem in a mains room we are working in .

The problem being the UPS modules display a general wiring fault whenever the ATS switches from generator back to mains .

So far the UPS manufacturer and generator company cannot find a cause for the problem . The fault is only displayed when switching one way .

TNS earthing system on both sources with a reference spike for the genny .

 

[pc1966]

Is it 3-phase and/or a bunch of UPS in parallel?

My suspicion would be the neutral is isolated during the switch over and they are seeing N-E volts going high on leakage. Try putting a 60W or smaller light bulb between UPS feed (i.e. output of ATS) N-E and see what happens during a test switch-over.
[automerge]1594674201[/automerge]
Another possibility the the ATS change-over contactor is not opening/closing all lives on 3-phase sufficiently simultaneously, but try above and let us know if it reveals anything.

 

[jackhammerJIM]

Is it 3-phase and/or a bunch of UPS in parallel?

My suspicion would be the neutral is isolated during the switch over and they are seeing N-E volts going high on leakage. Try putting a 60W or smaller light bulb between UPS feed (i.e. output of ATS) N-E and see what happens during a test switch-over.
[automerge]1594674201[/automerge]
Another possibility the the ATS change-over contactor is not opening/closing all lives on 3-phase sufficiently simultaneously, but try above and let us know if it reveals anything.

Yes it is 3 phase with 2 x 30kva ups running in parallel . The neutral has to be switched to satisfy g99 .

 

[pc1966]

Yes, most ATS will switch neutral for that reason. However some systems are not happy with a floating 'IT' supply during any prolonged change over so they recommend a transformer or similar between the ATS and UPS to ensure there is the N-E link on the secondary (UPS feeding) side so no floating supply but without violating the general rule on no TN-C supply arrangements (which hard linking N & E on the ATS output / primary side would create).

Also a delta-star (or zig-zag) of transformer can help control 3rd harmonics and lower Zs for the UPS supply loads, so there can be other reasons for looking at that. Specialist companies such as R Baker (Electrical) Ltd can make transformers to specification if needed.

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Another possibility is the switch-back from generator to mains is "too fast" in the sense the UPS isa not recognising it as a break but as a glitch on the supply voltage/phase. Some ATS controllers have various programmable parameters and it might be possible to set a defined gap of 0.1s or similar for going back to the mains.

But really the UPS supplier should be able to explain what triggers the "wiring fault" message in the first place!