Without seeing it it is not obvious, but I'd start by looking to see if there is a breaking order to the poles. It is possible that the switch that trips is not breaking neutral last and making it first, and the one that does not cause a trip always avoids a 'flying neutral' condition, such that on one or more phases is connected and neutral is not.

On 4 pole switchgear with one intentional  'late' pole, that is specifically labelled 'N' - whereas if all poles break together, then there is no such indication, and at the millisecond level , what happens first is a lottery each time.

Even so this is only a problem where there is a significant capacitance between neutral and earth (or of course a direct connection, but that would probably trip all the time and you would have found it by now) During the sudden step in neutral to earth voltage, there is a charging of that capacitance, and the RCD-like part of the trip sees an imbalance.

The problem is that this is a transient effect and quite hard to prove, and worse may not always happen every time...

It may well not be sensible, but (only!) if it is safe to try, then bypassing the neutral pole of the offending switch with a solid link, and then seeing if the trip no longer occurs will at least clinch it as that mechanism.

What you do next if that is or is not the case, of course is another matter.

Mike.

I'm not 100% clear if the disconnector is before or after the UPS - if before, questions about the N-PE reference of the output come to mind.

Otherwise, I'd agree with Mike about the order of poles breaking. It's common enough on single pole supplies to have RCDs trip when switching off DP switches - if the N opens first and there's a relatively high impedance fault between N and PE. In normal working conditions there's only a small voltage difference between N and PE so the leakage current is often small enough for the RCD to tolerate - but if the N contact opens first, the downstream N conductors are pulled towards line voltage by the load, so so the increased voltage between N and PE results in more current leaking - often enough to go beyond the RCD's threshold. 3-phase could be similar if the loads aren't well balanced (or otherwise have non-cancelling N currents - e.g. from single phase switch-mode PSUs).

  - Andy.

Hi Mike,

Thanks, I think you got the right cause in that it is likely that the disconnector switch is opening the neutral at the same time or slightly before the Phases are disconnected.

Currently we have this repeated over multiple sites and it seems consistent that the disconnecting switch is the only item that is causing the RCCB trip.

Will try the bypass after consulting with the upstream/supply Person In Charge.

I guess the easiest solution would be to change the 4th Pole config to a Solid Neutral link config. (SIRCO M seem to have this option) 

Do you think a change in RCCB to a S (Delay) type will solve this symptomatically assuming it is a transient issue?

You mentioned about breakers with intentionally late Neutral, would this not be true for Circuit Breakers in general or is it something we need to specify? Where would I see or how can I verify this when selecting a product?

Regards

Thanks Andy,

The disconnector is before the UPS (at the input to UPS), on the N-PE reference of the output are you referring to when the UPS is in battery mode?

Or is there something else to be considered?

Regards

are you referring to when the UPS is in battery mode?

Indeed. Some UPS don't establish their own N-PE link but rely on the N-PE loop of the supply even when in battery mode (not a good policy in my opinion, but apparently not uncommon) - so breaking the supply N might have the effect of separating load N from PE ... which can again cause odd effects as the N is no longer held close to 0V but all the conductors get to be loosely referenced to Earth via filters and so on.

   - Andy.

Well if it works with 3 poles and solid N, and your safety case allows, it may be best/simplest to do that permanently. If there  is a specific offset contact made for neutral switching or not seems to be a bit pot luck - some makes seem very attuned and the catalogue mentions it as a matter of course, others seem to have never even heard of the problem. I suspect you will have to chat to the makers reps to find out if they do not highlight it. It is most serious on rural overhead TT, where you really do want to isolate the N during servicing.

(not quite what you need but shows how some makers refer to it.)

I'm not au fait with sirco at all I'm afraid - it may be best to call them and ask what they do.

Mike.