Switch Disconnector for UPS causing RCCB Trip

Question

Hi, First time on the forum. 
 I am working on a UPS that is connected via a 4P disconnector switch (Socomec Sirco M AC-21A) 
 Upstream in series to the Disconnector Switch is a 4P 40AT MCCB and a RCCB-AC. 
 When I switch off the disconnector switch the RCCB tripped. 
 When I switch off using 4P MCCB, the RCCB is not tripped. 
 The UPS is a dual input type but we are looping the single source to Mains and Aux. input 
 
 What am I looking at that is causing the RCCB trip and why the MCCB did not cause the same issue?

mapj1 · Accepted Answer

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.

AJJewsbury · Answer

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.

Switch Disconnector for UPS causing RCCB Trip

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