RCBO Issues

Question

I have a wylex split consumer unit with two WRS80/2 30mA RCDs feeding a series of final circuits including ring mains and lighting. These are fed by NHXB32 and NHXB6 MCBs respectively. Currebt system works fine and there are no earth faults on the system. 
 I have fitted RCBOs in place of the MXCBs as follows: 
 
 NHXSBS6B on the lighting 
 NHXSBS32B on the ring mains 
 In place of the WRS80/2s I have fitted WRDMT100/2 time delayed RCDs for discrimination. 
 
 Ring main RCBOs work fine. The RCBO on the lighting circuits trip immediately I switch the breaker on. This happens on all three lighting circuits. 
 
 Lighting circuits have a mix of LED and halogen lamps. Total LED load on each is around 120W (12x6W) 
 Each RCBO is wired correctly and the correct meutrals are wired into the RCBO. The neutral flying lead is on the neutral bar and the functional earth is connected to the earth bar. 
 
 Can anyone suggest why the RCBO is tripping? My only suggestio wuld be a borrowed neutral. Given all three lighting circuits use the same neutral bar and the problem has only arisen since I fitted RCBOs then I'm wondering if neutrals have been shared between circuits (would not have been an isue before as all three circuits shared the same WRS80/2 RCDs. 
 Thanks in advance

Simon Barker · Accepted Answer

Maybe a shared neutral on a stairwell lighting circuit. When all the lighting circuits were on the same RCD, it wouldn't have been a problem. With all RCBOs, the current could be returning through the wrong neutral.

AJJewsbury · Answer

My only suggestio wuld be a borrowed neutral. 
 
 That was my 1st guess too - a common 'trick of the trade' of yore was to drop a twin & earth to the hall light switch, bridge com on that com on the landing switch and run another twin & earth as "strappers" to the landing light switch.Nett result the landing light was connected to upstairs N and downstairs L. Wall lights were another common one, but they tended to be connected to the socket circuit N, which would probably have shown up with the ring RCBOs tripping as well. 
 2nd/3rd guesses would involve some incompatibility with the loads (e.g. LED ballasts) or switch-on surges tripping the magnetic part of the circuit breaker (try with all the lights switched off and see if the RCBO distinguishes between overload and residual trips - some trip to a centre position for residual), Failing that perhaps a bad batch of RCBOs - try the RCBO with its output disconnected. 
 - Andy.

mapj1 · Answer

which lights are the 2 RCBOs supplying, and does the problem go away if you feed all 3 outbound cables from the same RCBO ? 
 As others have also, I suspect that where these circuits come close, such 2 way switches sharing a faceplate between up and down, or wall lights, there is indeed a cross connection. The other way to check for this is to turn on all lights, and then meter between L+N cores shorted on one circuit, to L+N cores shorted on each other in turn. M

AlexG · Answer

Thanks for your reply. They are all cceiling lights - a mix of halogen and LED (mostly the latter). Yes the problem goes away when they are all connected to the same RCBO. I've got a three gang swtich in the kitchen which is fed by two circuits - the problem might well be there. I'll go ahead and try what you suggest.

RCBO Issues

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