rcbo only resets when other mcb is off

[copey84]

Hi there, i have recently wired a lighting circuit and a radial circuit for two socket outlets. Run the feeds for both circuits using a 5 core 2.5mm high tough cable from the distribution board to a jointbox. From the j/b i took my feed for lights and feed for sockets, used same earth for both circuits. Lighting circuit is on a 10a mcb and radial for sockets is on a 20a rcbo.
Completed wiring everything worked fine until i hit test button on rcbo and found it would not reset. Only when i switched off the 10a mcb for lighting circuit would the rcbo reset, found this very strange! Have tried different rcbo, but same thing happens, checked all wiring everything looks ok. I know using the same earth for both circuits is not ideal but can't see how it could effect the test button. Really not sure about this one any advice would be welcome.

 

[ipf]

For real ??? Well if all the wiring looks ok ???

 

[Engineer54]

Is your wiring method acceptable to BS7671?? I can't see how it can be, as you need to isolate 2 separate OCPD to isolate this cable.

 

[D Skelton]

Is your wiring method acceptable to BS7671?? I can't see how it can be, as you need to isolate 2 separate OCPD to isolate this cable.

I'm pretty sure his wiring method is acceptable Eng, see Section 521.7.

Hi there, i have recently wired a lighting circuit and a radial circuit for two socket outlets. Run the feeds for both circuits using a 5 core 2.5mm high tough cable from the distribution board to a jointbox. From the j/b i took my feed for lights and feed for sockets, used same earth for both circuits. Lighting circuit is on a 10a mcb and radial for sockets is on a 20a rcbo.
Completed wiring everything worked fine until i hit test button on rcbo and found it would not reset. Only when i switched off the 10a mcb for lighting circuit would the rcbo reset, found this very strange! Have tried different rcbo, but same thing happens, checked all wiring everything looks ok. I know using the same earth for both circuits is not ideal but can't see how it could effect the test button. Really not sure about this one any advice would be welcome.

Firstly, you need to check, check, then triple check that your neutrals are not crossed over at any point throughout the circuit/s. My bet is that they are and you've missed it.

Secondly, I'm assuming you've done your testing? What were your results for the two circuits?

 

[copey84]

Hi there thanks for your reply. Have checked wiring many times definitely ok. Done an insulation resistance test and all results were fine. Problem is with test button not closing after operation, you would think there is a fault on the cable but there can't be as surely rcbo would be tripping out all the time. Hate not knowing what is causing it, if you can think of anything else let me know, thanks again.

 

[wallyanker]

In the jointbox where all 5 wires meet, are the neutrals kept seperate for the two circuits??

 

[Knobhead]

Copie, do an IR from the CU between the neutrals. Wally may have found it for you.

 

[copey84]

yeah neutrals are seperate. used a 5 core high tough 2 lives 2 neutrals and 1 earth for both circuits.

 

[copey84]

neutrals are seperate, done IR test results ok.

 

[Rockingit]

Not that I can immediately think it's relevant, but what's the earthing arrangement?

 

[jedisparks]

I hope not to offend with this question - Is the RCBO wired correctly? Easy to forget and put the circuit neutral into the neutral bar instead of into the RCBO. That could cause this to happen. Whenever I have come across instantaneous RCD or RCBO operation in relation to another circuit being swtiched on, it has always been neutrals related (if my memory serves me correctly - not always the case).

Best of luck

 

[D Skelton]

Probably worth checking the individual sockets. Take each faceplate off and disconnect. Then run an IR test between each of the terminals.

I only suggest this as I once had a similar problem with an RCD not setting on a newly installed 20A radial. After hours of head scratching it turned out it was a faulty socket outlet!

 

[D Skelton]

I hope not to offend with this question - Is the RCBO wired correctly? Easy to forget and put the circuit neutral into the neutral bar instead of into the RCBO. That could cause this to happen. Whenever I have come across instantaneous RCD or RCBO operation in relation to another circuit being swtiched on, it has always been neutrals related (if my memory serves me correctly - not always the case).

Best of luck

Dammit, I just thought of that but you beat me to it!

 

[copey84]

Its on a tt earthing system. earth spike is reading 54 ohms

 

[copey84]

rcbo is wired correctly, live and neutral of circuit into rcbo, neutral lead into neutral bar and functional earth into earth bar. no offence taken!

 

[tony mc]

Head scracter.

If your dead tests are good

Does the RCBO test OK at 5 & 1 times.

When you do switch on the RCBO and then the lighting cct I take it that the RCBO does not see a imbalance. I take it you have switched the lights on and off and tested a load on the sockets making sure it holds?

 

[jedisparks]

Are there any other existing circuits on the board that you have put these on? If so, have you tested IR on these circuits? I know that it shouldn't be relevant but RCBOs are only single pole, any N - E short could be causing it to behave strangley. Maybe clutching at straws here............but it's worth a look.

 

[copey84]

yeah rcbo stays on when lights are on and load plugged into sockets. rcbo tests are good.

 

[Ponty Massive]

As it's a TT system, I'm guessing the MCB is covered by it's own RCD. Are you 100% certain the RCBO hasn't been installed in some way incorporating the RCD's neutral supply?
Is the RCBO neutral completely independant of the MCB (RCD) neutral?

 

[electromonkey]

Pretty much what Ponty just said. Got a photo of your board? My money's on a dodgy neutral connection at the board.