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.

 

[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.

 

[Snapester]

I don't think your installation is contravening regs as long as you are satisfying 543.1.2
I was thinking along the lines of Ponty for the cause of the fault?

 

[electromonkey]

I don't like the idea of a single earth being split for 2 circuits. Unless it's made clear where the JB is and where it is connected, you'll cause a headache for some poor spark in the future I'm sure.

I'm now thumbing through the regs to see if I can back this up. If it's deemed satisfactory to do it, fine. I just personally don't like the idea.

 

[Sav]

so your ZE is 54 Ohms ??

Thats very high. I think thats your problem.

Need to get your ZE below 21 Ohms

Sav

 

[Snapester]

You should label the earth connection and state is supplies two circuits as someone might come along and disconnect one of the circuits and think its the earth and voila!! Circuit without earth and someone shocked...
Sharing 2 circuits with one CPC is not the best practice imho.

 

[snowhead]

Another negative vote for combined earth.

If isolating the MCB allows the RCBO to be reset, it must be that the MCB live is causing an inbalance in the rcbo cct by induction.

My next move would be to temporarily swap the cct connections at the Mcb / Rcbo, so the sockets are on the Rcbo and lights on Mcb and repeat the test, it may be a peculiarity of how the cable cores are layered.

I think you'll end up using 2 cables tho'

 

[Snapester]

Have you measured the leakage on the RCBO circuit?

 

[mikehopkins]

What is the configuration of the fuse board ,is it a split load ?

 

[mikehopkins]

What is the fuse board configuration, is it a split load type?

 

[Brightspark2]

I don't like the idea of a single earth being split for 2 circuits. Unless it's made clear where the JB is and where it is connected, you'll cause a headache for some poor spark in the future I'm sure.

I'm now thumbing through the regs to see if I can back this up. If it's deemed satisfactory to do it, fine. I just personally don't like the idea.

A common cpc between circuits is acceptable as long as it meets the requirements of all circuits, size wize etc

 

[D80]

so your ZE is 54 Ohms ??

Thats very high. I think thats your problem.

Need to get your ZE below 21 Ohms

Sav

Where does 21 Ohms come from? I've only heard of 200Ohms for TT system being "acceptable". Not come across the 21 ohms figure before?