Urgent - rcbo tripping

[sidekick]

I have just replaced mcb's with RCBO's on upstairs and downstairs light circuits to cover work on bathroom lights and fan.
All work ok and tested ok etc. However, customer just phoned to say when they switch on landing light both RCBO's trip.
What could be causing this. Is it to do with the 2 way strap between the landing light switches up and downstairs ????

Soonest help appreciated as I need to put MCB'S back if I cannot easily sort.

 

[WayneL]

Any reason why I cannot replace the old mcb's and put bathroom lights and fan on rcd spur outside bathroom (apart from additional cost and loss from 2 rcbo's)

It wouldn't be immediately dangerous to your customer or the installation - there's probably loads of similar situations all over the country.


I've been trying to think of a way of explaining the danger - I came up with this:


If the borrowed neutral for the landing light comes from the bedroom ceiling rose.

At a later date, the customer wants the light fitting changing, so they call in a spark.

He safely isolates the upstairs lighting circuit and confirms with his tester at the rose.

Even if, at this stage, someone switched the landing light on (powered from downstairs), there still

wouldn't be a voltage apparent at the rose!

BUT, ...and this is the killer, the minute he disconnects the borrowed neutral from the other neutrals in

the rose, if the landing light is switched on, the borrowed neutral rises to 230v.

 

[chris gray]

why dont you get your feed off the upstairs lighting?

 

[sidekick]

Can u not just put both lighting circuits on the same rcd and re-jig some circuits?

I know it doesnt meet discrimination requirements but at least it will still have RCD protection which I would have thought is more important. Obviously note on the cert etc.

Thanks Super Stuey,
Did think of that but each rcbo already has x 2 line conductors in so would be x 4 into one rcbo and already struggled with the 2 on a memra 2000 board.

It wouldn't be immediately dangerous to your customer or the installation - there's probably loads of similar situations all over the country.


I've been trying to think of a way of explaining the danger - I came up with this:


If the borrowed neutral for the landing light comes from the bedroom ceiling rose.

At a later date, the customer wants the light fitting changing, so they call in a spark.

He safely isolates the upstairs lighting circuit and confirms with his tester at the rose.

Even if, at this stage, someone switched the landing light on (powered from downstairs), there still

wouldn't be a voltage apparent at the rose!

BUT, ...and this is the killer, the minute he disconnects the borrowed neutral from the other neutrals in

the rose, if the landing light is switched on, the borrowed neutral rises to 230v.

Thanks for that Waynel,
Danger noted.

why dont you get your feed off the upstairs lighting?

Can do Chris and that would be the ideal way but that would require the new cable down to the landing switch and the three core from there to the downstairs switch and also finding and disconnecting the live feed from downstairs.

 

[gaz2212]

put both the circuits on the same rcbo for tonight.. sholdnt trip then... unless theyve taken the neutral off the shower lol

if it dont trip then its gotta be borrowed neutral?

 

[Hawk81]

thats because dust will build up in a vent fan and when it has to draw moist air from an bathroom it will track to earth and trip RCBoS mate. There is some fab advice on this thread for you keep it all in mind, you will need it al to solve faults, always start with the ones that require minimum effort to test, like geting the crap out of a fan etc.

If the fault was in the fan surely it would only trip 1 rcbo. I agree there is great advice on this on this thread but most of it agrees with what i said about a borrowed neutral. I also agree that it is best to try the easy solutions first but did not want to have the fella doing pointless exercises. He had stated that he had fitted a new fan in his original post.

 

[chris gray]

got a plan:
bin the borrowed neutral, tap a neutral off the upstairs smoke, and double the smokes and downstairs lighting up?

why dont you get your feed off the upstairs lighting?

yeah that was never gna work

 

[sidekick]

got a plan:
bin the borrowed neutral, tap a neutral off the upstairs smoke, and double the smokes and downstairs lighting up?

Thanks Chris,
However, no smokes currently installed in house so have gone with RCD spur outside bathroom and pointed out requirement to eliminate the borrowed neutral should customer want to go down that route.