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.

 

[Hawk81]

i would check for a borrowed neutral. could be a fault on the straps.

 

[Jason Smith]

right mate, get a paint brush and get all the dust and crap out of that fan. Re test. No joy, disconnect two way switching, re test. No joy, get switches off and check wiring. check roses for a crossed wire. if no joy, start to break down the legs as you would on a ring.

obviously, start with the landing light, the feed from this is often used for a live feed to a fan in an adjacent bathroom.

 

[WayneL]

I would agree with Hawk81 -

If it only happens when the landing light is switched on, then you have your line from one RCBO, but

the neutral is connected to the other - gives you unballanced current in both, so they both trip

 

[Hawk81]

right mate, get a paint brush and get all the dust and crap out of that fan. Re test. No joy, disconnect two way switching, re test. No joy, get switches off and check wiring. check roses for a crossed wire. if no joy, start to break down the legs as you would on a ring.

obviously, start with the landing light, the feed from this is often used for a live feed to a fan in an adjacent bathroom.

i dont undestand why you have started by saying get the dust out of the fan. his fault is tripping 2 rcbo's.

 

[Jason Smith]

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.

 

[sidekick]

The fan is new and so is the landing pendant light so all wires connected ok and safe there. Also checked landing light switch connections etc at time of replacing light.
All was ok on old mcb's so thought it may borrowed neutral.
Iassume it would have to be borrowed from one circuit to the other though to cause both to trip and not just within the upstairs circuit.
Both circuits have x 2 line and x 2 neutral wires leaving consumer unit.
Apart from the excessive 3 foot of insulation in the loft I should be able to trace the wires for the upstairs circuit but downstairs along with several outside lights more of nightmare.
Whats the best way to identify the problem now.

 

[WayneL]

You will probably find that the neutral for the landing light is taken from the upstairs lighting circuit ( RCBO 1)

BUT the live feed will be fed into the downstairs switch from the downstairs lighting circuit (RCBO 2).

This will give an imbalance in both circuits and trip both RCBOs

 

[Deleted member 9648]

You'll probably find the landing light is fed N and earth off the upstairs circuit but the switchline comes from the down stairs circuit, hence borrowed neutral,was common on 60's wired houses and now a right pain in the arsenal. The feed will come from the downstairs circuit at the downstairs switch,two strappers to the upstairs switch and a switch line from there to the landing ceiling rose. Only real option is to rewire the 2 way so it is on the upstairs circuit.
Re the fan dust tripping the rcd...not come across that one before and as most fans are double insulated and not earthed you wouldnt expect it to be a problem,logged for future reference tho'.

Edit....pretty much wot wayne posted while I was typing.

 

[sidekick]

You will probably find that the neutral for the landing light is taken from the upstairs lighting circuit ( RCBO 1)

BUT the live feed will be fed into the downstairs switch from the downstairs lighting circuit (RCBO 2).

This will give an imbalance in both circuits and trip both RCBOs

Thanks Wayne,
I assume the only way to cure this is to run a neutral from downstairs to the landing light and therefore put the landing light on the downstairs circuit or replace the mcb's whch would not give the required rcd protection.

 

[WayneL]

Thanks Wayne,
I assume the only way to cure this is to run a neutral from downstairs to the landing light and therefore put the landing light on the downstairs circuit or replace the mcb's whch would not give the required rcd protection.

Best try to cure it, mate, borrowed neutrals are dangerous.

Ideal situation is the line and neutral taken from the upstairs lighting circuit, with a three core&E

dropping down to the two way in the hall - but that means running a new cable, coz I'll bet good money

that you only have a T&E running between the 'two-ways' at the moment.(That's why they take the

live feed from downstairs).

Your idea of running a neutral gives you the same headache

I don't know what else to suggest, though. Maybe someone else will have a better idea.

This problem will crop up more and more now with the new 17th regs.

 

[sidekick]

Best try to cure it, mate, borrowed neutrals are dangerous.

Ideal situation is the line and neutral taken from the upstairs lighting circuit, with a three core&E

dropping down to the two way in the hall - but that means running a new cable, coz I'll bet good money

that you only have a T&E running between the 'two-ways' at the moment.(That's why they take the

live feed from downstairs).

Your idea of running a neutral gives you the same headache

I don't know what else to suggest, though. Maybe someone else will have a better idea.

This problem will crop up more and more now with the new 17th regs.

Think its going to be difficult if not impossible to talk customer into new cable via newly decorated hall and landing.

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)

Any one else got a better less disruptive idea???

 

[Deleted member 9648]

Sounds like a good idea.....certainly the easiest option if the customer will not contemplate altering the wiring....note on the cert the full story to cover your self.

 

[super_stuey]

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.

 

[Jason Smith]

well done stuey, wish I had thought of that !- it can be noted on the cert as a deviation.

 

[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.