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Piratepete

Hi Guys
Semi detached with 16th Ed box. Upstairs and downstairs lights on seperate ccts and mcbs

Just extended the downstairs lighting cct into a conservatory and fitted an RCBO after checking (not very well, it seems) whether the landing light was on a borrowed neutral. All tested Ok. All lights worked.

A couple of days later, customer rings to say that RCBO sometimes trips when the landing light is switched on. Seems landing light is a 7 watt CFL which equates to about 30mA.

When I explained the problem and the mess I was going to make either to feed an upstairs live to the downstairs switch or a downstairs neutral to the pendant, he thought a bit and says 'no problem - I'll fit a 3watt LED bulb and it'll never trip - go away!' So I do making a mental note to check landing light ccts more thoroughly.

Couldn't think of a suitable reply! Can you?

Pete
 
I always say it is good that the rcbo/rcd is tripping because it shows it is working correctly.......safety device....safety device!!! Luckily I deal with a lot of bungalows and flats without shared/borrowed/nicked N's..
 
Prove the ciruits are either separate or not, drop the neutrals from the bar and IR between the neutrals from each circuit and then the lives, you are checking to make sure you have two independent circuits. It is usually the live that is linked at one of the landing switches giving an inbalance.
 
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Not really a solution as RCD will be sat under threshold, needs to be around 3mA max. When he fits the LED be interesting what happens when he first turns it on. Cct needs to be moved onto same RCD otherwise its simply wrong.
 
No! The interconnection needs to be found and fixed, the dangers associated with a borrowed neutral will still be present even if the two circuits are on the same RCD.

Agreed, although not so much of an issue if on same rcbo as in this case - though not ideal (landing not separate/No. of lights on one circ etc)

The main danger with borrowed neutrals if left (as I see it) is when some unsuspecting sparky carefully isolates upstairs circ (locks off etc) then proceeds to change fittings in the bedrooms or bathroom - then someone (the customer on the way up to offer him tea) switches the landing light on....result - he gets fried!!

If you're gonna leave it you should at least label it on CU.
 
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Millions if not trillions of "17 edn" boards (and 16th) out there with exactly that --- both on the same MCB! Clearly whoever was changing them at the time hadn't done proper checks or wasnt too concerned about no seperation of circuits. Although as one learned 'electrician' once told me, "they've got sockets and standard lamps" .....
 
Thanks, guys, for your interesting replies.

1. RCBO required because cables to new wall lamps buried less than 50mm below the surface.

2. Testing - IR testing between circuits is useless if there is no light bulb in the landing light and it's not switched on! I fkd up!
A good indication is to check if there's a live strap across both switches of the usual 2 gang downstairs switch point. I didn't - in too much of a hurry!

3. Solutions - I prefer to feed an upstairs live down to the downstairs switch. If the upstairs pendants are all looped it can get messy separating out the upstairs neutrals and feeding in a downstairs neutral inside the pendant, apart from the safety issue of 2 circuits within one pendant. Inside the switch it is easy to leave an isolation warning note.

4. Not happy. I shall go back to the customer, either to persuade him to have the remedial work done or to put both ccts on the same RCBO as a Risk Assessment allows.

Cheers
Pete
 
Thanks, guys, for your interesting replies.

2. Testing - IR testing between circuits is useless if there is no light bulb in the landing light and it's not switched on! I fkd up!
A good indication is to check if there's a live strap across both switches of the usual 2 gang downstairs switch point. I didn't - in too much of a hurry!

You don't need a lamp fitted or the switch to be on to find a borrowed neutral by IR testing!

If the switch is operating in the neutral instead of the live then they have a very serious and potentially lethal problem.

Disconnect the neutrals at the CU and carry out an IR test between the circuit neutrals, if they are connected then you have a borrowed neutral.

If this test shows the neutrals as not being linked between the two circuits then the symptoms could also be of a neutral - earth fault on the circuit which is not on the RCBO.
 
You don't need a lamp fitted or the switch to be on to find a borrowed neutral by IR testing!

If the switch is operating in the neutral instead of the live then they have a very serious and potentially lethal problem.

Disconnect the neutrals at the CU and carry out an IR test between the circuit neutrals, if they are connected then you have a borrowed neutral.

If this test shows the neutrals as not being linked between the two circuits then the symptoms could also be of a neutral - earth fault on the circuit which is not on the RCBO.

I think that you've misunderstood what is meant by 'borrowed neutral' though I think the that it is not a good description. It describes the scenario where a light (usually on the landing) takes it's live via the switch from one circuit and it's neutral from another. Clearly if there is no bulb or the switch is off there will be no link between the two circuits. Doing an IR test between the two circuits at the CU will not, therefore, show a link.
Pete
 
Whoops!

sorry about that, its been a long day!

Borrowed neutral is a very good description, does exactly what it says on the tin. I just typed before engaging my brain properly.
 

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