Easy Solution to Borrowed Neutral?

[Hawkmoon]

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

2. Testing for b/neutral - forget all that palava, just turn on landing light and use clamp ammeter at the CU to determine which neutral is carrying return current - quick/easy/failsafe (takes about 1 minute if you're slow)

3. Better to take the neutral upstairs, that way you are keeping the landing on separate circuit from the rest of upstairs - why not just jb (separate) the upstairs circuit outside the pendant? (assuming in loft)

 

[Hawkmoon]

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.

This won't work unless you have both the landing light switched on (lamp in) and at least one downstairs light switched on (lamp in)

In the typical 'borrowed neutral' scenario the neutrals are not directly linked...



Didn't bother drawing in switches but:


...notice you will have to have some lamps in and switches closed for your N-N IR test to reveal it

 

[telectrix]

easiest way i've found is to switch off the MCB for the upstairs lights, then voltmeter at landing light with switch on, meter across L and E. if you get 240V then the L is from the downstairs. don't even need to open the CU.

 

[Hawkmoon]

easiest way i've found is to switch off the MCB for the upstairs lights, then voltmeter at landing light with switch on, meter across L and E. if you get 240V then the L is from the downstairs. don't even need to open the CU.

But won't you have the same result if it is wired correctly? (neutral also from downstairs)

 

[Octopus]

easiest way i've found is to switch off the MCB for the upstairs lights, then voltmeter at landing light with switch on, meter across L and E. if you get 240V then the L is from the downstairs. don't even need to open the CU.

Try that in my house and you'll get voltage showing BUT the neutral is also from downstairs so what would your test show? just asking!

 

[telectrix]

thin k i got it wrong there. let me rethink. yep, my thinking is that 99% of the time it's L from downstairs and N from upstairs , when you have borrowed.

 

[Richard3009]

The only test you need to do is to find out if you have two completely separate circuits IR test between the neutrals from each circuit then the lives, all done at the CU dont forget to take the neutrals off the bar
(Nudge)

 

[Hawkmoon]

The only test you need to do is to find out if you have two completely separate circuits IR test between the neutrals from each circuit then the lives, all done at the CU dont forget to take the neutrals off the bar
(Nudge)

Nobody's saying you can't do it that way...but if you do you have to make sure lamps/switching are in place

 

[Badged01]

Thats if you can decepher what circuit is what in the spiders nest of wires wrapped around inside the 3036 board ..... or most of the other boards Ive seen for that matter!

 

[Richard3009]

Thats if you can decepher what circuit is what in the spiders nest of wires wrapped around inside the 3036 board ..... or most of the other boards Ive seen for that matter!

I know what you mean, when confronted with a rats nest I like to remove each circuit and marker flag them, straighten everything out so its all clear which cable is what, then start methodicaly testing and re-connect the circuits that are ok, then deal with the crap, otherwise you can get seriously confused.

 

[davesparks]

The only test you need to do is to find out if you have two completely separate circuits IR test between the neutrals from each circuit then the lives, all done at the CU dont forget to take the neutrals off the bar
(Nudge)

You've made the same mistake I did earlier, testing neutral to neutral won't identify a borrowed neutral.
Testing live of 1 cct to neutral of another with loads connected is the only way. A borrowed neutral is where a live is connected to 1 cct but the neutral of another cct is used.

The classic example is the upstairs/downstairs 2 way. A 2 gang sw at the bottom of the stairs has a T+E from the hallway light connected as live and sw live, this live is used to feed the 2 way to upstairs.
A twin is run from this switch to the upstairs switch and forms the strappers.
A sheathed single is run from the upstairs switch to the upstairs light.
The upstairs light gets its neutral from the upstairs feed, often another sheathed single.

End result is a live from one circuit and a neutral form another.

 

[Richard3009]

Ohhhh yes when they use the singles method fair point

 

[Hawkmoon]

In my neck of the woods (Portsmouth) this must have been the 'install method of choice' back in the 60s/70s - bottom switch fed, strappers only between switches, neutral from upstairs.

If you work in an area where this is common you are less likely to be caught out/confused by it - like everything in this game, the more experience you have in a particular area, the more 'competent' you are in it.

 

[dazza1981]

In my neck of the woods (Portsmouth) this must have been the 'install method of choice' back in the 60s/70s - bottom switch fed, strappers only between switches, neutral from upstairs.

If you work in an area where this is common you are less likely to be caught out/confused by it - like everything in this game, the more experience you have in a particular area, the more 'competent' you are in it.

I personally have not come across this yet hawkmoon and must admit it has taken a few reads to get my head round it.... but i feel armed with a bit of knowledge to tackle it if and when i do come across a borrowed neutral..... cheers buddy

 

[davesparks]

Ohhhh yes when they use the singles method fair point

By any method a borrowed neutral won't show up by testing between neutrals of different circuits.
A borrowed neutral is where a load is connected between a live of one circuit and a neutral of another, not an interconnection between two circuit neutrals.

 

[7029 dave]

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

This was a very common practice, 2way lighting, no 3core bridge the live on downstairs lighting circuit (com), twin to upstairs (strappers) s/l for landing light. (com)