Shared Neutrals
- Thread starter Surhills
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Identify where the borrowed neutral is, disconnect it. Identify which part of circuit live,where feed is from and where the neutral is connected on that circuit, and run new cable from power source, disconnect the existing live, if using twin and earth. Providing switch arrangement allows.
MCB or RCD?
Shared neutrals are a problem on two or more different circuits if not covered by the same RCD.
Shared neutrals are most commonly found on lighting circuits - usually between the hall and landing area.
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The term borrowed neutral can be a little confusing as often it is a borrowed line in real terms.
Items do share neutrals but the safety rules require that RCD and/or MCB should never supply a line to a circuit using a alternative return path.
However in many houses the demand for more and more lights with the ceiling rose limiting the supply to 5/6 amp has resulted in circuits being split.
However in older houses before anyone really considered EMC it was common to use two wire not three wire strappers between the light switches and "borrow" the line from another switch on the same unit. Today we don't do this mainly as it causes mains hum on radios, TV's and cordless phones.
When both are done we get the borrowed neutral but although dangerous and against the rule book it would work OK. But as the RCD's started to be fitted then these faults started to show themselves.
So to cure.
1) Return to one MCB for all lights. With the new low power lights likely it no longer needs the extra power. But do remember ONE MCB not two MCB's from the same RCD although the latter works it's still against the rules.
2) Swap the strapper to a three core cable.
3) Remove the two way lighting.
Although removing the existing two way lighting may at first seem not an option what you must remember there are now radio controlled light switches. Something like
this
may save the day.
There are of course other borrowed neutral situations I remember one in a bank which took me a long time to find. However I was young then and did not have a clamp on ammeter which is likely the best tool for finding out what is causing it.
Items do share neutrals but the safety rules require that RCD and/or MCB should never supply a line to a circuit using a alternative return path.
However in many houses the demand for more and more lights with the ceiling rose limiting the supply to 5/6 amp has resulted in circuits being split.
However in older houses before anyone really considered EMC it was common to use two wire not three wire strappers between the light switches and "borrow" the line from another switch on the same unit. Today we don't do this mainly as it causes mains hum on radios, TV's and cordless phones.
When both are done we get the borrowed neutral but although dangerous and against the rule book it would work OK. But as the RCD's started to be fitted then these faults started to show themselves.
So to cure.
1) Return to one MCB for all lights. With the new low power lights likely it no longer needs the extra power. But do remember ONE MCB not two MCB's from the same RCD although the latter works it's still against the rules.
2) Swap the strapper to a three core cable.
3) Remove the two way lighting.
Although removing the existing two way lighting may at first seem not an option what you must remember there are now radio controlled light switches. Something like
this
may save the day. There are of course other borrowed neutral situations I remember one in a bank which took me a long time to find. However I was young then and did not have a clamp on ammeter which is likely the best tool for finding out what is causing it.
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Many thanks for the detailed explanation. I have a house with two single phase supplies (off a three phase). On one I have put in a new lighting circuit for upstairs which led me to finding the shared neutral as the downstairs lights gave me a problem on the continuity tests. I have decided to split the circuits and put in a new power line for the downstairs lights.
For the other part of the house I have two light circuits, not upstairs and downstairs but right and left parts of the house. I will have to investigate that this has not been shared as the circuits will be on separate RCDs in a split load CU.
From your note, is it common for the circuits to be connected at the switch? Would you expect the circuits to be connected in the cable run?
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What you have to consider what would be the easy way. Most electricians will go for the easy way so ceiling roses, switch boxes and the like.
I was told should not have two phases within arms reach although never found rule on that and norm is never two phases in the same room. Although common sense must apply size of room etc.
I have found two phases to be a problem and the norm is to use some red, yellow and blue sticky dots (now I suppose black, grey and brown although not so easy to see which at a glance I liked old colours) but with the little dots stuck on it was easy to see if there was a problem with two phases being too close.
We wired a very large house was an old mill and we were not certain if the 100A supply would be enough so used a 200A three phase fused isolator to supply three consumer units but linked all inputs to fuses together so with 100A fuses in the box we could use standard consumer units and we could still use a larger than 100A supply or a three phase or split phase supply. As far as I am aware it had a 100 amp supply which turned out to be ample. But in that case it would be easy to change.
However in a normal build to use two or more phases can be a nightmare. Much depends on the DNO but often they will refuse to supply two phases until good reason is given for doing so.
Yes I have used three phases for lighting. Used lighting track across a factory this before HF units was required. However would not do it in a domestic.
I was told should not have two phases within arms reach although never found rule on that and norm is never two phases in the same room. Although common sense must apply size of room etc.
I have found two phases to be a problem and the norm is to use some red, yellow and blue sticky dots (now I suppose black, grey and brown although not so easy to see which at a glance I liked old colours) but with the little dots stuck on it was easy to see if there was a problem with two phases being too close.
We wired a very large house was an old mill and we were not certain if the 100A supply would be enough so used a 200A three phase fused isolator to supply three consumer units but linked all inputs to fuses together so with 100A fuses in the box we could use standard consumer units and we could still use a larger than 100A supply or a three phase or split phase supply. As far as I am aware it had a 100 amp supply which turned out to be ample. But in that case it would be easy to change.
However in a normal build to use two or more phases can be a nightmare. Much depends on the DNO but often they will refuse to supply two phases until good reason is given for doing so.
Yes I have used three phases for lighting. Used lighting track across a factory this before HF units was required. However would not do it in a domestic.
The "traditional" borrowed neutral associated with up/down lighting splits resulted in the landing light getting its switched live from the downstairs circuit.
It's not just RCDs you need to worry about - if you've got lighting circuits on different phases and you have borrowed/shared neutrals/lines (however you want to view it) you'll have 400V at affected ceiling roses, lamps suddenly in series between two phases if you disconnect a neutral....
Also be aware of having 400V at multi-gang 2-way switches if they are doing lights on different circuits.
What do you mean?
Hi thanks for the reply. I fully appreciate the worry about two phases. Luckily I have a house with two wings. Each wing on a different phase. I am making sure that no room as both phases in it.
My comment about connections at the switch and cable runs was because I was not fully understanding how it works. I think I do now. So what I have is a downstairs and landing light on the same switches. The downstairs switch is fed from one circuit and has a two core strapper up to upstairs switch. The downstairs light is on the same circuit as the switch. Upstairs the line comes from the upstairs switch but the neutral from a separate circuit. Both circuit are on the same CU which I am about to replace. I wish to put the two lighting circuits on different MCBs and even different RCDs.
To correct this properly seems to involve running new cables which will be time consuming and cause distruption. Are there any tricks to solving this? I have seen one suggestion of using wireless switches.
My comment about connections at the switch and cable runs was because I was not fully understanding how it works. I think I do now. So what I have is a downstairs and landing light on the same switches. The downstairs switch is fed from one circuit and has a two core strapper up to upstairs switch. The downstairs light is on the same circuit as the switch. Upstairs the line comes from the upstairs switch but the neutral from a separate circuit. Both circuit are on the same CU which I am about to replace. I wish to put the two lighting circuits on different MCBs and even different RCDs.
To correct this properly seems to involve running new cables which will be time consuming and cause distruption. Are there any tricks to solving this? I have seen one suggestion of using wireless switches.
Wireless switches, surface cabling, doing away with 2-way switching are all alternatives to chasing in new cable and making good.
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