Sharing Neutrals

[vernd64]

Please help out this poor uneducated American Electrician, in the states we can run whats called a multi-wire branch circuit, where we can run two or three lives and share one neutral. Im told in the UK its not done, I've looked in 7671 for this regulation but cannot seem to find it.

 

[supasparxs]

Off the top of my head, I'm not sure which EEG concerns shared neutrals.
However if you are running a 1.5 lighting circuit in an office the amount of lights on the shears circuits could overload the cables,
I have seen this happen on a bank in the city of London.

If each circuit has it's own live neutral and earth the the design load will be know. If you shared your neutrals then you may overload cables. The problem maynot arise when you carryout the work, it may arise in 5 years time when another electrician upgrades the installation,

Good luck with the course

 

[malcolmsanford]

In a lot of domestic installations in the past a way of doing two way lighting was to "borrow" the neutral from another circuit. This was prevelant with stair lighting. The upstairs light would be controlled with 2 switches one on the top the other on the bottom. The feed to the up light would invariably be taken form the downstair lighting circuit and via the two switch contol the upstair light. The neutral for the light would then be taken from the nearest place which is the nearest upstair light.

With old protection devices ie fuses or MCB this never created a protection problem, but with the advent of RCDs you can have a problem with this. As most new distribution boards are a "split" type ie with 2 main RCDs protecting the mcbs most sparks try to divide the installation up ie upstairs light on 1 rcd downstairs light on another. So if the RCD trips you won't have all the lights off just either upstair or downstairs. With a borrowed neutral if you did this you would trip the RCD as you got an inbalance on the circuit.

Apart from that there is also the joy as most sparks found over time of that borrowed neutral giving you a shock. You isolate the the lighting circuit downstairs to work on it and if there is a borrowed neutral and the lights are on upstairs you get a nice little shock working on the downstairs neutral.

We do like the States run a 3 phase supply with a Neutral. I would class a shared neutral as against Reg 314.1

 

[youngpadawan]

First of all, welcome back to the motherland!

Now I would guess that this is mainly down to our mysterious 3rd wire we like to call the earth, or 'CPC'. With the addition of this wire, we can use a device on the consumer unit called an RCD to measure the current on the live and neutral wires, with the device cutting off the electric the moment that the current becomes inballanced. The idea behind this, is that if you get a fault where something becomes live that shouldn't be, the current will travel down the CPC and the system will cut off before you learn to fly.

Which brings us onto mixed neutrals. It is common place to find boards which offer a split between RCD use and not (or even multiple RCD's, or both). This is so that even in an earth fault, some equipment will still work, important stuff like fire alarms and kettles (if i'm going to die I want a brew!). Now imagine that the neutral for your sockets is mixed up in your fire alarm neutral, where the fire alarm is not RCD protected but your sockets are. Some of that current is going to flow down the fire alarms neutral, so as soon as you plug something in, the RCD measures different amounts of current and off goes the power.

And that's at least one reason why it's generally a bad idea to mix neutrals.

It's also common place to have the live wire from the light at the bottom of the stairs taken up to the top of the stairs in this country, meaning that the downstairs circuit is connected to the neutral of the upstairs. Which works fine, till you throw an RCD into the equation

Also, what he said ^^

 

[vernd64]

I phoned a freind, its 314.4 and 521.8

 

[vernd64]

Thanks all

 

[Darkwood]

In a lot of domestic installations in the past a way of doing two way lighting was to "borrow" the neutral from another circuit. This was prevelant with stair lighting. The upstairs light would be controlled with 2 switches one on the top the other on the bottom. The feed to the up light would invariably be taken form the downstair lighting circuit and via the two switch contol the upstair light. The neutral for the light would then be taken from the nearest place which is the nearest upstair light.

With old protection devices ie fuses or MCB this never created a protection problem, but with the advent of RCDs you can have a problem with this. As most new distribution boards are a "split" type ie with 2 main RCDs protecting the mcbs most sparks try to divide the installation up ie upstairs light on 1 rcd downstairs light on another. So if the RCD trips you won't have all the lights off just either upstair or downstairs. With a borrowed neutral if you did this you would trip the RCD as you got an inbalance on the circuit.

Apart from that there is also the joy as most sparks found over time of that borrowed neutral giving you a shock. You isolate the the lighting circuit downstairs to work on it and if there is a borrowed neutral and the lights are on upstairs you get a nice little shock working on the downstairs neutral.

We do like the States run a 3 phase supply with a Neutral. I would class a shared neutral as against Reg 314.1

A shared/borrowed neutral i would class as a neutral that spans more than one isolatable circuit, where you have a 3phase supply and 1 common neutral with it, it in itself is the circuit and if isolated all three phases are lost, if fused correctly then loading 1ph up only will give a max neutral current equal to the phase load but loading up one or two off the remaining phases will have a balancing effect and reduce the neutral current until it reaches zero when all three phases match so if all is fused and sized correctly then there is no risk of overloading neutral.
At no point does a 3phase and N circuit if installed correctly contravene reg 314.1 regardless of 1 neutral been common to all 3phases.

Sorry Malc re-read your last line and misunderstood your point, ignore the above as a reply to you but il leave it as info anyway.

 

[malcolmsanford]

No see why it would Darkwood, I wrote it and had to re read it again lol. Laying out text and punctuation was never my strong point.

 

[Ponty Massive]

Out of curiosity, how do you guy's find the easiest way of getting rid of the shared neutral problem in the upstairs / downstairs lighting?

 

[Darkwood]

no easy way, got to run a neutral from the circuit it should be on.

 

[Ponty Massive]

...or put both circuits on the same RCD

 

[mattyevz]

i put them both on the same rcd, when the customer doesnt want to pay the extras. This is O.K but not ideal correct?

 

[Darkwood]

i put them both on the same rcd, when the customer doesnt want to pay the extras. This is O.K but not ideal correct?

This kind of problem should always be rectified as their is a high risk of shock injury or even death with the nature of this fault, both circuits will have to be put on same rcd anyway or your rcd wouldnt set havin neutral not been balanced with its live, when the customer cant afford then best to couple both lighting circuits onto one mcb, lighting loads are now been reduced with energy efficient lamps and doing this will leave the job safe at no extra cost, you are responsible for the safety of the part of installation if you get involved with it, and the whole installation if changing the dist board.
If loading dosnt permit the coupling up of the 2 circuits then as a last resort have it noted on the test cert' forward a letter to the customer explaining the risk and mark the board up with appropriate warning about the borrowed neutral, this way you have done everything you can to avoid others falling foul of this dangerous situation.

 

[ukpablo]

and how would u go about finding the borrowed neutral...................ie where would u start?

 

[telectrix]

on the landing 2 way light scenario, it's usual to find that the neutral is from the upstairs lighting circuit and the live from the downstairs, so i would call this a borrowed live, not a borrowed neutral!!!!! i have found that the best way ( not always practical ) is to remove the live feed from downstairs , switch wire from landing light to upstairs 2 way switch, 3 core to donstairs 2 way switch. phone plasterer