Neutral lighting wire

[John D v2.0]

I can't see how it can be on a different circuit/RCD if, as we have been told (twice), the proposed source of the neutral shares the same MCB.

Neutrals don't really have an MCB as the MCB is single pole, they are just all connected into the neutral bar (for that RCD) together.

As an interesting thought, if your install is wired in singles and you loop neutrals through the switch, and you have a 2 gang switch for upstairs and downstairs on different circuits, you would wire one neutral to each switch in parallel, but as the neutral goes to the first switch, which "circuit" is it on?

Electrically there's no difference until it goes off to the light.

 

[EFLImpudence]

if your install is wired in singles and you loop neutrals through the switch, and you have a 2 gang switch for upstairs and downstairs on different circuits, you would wire one neutral to each switch in parallel, but as the neutral goes to the first switch, which "circuit" is it on?

The neutral for the landing light will be upstairs - the correct position.
That's why some think a more accurate name is a "borrowed line".

If you wire it as you are saying then the landing light will be on the downstairs circuit - with no borrowed neutral or line.

 

[333rocky333]

You would use the same number on the neutral bar as the supply mcb surely

 

[JohnW2]

I'll give you one bit of advice, improve your teaching of electrical knowledge and don't get so uppity when your protoge is criticised for being wrong.

I don't teach 'electrical knowledge', but what you wrote made absolutely no sense to me, and you seem unable or unwilling to explain to me what you meant.

If two circuits are protected by the same RCD, or if neither is protected by an RCD, then the concept of 'borrowed neutral' is simply topological, rather than electrical, since both circuits' neutrals are connected to the same neutral bar in the CU. When two circuits are protected by different RCDs, or only one circuit is protected by an RCD, the only electrical difference between a 'borrowed neutral' and a 'non-borrowed neutral' is that in one case the neutral currents of the two circuits go through separate RCDs to the N incomer, whilst in the other case the relevant part of the neutral current goes through an RCD which is not carrying the corresponding L current. Whilst that will usually cause the RCD to operate, I can see absolutely no way in which it could be "an electrical shock and fire hazard" which has "caused many associated injuries and deaths.".

Kind Regards, John

 

[John D v2.0]

The neutral for the landing light will be upstairs - the correct position.
That's why some think a more accurate name is a "borrowed line".

If you wire it as you are saying then the landing light will be on the downstairs circuit - with no borrowed neutral or line.

It was a hypothetical question, let's remove some unnecessary details.
Imagine you have a single RCD singles install with 2 lighting circuits each with a single separate light on a single 2 gang switch. The wiring is singles and goes cu -> switch -> lights.
At each light you have a line and neutral for the same circuit. However at the switch, if you were to swap the neutrals around, your circuit would be exactly the same electrically.

Have I missed anything?

 

[JohnW2]

Isnt that what a borrowed neutral is though, with NO rcds in the mix, the live source mcb would still do both 1st floor and passage light, but the neutral could still be bunched to the other ground floor circuit, the problem arises when working on that ground floor circuits neutral.

As I've just written, if there are no RCDs, or if two circuits are protected by the same RCD, then the concept of a "borrowed neutral" is meaningless, since both circuits' neutrals are connected to the same neutral bar in the CU.

Kind Regards, John

 

[EFLImpudence]

if there are no RCDs, or if two circuits are protected by the same RCD, then the concept of a "borrowed neutral" is meaningless, since both circuits' neutrals are connected to the same neutral bar in the CU.

No, it isn't (meaningless).

It is dependent on which MCB supplies the different lights.

 

[JohnW2]

Sorry, just for clarity I was meaning these "they" being the same thing, not any reference in the post I made!

Apologies. I must be getting old, since I still don't really get what this "they" was meant to refer to, given that the neutrals of two circuits on different RCDs will, of course, go to different places (and hence are not 'the same neutral'). To remind you, you wrote:

Do you have RCDs in your consumer unit? Are the lighting circuits on different RCDs? If yes to both, then they are the same neutral.

Kind Regards, John

 

[JohnW2]

No, it isn't (meaningless). It is dependent on which MCB supplies the different lights.

As I wrote in the previous post, it has 'topological' meaning (the neutrals travel in different cables) but is electrically meaningless, since they are joined at a neutral bar in the CU.

Kind Regards, John

 

[EFLImpudence]

Imagine you have a single RCD singles install with 2 lighting circuits each with a single separate light on a single 2 gang switch. The wiring is singles and goes cu -> switch -> lights.
At each light you have a line and neutral for the same circuit. However at the switch, if you were to swap the neutrals around, your circuit would be exactly the same electrically.

Yes, but if on the same MCB, you wouldn't need two neutrals or two lines.
They would both be on the same circuit.
Singles or twin and earth makes no difference.

If they come from sparate MCBs, then you would have two lines so should have two neutrals and should not swap them.

Have I missed anything?

Yes.

The reason for a borrowed neutral (or borrowed line) is because there are only two strappers - hall switch to landing switch.
If you have another wire available to send the neutral to the landing light, then you have enough wires for three strappers and should wire it properly with the line and neutral at the landing switch, and just the three line strappers to the downstairs switch.

 

[EFLImpudence]

As I wrote in the previous post, it has 'topological' meaning (the neutrals travel in different cables) but is electrically meaningless, since they are joined at a neutral bar in the CU.

That doesn't matter.

Borrowed neutrals arise with regard to line on separate MCBs - actually borrowed lines, not whether on the same RCCB or not.

 

[333rocky333]

As I've just written, if there are no RCDs, or if two circuits are protected by the same RCD, then the concept of a "borrowed neutral" is meaningless, since both circuits' neutrals are connected to the same neutral bar in the CU.

Kind Regards, John

Im getting confused again, the reason its frowned on is when your working on the single pole assumed isolated "other circuit" and get a shock when parting the neutrals due to the return current in neutral from the other circuit

 

[333rocky333]

Apologies. I must be getting old, since I still don't really get what this "they" was meant to refer to, given that the neutrals of two circuits on different RCDs will, of course, go to different places (and hence are not 'the same neutral'). To remind you, you wrote:


Kind Regards, John

Sometimes johnw no disrespect i think you read things to literally and correct if thats the right words.

By "They" I took it to mean the N to the first floor lights AND the N to the passage light, the OP was trying to get confirmation THEY were on the SAME circuit and not a borrowed neutral, which is not evident just because the same mcb controls them, the passage light could still go off with the same 1st floor mcb, with the Passage N connected to either the 1st floor lights or the Ground floor lights,

As you say above, 2 circuits on different Rcds are NOT the same Neutral , so if the OPS Ground and 1st floor lights are NOT the same N, that surely confirms the Passage and 1st floor lights that are on the same Mcb are as Johnd2 says are indeed the SAME Neutral.

The op was concerned that he did not want to connect his new gizmo to the 1st floor switch and the N to the passage light without confirming the passage N was NOT connected to the Ground floor lights

 

[JohnW2]

Neutrals don't really have an MCB as the MCB is single pole, they are just all connected into the neutral bar (for that RCD) together.

I know, which is why I wrote:

I can't see how it can be on a different circuit/RCD if, as we have been told (twice), the proposed source [i.e. the proposed source circuit] of the neutral shares the same MCB.

... my point obviously being that if (the L of) the light/accessory from which it was proposed to obtain a neutral has the same MCB as the light for ehich the neutral is required, then the two can't possibly be on different MCBs - an MCB cannot get it's supply from more than one RCD (unless, I suppose, some very wrong wiring has been done in the CU ).

As an interesting thought, if your install is wired in singles and you loop neutrals through the switch, and you have a 2 gang switch for upstairs and downstairs on different circuits, you would wire one neutral to each switch in parallel, but as the neutral goes to the first switch, which "circuit" is it on? ....

I don't really understand. In particular, I don't understand what you mean by "you would wire one neutral to each switch in parallel".

Kind Regards, John

 

[JohnW2]

Im getting confused again, the reason its frowned on is when your working on the single pole assumed isolated "other circuit" and get a shock when parting the neutrals due to the return current in neutral from the other circuit

Indeed. However, I was talking about an intact circuit - in which case a 'shared neutral' between two circuits on the same RCD, or two circuits without RCD protection will be electrically identical to the situation without a 'shared neutral' (since the two neutrals are joined at the CU.

However, IF you break the cross-connection between neutrals (in which case one of the loads no longer has a connection to neutral at all) then the situation will be as you describe.

Kind Regards, John