Neutral lighting wire

[EFLImpudence]

 

[John D v2.0]

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

Yeah I realise what I actually wrote was a bit unclear/ambiguous, but that's why in the clarification I quoted the two words "they" in bold to illustrate that its the same they I was referring to. If you go to that post and look at the two quotes with the bold word it should make more sense.

 

[John D v2.0]

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

In general yes, but it's the balance issue again, the MCB should be protecting both the line and neutral for overload. A lighting circuit with an extra light isn't going to be a problem but imagine the extreme case where the shower circuit is borrowing the lighting circuit neutral. The 1mm neutral isn't going to last long.

 

[John D v2.0]

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

We're on the same page now, this is exactly what I'm getting at.
Would you mind explaining exactly why you shouldn't swap them?
Remember it's 2 MCBs with no separate RCD protection and the neutrals only run to the switch and on to their respective light fittings.

 

[JohnW2]

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, ... 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

Right, I think I've got it now, and I think I understand why I have been talking at cross-purposes with some of you ...

I thought that the OP was concerned that he might create a 'shared neutral' by doing what he proposes (and was assuming that there were no pre-existing 'shared neutrals') but it seems that he was concerned that there might have been a pre-existing 'shared neutral'. Does that sound right?

Kind Regards, John

 

[ban-all-sheds]

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.

What he is unwilling to do is to admit that he made a mistake - he misinterpreted what John D said, and now he will not back down.

I did the same, but I realised my error straight away, hence Post #3.

 

[JohnW2]

In general yes, but it's the balance issue again, the MCB should be protecting both the line and neutral for overload. A lighting circuit with an extra light isn't going to be a problem but imagine the extreme case where the shower circuit is borrowing the lighting circuit neutral. The 1mm neutral isn't going to last long.

Sure - but, needless to say, I was thinking and writing about the only context in which this situation - i.e. two lighting circuits (both with over-engineered cable sizes!).

Only a complete idiot would even dream of using a lighting circuit's neutral as the return path for a shower!

Kind Regards, John

 

[JohnW2]

Yeah I realise what I actually wrote was a bit unclear/ambiguous, but that's why in the clarification I quoted the two words "they" in bold to illustrate that its the same they I was referring to. If you go to that post and look at the two quotes with the bold word it should make more sense.

Yes, I've got it now. Thanks for your patience.

Kind Regards, John

 

[EFLImpudence]

We're on the same page now, this is exactly what I'm getting at.
Would you mind explaining exactly why you shouldn't swap them?
Remember it's 2 MCBs with no separate RCD protection and the neutrals only run to the switch and on to their respective light fittings.

Well, just very bad practice, I suppose.

If not noticed it could lead to confusion and some danger if the wrong neutral is removed from the CU.

 

[JohnW2]

.... Would you mind explaining exactly why you shouldn't swap them? ...
Remember it's 2 MCBs with no separate RCD protection and the neutrals only run to the switch and on to their respective light fittings.

IF those neutrals both came straight from the CU and went nowhere other than to their one respective light, then I don't think there would be any problems swapping them (the two feeds from the CU would be effectively interchangeable). However, if there are other loads connected to either of the neutrals, then EFLI's diagram could come into play (IF one cut some part of the neutral path).

Furthermore, as I have said before, there is, as far as I can see, no problem with intact circuits - since 'swapping' two conductors which come from the same bar in the CU means nothing. The potential problem/danger arises if there are multiple loads and one breaks the neutral path at some point.

Kind Regards, John

 

[John D v2.0]

Thanks all, so to summarise, as we all know, the line for a circuit is defined by all things connected to the load side of the given MCB.

The neutral for a circuit (and I suppose the CPC) is defined by continuing the line core through all the connected devices/accessories and back to the neutral bar.

If the set of line, neutral and CPC cables doesn't match directly, then there's some borrowing going on (although in which direction and which conductors) the borrowing is, might be a matter of philosophical debate)

There's also the good practice of making sure it's connected to the expected place on the neutral/earth bar, if marked.

 

[EFLImpudence]

IF those neutrals both came straight from the CU and went nowhere other than to their one respective light, then I don't think there would be any problems swapping them (the two feeds from the CU would be effectively interchangeable).

That may be electrically true but from a safety point of view, very careless and potentially dangerous when working in the CU.
Why would you (not one) do it other than by accident?

However, if there are other loads connected to either of the neutrals, then EFLI's diagram could come into play (IF one cut some part of the neutral path).

Then, why do it?

Furthermore, as I have said before, there is, as far as I can see, no problem with intact circuits - since 'swapping' two conductors which come from the same bar in the CU means nothing. The potential problem/danger arises if there are multiple loads and one breaks the neutral path at some point.

So don't do it.

It's a silly argument.

 

[333rocky333]

I once found a circuit with live from one board and a neutral from another board, they were both sub mains and some one working on one of the switchfuses wasnt happy when he got a shock of the exposed neutral bar

 

[JohnW2]

That may be electrically true but from a safety point of view, very careless and potentially dangerous when working in the CU.

JohnD introduced this hypothetical offshoot of the discussion by talking about singles. In that context (and in the 'single loads' scenario I mentioned), about the only meaning that 'swapping' has is in relation to which terminals of the (same) neutral neutral bar they are terminated into - is that what you mean>

Why would you (not one) do it other than by accident?

I wouldn't. Even if I did (by accident), it's pretty likely that a RCD or two would bring the 'accident' to my attention.

So don't do it. It's a silly argument.

What is a silly argument? - JohnD's hypothetical question? I hope you don't think that either he or I are advocating deliberately installing 'borrowed neutrals', or suggesting that anyone should deliberately install them today, do you?

Kind Regards, John

 

[mwatsonxx]

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.

Just to reaffirm, this advice is nonsense. If there was one RCD then the advice would be correct. But he said RCD's ie more than one, that means the neutrals associated with either RCD have to be separate.

sometimes the landing light if it has 2 way switching is on the downstairs live and upstairs neutral, but that wouldn't cause you an issue.

That is a descrption of a borrowed neutral. Borrowed neutrals are an electrical shock and fire hazard and have caused many associated injuries and deaths. And anyone that can't understand that dangererous situation, should not be carrying out electrical work.