Split neutral

[r.bartlett]

Chaps I was watching a sparks last week and he was removing an old bit of kit on 3 p&n 20a supply on 6mm cable 15m run but really difficult access and fitting 3 x singles 1 x 10a 2 x 16a.

H said he did not want to rewire from the board as it was a rush job to get the new stuff up and running so fitted 3 x new breakers and kept just the 1 neutral

I didn't comment as it wasn't my business but is this correct and legal?

commercial property.

just a curious bystander

 

[DaftPunk]

What is it now supplying?

 

[r.bartlett]

appears it's called a shared neutral. Been looking around and there is much debate with conflicting views....

 

[EFLImpudence]

What is it now supplying?

 

[maltaron]

It's normally OK as long as the socket outlets are separated by at least spread arms, as there will be 440v across the lives on separate phases. The neutral will never carry more current than the highest current on one phase.

 

[r.bartlett]

It's normally OK as long as the socket outlets are separated by at least spread arms, as there will be 440v across the lives on separate phases. The neutral will never carry more current than the highest current on one phase.

thanks by socket outlet do you mean isolators as this is an external installation of 3 pump sets.?

 

[r.bartlett]

What is it now supplying?

sorry missed both these requests.. 2 x 1.5kw+ 1 x 1.0kw pumps (approx I think etc)

 

[JohnW2]

It's normally OK as long as the socket outlets are separated by at least spread arms, as there will be 440v across the lives on separate phases.

It always makes me laugh a bit when I see people citing that traditional wisdom (I'm not sure whether it actually exists in regs), since it seems to imply that having one's outstretched arms touching two things which 'only' have a 230V pd between them would, in some sense, be 'OK'! However, you're talking here about the acceptability of having single-phase sockets run off a 3-phase supply (and the OP might well not be talking about sockets, anyway), not the acceptability of having a 'common neutral', which is what concerned the OP.

The neutral will never carry more current than the highest current on one phase.

Indeed so. For those who can't be bothered with the maths, they really only need to look at the supply cable to the installation. Even if all three phases are going to supply just single-phase loads, you'll never see the neutral conductor (which is 'common' to all single-phase loads) being bigger than the line ones - because, as you say, it could never carry more current than that in the greatest-loaded line one.

Kind Regards, John

 

[JohnW2]

...they really only need to look at the supply cable to the installation. Even if all three phases are going to supply just single-phase loads, you'll never see the neutral conductor (which is 'common' to all single-phase loads) being bigger than the line ones - because, as you say, it could never carry more current than that in the greatest-loaded line one.

Having said that, at least one meter operative does/did not seem to understand this concept. I have a 3P+N supply (which only serves single-phase loads) but whoever installed the meter used 16mm² (or imperial equivalent) tails for the three line conductors but a 25mm² (or equivalent) tail for the neutral one!

Kind Regards, John

 

[maltaron]

Thanks John, for clarifying that point, perhaps not worded well. Certainly 240v PD can kill, but 440v is much more likely not to give a second chance.

 

[JohnW2]

Thanks John, for clarifying that point, perhaps not worded well. Certainly 240v PD can kill, but 440v is much more likely not to give a second chance.

It's not just you - very many people (and even possibly the regs) say it. I have to say that I'm far from convinced that, in practice, the 440V pd results in any significant increase in risk. Apart from anything else, it would require an incredibly rare/unlikely set of circumstances to result in one coming into contact with the line conductors of two simultaneously touchable sockets!

It's a bit like those labels saying "danger 400V inside" or something like that. What precautions do 'they' envisage that one would take in the presence of 400V that one would not also exercise in the presence of 230V?!

Kind Regards, John

 

[RF Lighting]

It's normally OK as long as the socket outlets are separated by at least spread arms, as there will be 440v across the lives on separate phases.

It was added to the wiring regulations in 1966 and removed in 1981.
Phase separation has in fact never been a requirement of BS7671.

The 13th Edition of the IEE Wiring Regulations made no mention of it at all.

The 14th Edition of the IEE Wiring Regulations (1966) added the following text:

“A.20 All socket-outlets in any one room shall be connected to the same phase (or pole of a 3-wire system).
Exemption.- In non-domestic premises, if it is clearly impractical to comply with Regulation A.20, more than one phase (or pole) of the supply may be utilized provided that all socket-outlets on one phase (or pole) are grouped together and are not intermingled with socket-outlets connected to a different phase (or pole); and provided that in no circumstances may a socket-outlet be installed at a distance less than 6 feet from any socket-outlet connected to a different phase (or pole).”

In 1970, “6 ft.” was revised to “2m.”

In 1981, the 15th Edition of the IEE Wiring Regulations deleted the above section.

In 1992 the (as-then-revised) 15th Edition became BS7671.
This section remained deleted, and has remained absent ever since.

 

[JohnW2]

It was added to the wiring regulations in 1966 and removed in 1981. Phase separation has in fact never been a requirement of BS7671.

Thanks for clarifying - as I said, I wasn't sure whether it had ever been in the regs, even though it's not all that usual to hear people citing it.

As I've said, I wouldn't really see a lot of point in it, primarily because it is so unlikely that people would come simultaneously come into contact with the line conductors of two nearby sockets (whether they were wired to the same phase of different phases).

Of course, should that (seemingly 'exceedingly improbable') situation arise, it could represent a significant hazard, not so much because there was a pd of 400V rather than 230V, but because (just as with L-N shocks) the victim would not enjoy protection from any RCD(s) on the circuit(s).

Kind Regards, John

 

[EFLImpudence]

In 1970, “6 ft.” was revised to “2m.”

Why was it thought necessary to increase the distance by 6.74 inches?

 

[JohnW2]

In 1970, “6 ft.” was revised to “2m.”

Why was it thought necessary to increase the distance by 6.74 inches?

Maybe because they liked round numbers and felt that an increase of 6.74" was preferable to a decrease of 32.63"? ... Or maybe, due to improved diets, human armspan had increased by that amount!

Kind Regards, John