Shared Neutral (not borrowed) 3 phase

[Tony S]

As we don’t know the loading it would be pure conjecture to assume there will be a problem with harmonics.

My memory isn’t so bad that I can’t remember our little tiff over harmonics in a theatre’s distribution system.

Another time, another place.

 

[steptoe]

strange one,

yep, imo its one circuit, therefore 1 N is OK,

but, that N needs to be large enough to carry the full current of all points,

ie: you couldnt have 3 x SP 16A commandos on a 2.5 circuit protected by a 16A OCPD, the N could be expected to carry 48A,  not good, :shakehead

 

[Marvo]

Certainly unorthodox and poor design but I'd see it technically as a single circuit so as long as the neutral is rated to carry the maximum theoretical unballanced load current then it's electrically acceptable unless there's a regulation specifically forbidding it.

 

[Phoenix]

Its an interesting one, I'd agree with others that the 3pole MCB makes it one three phase circuit, would like to see it clealy indicated as such on the schedules though so no one is under the illusion that it is anything else and changes the breakers out to 3xSP

I did think to myself, what if it was a HRC fuseboard rather than an MCB board, where a 3phase circuit is protected no differently than 3x single phase circuits, could you have a legitimate circuit as the OP describes from such a board?  -I'd say yes, but only if you have a 3phase isolator in there somewhere before the circuit branches to single phase, I think BS7671 does actually state this, but only for 3phase lighting circuits, I'd guess that similar circuits supplying loads other than lighting would meant to be caught by this, but they weren't due to not being considered as a possibility by the authors

 

[Evans Electric]

It is an interesting question ...one which I've never considered before , however   lets say you are asked to install  three separate commando sockets  ...each on a different phase      surely  you would  feed them from  3  X  SP  MCBs  , each socket having it's own neutral .        You wouldn't consider using a TP  MCB...............or would you? 

 

[Risteard]

I would argue it is a single circuit with multiple outlets and therefore I can’t see a problem.

I would agree and don't believe that any other interpretation would be correct.

 

[phil d]

Personally I would take it as one circuit, therefore one neutral, however I must say that I think it's a very poor design.why use a 3p breaker to control 3 individual sockets? it's as bad as using one rcd to protect a whole board, if one socket takes a fault then it takes all of them out, very silly imo. Was it originally like this, or was it designed to supply something requiring TPN and it got reused, I wonder?

I remember one job many years ago in a training centre for electricians of all places, there was a single metalclad socket mounted directly below a db, someone had decided they needed another, so, rather than replace the single with a double, they attached another single below the first one using 2 bushes and a coupler, they extended the neutral and earth cables from the first socket, however for some inexplicable reason they thought it was a good idea to feed second socket off it's own fuse.

In the backbox for the first socket you had a red, a yellow, a black and a green/yellow, the red,black and g/y fed the first socket, Red was fed from a fuse on red phase in the db, the yellow fed the second socket, and was fed from a fuse on the yellow phase in the db! So you had 415v in the back of a backbox for a 13a socket outlet, not even a warning label,needless to say  I removed it all and replaced the lot with one twin socket fed from one phase.

 

[Richard Tawn]

Wow thank you for the replies on this, Im glad it pulled up a healthy debate as I have yet to find info on this scenario in a UK context, seems more common place in the states,

So to recap it seems we are looking at legitimate according to regs but simply not best practice,

this result based on the circuit being seen as one from the Dis board side,

downside being that if one rcd trips then all 3 trip,

for perspective these 3 sockets supply 3 identical machines although one would assume eventually these will change.

Many thanks !

 

[Lurch]

strange one,

yep, imo its one circuit, therefore 1 N is OK,

but, that N needs to be large enough to carry the full current of all points,

ie: you couldnt have 3 x SP 16A commandos on a 2.5 circuit protected by a 16A OCPD, the N could be expected to carry 48A,  not good, :shakehead

Erm, no. If they were all on a single phase then yes, but you will not be pulling 48A if the 3 sockets are on 3 phases.

lets say you are asked to install  three separate commando sockets  ...each on a different phase      surely  you would  feed them from  3  X  SP  MCBs  , each socket having it's own neutral .        You wouldn't consider using a TP  MCB...............or would you? 

That was my take on it, 3x single phase socket outlets. I have never, and am fairly certain I never will, seen anyone use a TP MCB for this. It's not even an edge case AFAIAC, I don't think you would find a reference to doing this anywhere.

 

[steptoe]

Erm, no. If they were all on a single phase then yes, but you will not be pulling 48A if the 3 sockets are on 3 phases.

So what would the load be then,?

16A per socket,  x3 

That's 48A to me that the N needs to be capable of carrying. 

 

[Rob.]

So what would the load be then,?

16A per socket,  x3 

That's 48A to me that the N needs to be capable of carrying. 

Phase angles....

 

[steptoe]

Phase angles....

Ye,

But if single phase unbalanced,

Surely by design it should be capable of carrying each loading separately.?

Its not something I've ever come across, and maybe (obviously) don't totally understand how this works using a 3phase circuit for S/phase loads.

Or is it just S/phase x 1.68,?

 

[Richard Tawn]

Erm, no. If they were all on a single phase then yes, but you will not be pulling 48A if the 3 sockets are on 3 phases.

That was my take on it, 3x single phase socket outlets. I have never, and am fairly certain I never will, seen anyone use a TP MCB for this. It's not even an edge case AFAIAC, I don't think you would find a reference to doing this anywhere.

3 single rcbo's are integral to the 3 socket outlets being fed by the 3p breaker

its the equivalent of a single dis board on paper but separated in reality by steel conduit, the steel conduit I believe is important as it unifies the install as single circuit

 

[Lurch]

Ye,

But if single phase unbalanced,

Surely by design it should be capable of carrying each loading separately.?

Its not something I've ever come across, and maybe (obviously) don't totally understand how this works using a 3phase circuit for S/phase loads.

Or is it just S/phase x 1.68,?

Balanced or unbalanced it makes no odds.

Think of it this way, you have a 50A submain to a DB, and that submain is full of single phase circuits. Do you you fit a neutral on the submain that is 3x the phase conductor?

Balanced or unbalanced just means it either needs or doesn't need a neutral. If it is unbalanced it does, but the phase angles means that each phase is only live when the other 2 aren't (over simplified), so the neutral is only used for any one phase at a given time.

 

[Rob.]

3 single rcbo's are integral to the 3 socket outlets being fed by the 3p breaker

its the equivalent of a single dis board on paper but separated in reality by steel conduit, the steel conduit I believe is important as it unifies the install as single circuit

Nothing to do with the containment. Multiple circuits can be run through the same conduit/trunking etc...

Ye,

But if single phase unbalanced,

Surely by design it should be capable of carrying each loading separately.?

Its not something I've ever come across, and maybe (obviously) don't totally understand how this works using a 3phase circuit for S/phase loads.

Or is it just S/phase x 1.68,?

If the circuit is unbalance there will be current flowing in the neutral conductor. The currents add up using vector maths so they start cancelling each other out.

It's not something I would ever design. Nothing wrong with it though. 

 

[Lurch]

3 single rcbo's are integral to the 3 socket outlets being fed by the 3p breaker

its the equivalent of a single dis board on paper but separated in reality by steel conduit, the steel conduit I believe is important as it unifies the install as single circuit

This is where the confusion comes. On paper (from a theoretical PoV) it is fine, it works exactly the same as a single phase DB, which is why I said if you fit a DB in the middle it is all fine.

From a single phase circuit PoV (which is what it is when it is a single phase socket outlet) it is wrong. You cannot have a single socket outlet tapped onto a 3 phase supply, so this is wrong.

Electrically, no probs. Officially it is a no-no. If yoou are classing it as a 3 phase circuit then it shouldn't be feeding single phase sockets.

 

[binky]

so that's how that works, I've seen similar things with shared neutrals and wondered how the neutral hasn't melted.

Is there a way to work out the neutral load under such circumstances?

 

[Lurch]

Is there a way to work out the neutral load under such circumstances?

Open up your cable calc software!

 

[binky]

Electrically, no probs. Officially it is a no-no. If yoou are classing it as a 3 phase circuit then it shouldn't be feeding single phase sockets.

seen quite a few single phase sockets on 3 phase machinery, but then I suppose those are fed from the machine control cabinet most of the time

Open up your cable calc software!

 Pen and pencil and regs book! I don't normally get involved in such things, so have neevr bought software.

 

[Lurch]

seen quite a few single phase sockets on 3 phase machinery, but then I suppose those are fed from the machine control cabinet most of the time

Yeah, again I mentioned that originally. Electrically there is nothing wrong with it, it's just the technicalities of circuit arrangements.