Split neutral

[EFLImpudence]

Yes, I remember Blue Peter's recipes after metrication:

"Take 57 grammes of butter ..."

 

[endecotp]

(just as with L-N shocks) the victim would not enjoy protection from any RCD(s) on the circuit(s).

If i'm standing with my feet on the ground and each hand on a live wire on different phases, I'd hope that enough current would go through my feet to trip an RCD....

If not, yes a 3-phase RCD would not trip - but if there were separate single-phase RCDs on each circuit then I would expect a trip. How would an RCD-protected 3-phase (e.g. office building, school etc.) normally be wired in this respect?

 

[EFLImpudence]

Surely touching two 230V line conductors on the same phase is the same as touching one.

However, touching two 230V line conductors on different phases is going to send 400V through you.



Any current to earth (>30mA nominal) will trip the RCD.

 

[JohnW2]

(just as with L-N shocks) the victim would not enjoy protection from any RCD(s) on the circuit(s).

If i'm standing with my feet on the ground and each hand on a live wire on different phases, I'd hope that enough current would go through my feet to trip an RCD....

I think many (maybe even most) people probably seriously underestimate the insulating properties of most floors and most shoes. I'm certainly not going to do the experiment, but I very much doubt that any significant current (certainly nothing like 30mA) would flow to earth if I did as you describe whilst standing on a normal (dry) floor whilst wearing normal (dry) shoes. However, the 400V between my two hands would definitely result in a very significant current through that path!

If not, yes a 3-phase RCD would not trip - but if there were separate single-phase RCDs on each circuit then I would expect a trip.

Well, yes, if they were separate single-phase circuits, with separate neutrals and separate single-phase RCDs then, yes, one would expect at least one, quite probably both, of the RCDs to operate. However, that is obviously not do-able if, per this thread, there was only a 'common neutral'.

How would an RCD-protected 3-phase (e.g. office building, school etc.) normally be wired in this respect?

In such buildings, all (or virtually all) of the final circuits would be single phase, presumably with separate neutrals for each phase and hence, as above, 'standard' single-pole RCD protection, wouldn't it? Although it's only a house, that's how my installation certainly works.

Kind Regards, John

 

[JohnW2]

Surely touching two 230V line conductors on the same phase is the same as touching one. .... However, touching two 230V line conductors on different phases is going to send 400V through you.

Indeed and Indeed.

Any current to earth (>30mA nominal) will trip the RCD.

... and a third Indeed. However, as I've just written, the question is whether there would be anything like 30mA flowing to earth through shoes and floor etc. if one were touching (either one or two) line conductors. I personally doubt it (in most situations!).

Kind Regards, John

 

[JohnW2]

However, as I've just written, the question is whether there would be anything like 30mA flowing to earth through shoes and floor etc. if one were touching (either one or two) line conductors. I personally doubt it (in most situations!).

I've just attempted to do the experiment (well, a safe version thereof!) by attempting to measure the IR of the sole of the (leather-, not rubber-soled) shoes I've been wearing all day (so probably not totally dry ). I clamped sheets of metal inside and out, and got an IR of about 16 MΩ at 1000V between them.

Kind Regards, John

 

[EFLImpudence]

Surely touching two 230V line conductors on the same phase is the same as touching one. .... However, touching two 230V line conductors on different phases is going to send 400V through you.

Indeed and Indeed.

Any current to earth (>30mA nominal) will trip the RCD.

... and a third Indeed. However, as I've just written, the question is whether there would be anything like 30mA flowing to earth through shoes and floor etc. if one were touching (either one or two) line conductors. I personally doubt it (in most situations!).

My points were in response to yours thinking there was little difference between touching single phase twice and two separate phases.

 

[JohnW2]

My points were in response to yours thinking there was little difference between touching single phase twice and two separate phases.

I'm not sure I understand. If one removes the 'complication' of there also being a path from one's body to earth, then there is obviously all the difference in the world - in one case the pd between ones two hands (or whatever points of contact) is zero, whereas in the other case it is 400V! Did you think I was saying something different from that? If not, I think I must be misunderstanding you, or something!

... and I still think it almost 'vanishingly improbable' that one would simultaneously come in contact with line conductors of two different single-phase final circuits (except, perhaps in a DB). 3-phase circuits are obviously a very different kettle of fish, since all three phases will then be present within the same enclosures and accessories.

Kind Regards, John

 

[endecotp]

I wonder if the 6 ft / 2 m ex-rule was more to do with equipment being connected to two phases. For example, audio or computer equipment that is then interconnected with e.g. data cables. This ought to work, but I can imagine possible noise issues, and perhaps there could be subtle safety implications.

(Standards need rationale!)

 

[JohnW2]

I wonder if the 6 ft / 2 m ex-rule was more to do with equipment being connected to two phases. For example, audio or computer equipment that is then interconnected with e.g. data cables. This ought to work, but I can imagine possible noise issues, and perhaps there could be subtle safety implications. (Standards need rationale!)

I suppose that could have been the thinking - but probably a bit flawed, given that a fair bit of such equipment comes (at least these days) with 1.5m (~4.5' !) or longer power cables!

However, I suspect that it was primarily just the (probably fairly "ill-", at least in probabilistic terms) perceived issue of 'touching Ls of two different phases' that brought it about.

Kind Regards, John

 

[davelx]

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

Unless you have dimmers in the circuit. ...take a look at triplen harmonics. Can be a big issue in theatre lighting installations! Oversize neutrals are needed to accommodate the additional neutral current.

 

[JohnW2]

Unless you have dimmers in the circuit. ...take a look at triplen harmonics. Can be a big issue in theatre lighting installations! Oversize neutrals are needed to accommodate the additional neutral current.

Well, yes, I suppose that's true - but that's a highly specialised and unusual situation. I would suspect that theatre lighting is one of the very few situations in which there are dimmers big enough for that to be a significant issue, isn't it? What do the DNOs have to say about these neutral currents?

Kind Regards, John

 

[EFLImpudence]

My points were in response to yours thinking there was little difference between touching single phase twice and two separate phases.

I'm not sure I understand. If one removes the 'complication' of there also being a path from one's body to earth, then there is obviously all the difference in the world - in one case the pd between ones two hands (or whatever points of contact) is zero, whereas in the other case it is 400V! Did you think I was saying something different from that? If not, I think I must be misunderstanding you, or something!

I thought you did when you wrote:

It always makes me laugh a bit when I see people citing that traditional wisdom, ... 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'!

 

[ban-all-sheds]

Unless you have dimmers in the circuit. ...take a look at triplen harmonics. Can be a big issue in theatre lighting installations! Oversize neutrals are needed to accommodate the additional neutral current.

"In worst case (very worst case) The load may be 3 battery chargers, only conducting little chips of current between L-N on each of the 3 phases at the very crest of the voltage waves, the part when the transformed L-N voltage exceeds the battery voltage. This does not cancel at all, just gives 6 chips of current per cycle down the neutral that do not overlap in time, and 2 chips down each phase (I am assuming diode bridges , so one chip at crest of positive wiggle, one at negative for each single phase load)
In that extreme case of 3 bursty waveforms that dont overlap and cancel at all, Ineut = I1 +I2+I3.
in reality it will be somewhere in between - lots of electronic equipment tends to give this rectifier like current waveform, with a lot of the 9th and 3rd harmonics present. "

 

[JohnW2]

Did you think I was saying something different from that? If not, I think I must be misunderstanding you, or something!

I thought you did when you wrote:

It always makes me laugh a bit when I see people citing that traditional wisdom, ... 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'!

Yes, but (as you quote) I wrote "... touching two things which 'only' have a 230V pd between them..." (e.g. L&N or L&E of a single-phase circuit), not "... touching two things which both 'only' have a 230V pd relative to earth (or relative to anything else)..." (in which case the pd between them would obviously be zero). That surely was clear, wasn't it?

Kind Regards, John