DIY 3 phase power distro

[Robin0577]

if the loads were equal, they would presumably both see about 200V between their Phase and 'neutral'.

Kind Regards, John

Equal loading being the critical point here, If there is a significant imbalance in the loads on the three phases as is often unavoidable, especially with portable/temporary installations, then the un-referenced "neutral" will float around in proportion to the loading. You effectively have a three legged potential divider with a floating centre point. In theatre lighting, where the load balance often changes significantly with every scene change, this can be particularly disastrous.

I've seen it happen a few times, one of which involved about £50k worth of intelligent lighting releasing it's magic smoke.

 

[JohnW2]

if the loads were equal, they would presumably both see about 200V between their Phase and 'neutral'.

Equal loading being the critical point here, If there is a significant imbalance in the loads on the three phases as is often unavoidable, especially with portable/temporary installations, then the un-referenced "neutral" will float around in proportion to the loading. You effectively have a three legged potential divider with a floating centre point. In theatre lighting, where the load balance often changes significantly with every scene change, this can be particularly disastrous.

Thanks. That all makes sense.

Kind Regards, John

 

[ericmark]

Since this was started in 2009 I would hope it has been all done and dusted by now.

I'm sure it will have been, but it's rocky's current interest in these distribution boxes (see his new thread) which caused him to raise a question about something which was written in this very old one.

I remember back in 1980 working in Algeria we had container type cabins which had three rooms each room had one phase from the three phase supply from a line of 32 amp sockets laid out ready for the cabins to arrive. ... Theory was great and 99% of the time it worked but every so often the neutral would get a bad connection the AC would survive but lights would blow and also any thing the worker had plugged in.

Can you explain that to me? Each of the cabins would presumably be supplied with one phase and N. If the N failed, or even just became high resistance, they would presumably just lose (or suffer a deterioration in) their supply - not experience any high voltages (which only exist between phases)? Perhaps I'm missing something, but I'm not aware of any situation which can result in an abnormally high pd between a phase and neutral (or even between phase and earth).

Kind Regards, John

Each unit supplied with 32A three phase split to three fuse boxes with bottle fuses which in turn supplied lights and sockets. All units split into either two accommodation rooms with shared shower and loo or three accommodation rooms with shower and loo block at start of the run.

rocky's current interest is for a permanent supply so the problem of neutral loss does not worry him. It is only when the distribution boxes are carried from site to site problems with lack of neutral raise their head.

Voltmeters will show wildly changing voltages when neutral is lost but the question is would the user realise what was wrong?

 

[JohnW2]

rocky's current interest is for a permanent supply so the problem of neutral loss does not worry him. It is only when the distribution boxes are carried from site to site problems with lack of neutral raise their head.

As I've now come to realise (now that I am thinking about substantially unbalanced loads) a 'lost' (or high impedance) neutral would be a problem in a permanent installation (which was deriving three separate single-phase supplies from the 3-phase one), just as much as with a distribution box which was moved about.

Kind Regards, John

 

[Robin0577]

Yes, it absolutely would cause the same problems in a fixed installation, only last week I had to repair a 3 phase lighting in a fixed installation dimmer due to a neutral loss. It's just that it's a more likely/common occurrence in temporary systems due the the environment and rough handling of the equipment.

I should probably add to what I said earlier about the potential divider. It's quite easy to imagine what happens to the floating neutral based on pure resistive loads on each of the phases. In reality though, there'll often be an odd mixture of inductive and capacitive loads to mess with the phase angles, and a variety of things which behave in different ways when subjected to over or under voltage, some will shut down, some will draw more current than usual, others will fail open circuit. The result of course is the load balance will also change rapidly during the course of the fault until such time as enough things have either gone open circuit or shut down to prevent any further current flow.

 

[JohnW2]

Yes, it absolutely would cause the same problems in a fixed installation, only last week I had to repair a 3 phase lighting in a fixed installation dimmer due to a neutral loss. It's just that it's a more likely/common occurrence in temporary systems due the the environment and rough handling of the equipment.

Indeed. I got the impression that what eric was originally talking about was the risk that a 'portable' distribution box might get plugged into a 3-phase socket which didn't even have a neutral connection, let alone loss/degradation of a neutral which was 'meant' to be there. That would obviously result in the same problems. We are, of course, not talking about anything different from what can happen to a line of houses (variously using different phases) if there is a neutral fault in the network upstream of those houses.

I should probably add to what I said earlier about the potential divider. It's quite easy to imagine what happens to the floating neutral based on pure resistive loads on each of the phases. In reality though, there'll often be an odd mixture of inductive and capacitive loads to mess with the phase angles, and a variety of things which behave in different ways when subjected to over or under voltage, some will shut down, some will draw more current than usual, others will fail open circuit. The result of course is the load balance will also change rapidly during the course of the fault until such time as enough things have either gone open circuit or shut down to prevent any further current flow.

Yes, that makes sense. Trying to analyse a 3-phase circuit in which the loads were not only imbalanced but also partially reactive would probably be enough to boggle my mind for a bit. Although I would imagine that loss/degradation of the neutral connection is a pretty rare event, I suppose the only 'solution' would be to have over-voltage breakers on each of the single-phase outputs - but whether even they would operate quickly enough to 'save' all connected equipment if there were a sudden rise in voltage, I don't know.

Kind Regards, John

 

[Lectrician]

I have seen some nasty damage due to lost neutrals on TPN supplies. recently on a caravan park, several van owners lost kit as the voltages went haywire after a neutral burnt out. One phase was used for a row of hookups, but only one avenue was in use. The other avenues just had lighting columns running. Completely out of balance, would be near to 400v present at the vans.

In Distro type applications, you run into greater risks of the neutral becoming open circuit due to the onerous conditions and unknown servicing of kit. The phase protection/neutral loss relays are a real saviour!

 

[JohnW2]

I have seen some nasty damage due to lost neutrals on TPN supplies. recently on a caravan park, several van owners lost kit as the voltages went haywire after a neutral burnt out. One phase was used for a row of hookups, but only one avenue was in use. The other avenues just had lighting columns running. Completely out of balance, would be near to 400v present at the vans.

Fair enough. When you talk about a 'burnt out neutral' are you talking about the result of a poor/loose connection? As for the cable itself, if my 'mental trig' serves me right, the neutral current can never be higher that the highest phase current, so if neutral has the same CSA as the phase ones, the cable itself should not come to any harm (if the system is designed and constructed properly!).

Kind Regards, John

 

[Lectrician]

The neutral current (excluding harmonics) would not be more than any one phase, but could be almost equal to a phase if the others are lightly loaded.

The burnt out neutral was due to a loose connection on the bottom of a 4 pole incomer in a TPN board. The neutral probably never had to carry too much current until they closed two avenues for the winter, and then all of a sudden it's working hard. Someone didn't tighten their terminals too well.

 

[JohnW2]

The neutral current (excluding harmonics) would not be more than any one phase, but could be almost equal to a phase if the others are lightly loaded.

Sure - the 'worst case' is obviously that of zero current in two of the phases, in which case the neutral current will clearly be equal to the current in the third phase. It can't get any 'worse' than that.

The burnt out neutral was due to a loose connection on the bottom of a 4 pole incomer in a TPN board.

Fair enough.

Kind Regards, John

 

[plugwash]

As for the cable itself, if my 'mental trig' serves me right, the neutral current can never be higher that the highest phase current

That is true if the loads on all the phases have the same phase relationship between voltage and current and generate no harmonics.

The latter in particular became something of a concern with the rise of switched mode power supplies.

 

[JohnW2]

As for the cable itself, if my 'mental trig' serves me right, the neutral current can never be higher that the highest phase current

That is true if the loads on all the phases have the same phase relationship between voltage and current and generate no harmonics.

Indeed. My 'mental trig' relied on such assumptions. If they are not applicable, I think it would be beyond my 'mental' capabilities (and maybe even my 'on paper' ones!)!

The latter in particular became something of a concern with the rise of switched mode power supplies.

That I can believe.

Kind Regards, John

 

[Robin0577]

Yup, SMPSUs can do horrible, horrible things to your mains waveform, as can most common types of lighting dimmer. In fact the American wiring "code" has now begun to specify double sized neutrals for three phase theatre dimmer installations I believe.

 

[RF Lighting]

It's been spec'd in the UK for years

 

[Robin0577]

It's been spec'd in the UK for years

A spec. which seems to have been very widely ignored in almost every dimmer I've ever repaired!