Converting 3 phase to single phase

SimonH2 · 7 Aug 2020

Ooh, interesting one. I assume you mean you have a 3P circuit which you don't need to run anything 3P - so you stick (say) 3off BS1363 double sockets off the end of it, one per phase, all sharing one neutral.
As long as the phases all share a linked protection device, then I'm not sure there would be anything against it. You could not have spearate protection (i.e. 3off single phase breakers) because you are not allowed shared neutrals like that.
The only thing I can think of would be the requirement to provide segregation of circuits so as to minimise danger or inconvenience when one of them trips - in this case, if any of the 3 loads causes the breaker to trip, then all 3 will go off. However, since this is likely to be a radial, you could argue that it's not like having all the sockets in your house on one circuit - there are likely to be other sockets around - and in any case you aren't going to lose your lights.
In some ways it would be better than 3off single phase radials of the same characteristics. As long as more than one phase is loaded, the volt drops would be less than with separate circuits - and of course it uses less copper, could be 5 cores vs 9 cores for separate circuits.

JohnW2 · 7 Aug 2020

SimonH2 said:
Ooh, interesting one. I assume you mean you have a 3P circuit which you don't need to run anything 3P - so you stick (say) 3off BS1363 double sockets off the end of it, one per phase, all sharing one neutral. As long as the phases all share a linked protection device, then I'm not sure there would be anything against it.

As you say, interesting, and not really something I've ever had need to think about! I must say that I thought plugwash was talking about just one single-phase circuit, using just one phase and neutral (e.g. all 3 of your sockets fed by the same phase (and neutral)- but I can't see anything wrong with it in either case.

SimonH2 said:
You could not have spearate protection (i.e. 3off single phase breakers) because you are not allowed shared neutrals like that.

I don't really get that. It surely is precisely what does happen when (as very commonly) one has single-phase circuits, with their own single-phase SP MCBs, in a 3-phase board, isn't it? I don't understand what you mean by 'shared neutrals like that' (which you say are 'not allowed').

Indeed, even without a 3-phase board, one still only has one 'shared' neutral. In my house, some of the sockets are fed (via different CUs and MCBs) by L1, some by L2 and some by L3, but all have the same ('one and only') 'shared' neutral.

Kind Regards, John

SUNRAY · 8 Aug 2020

JohnW2 said:
I don't really get that. It surely is precisely what does happen when (as very commonly) one has single-phase circuits, with their own single-phase SP MCBs, in a 3-phase board, isn't it? I don't understand what you mean by 'shared neutrals like that' (which you say are 'not allowed').

Indeed, even without a 3-phase board, one still only has one 'shared' neutral. In my house, some of the sockets are fed (via different CUs and MCBs) by L1, some by L2 and some by L3, but all have the same ('one and only') 'shared' neutral.

Kind Regards, John

A sub board is considered to be a 3ph load and may be fed by 3 fuses or 3ph MCB and a common neutral.
At that point onwards single phase circuits with their own phase and neutral wires are not considered to be sharing a neutral as in this:

This is not permitted unless the OCD is a 3p MCB:

However borrowed neutrals on lighting circuits used to be permitted, have been around for many decades and work perfectly [until RCBO's etc are introduced] but no longer permitted.

So how far do we take this recent ban? 7 pin Bulgin connectors were produced in their thousands [possibly millions] to power 4 or 5 channels of lighting using a common neutral and common earth. Back in the 80's I used to manufacture bespoke cable and adapter boxes for mobile disco's and the most popular being a box with 2x 7 pin Bulgins and 4 or 8 colour coded flying leads with IEC sockets sharing a neutral and an earth. They would typically be fitted to T bars for tripods with 4 or 8 pinspots.
It was common to have a number of these daisy chained.
Also popular was a 19pin socapex with 19 core cable using 2 cores as a common neutral and one earth for 16 channels of mains lighting

ericmark · 8 Aug 2020

I had problems using three phase supplies for single phase machines, this socket

can give three phase or single phase and are common around factories, but I have found them where there is no neutral, and had to fit 400 volt to 230 volt transformers in the machine to work the control circuit as no neutral, it is clearly bad practice to use a 5 pin plug without a neutral but time and time again I have found it done.

As to single phase sockets close together from different phases I was also told should not do it, but can't find a regulations to say don't do it, however I also know where a premises is in the control of an ordinary person you should use type tested equipment, however again not found anything in BS7671 to tell me that.

I will guess either HSE or building regulations? But where does it say I can't use an adaptable box instead of a consumer unit, I know I can in a factory and can't in a private home, but never found the regulation. Like the 1 mm tips on meter leads that is GS 38 and had I not been told in collage when doing my C&G2391 it is possible I would never have found the document.

So we have to use consumer units in domestic premises, and the consumer unit is always single phase and never over 125 amp rated, so would guess the should not have two phases in the same room is some how linked to use of the consumer unit, and using type tested equipment. But as to where this is stated I don't know, would be interested to find out, as it may be England, Wales and Scotland are all different, and as to those not on the main land who knows?

SimonH2 · 8 Aug 2020

SUNRAY said:
So how far do we take this recent ban? 7 pin Bulgin connectors were produced in their thousands [possibly millions] to power 4 or 5 channels of lighting using a common neutral and common earth. Back in the 80's I used to manufacture bespoke cable and adapter boxes for mobile disco's and the most popular being a box with 2x 7 pin Bulgins and 4 or 8 colour coded flying leads with IEC sockets sharing a neutral and an earth.

Still allowed IF sharing protection (fuse). But since most dimmers I can recall tend to have a fuse on each output, that's not the case. There's also the factor of these not being in scope of BS7671
And weren't they 8 pin - 7 plus a centre earth ?

ericmark said:
it is clearly bad practice to use a 5 pin plug without a neutral but time and time again I have found it done.

Worse than that. At a previous job I went out a few time with my portable genny to keep a client's systems going during a scheduled power cut. One time they phoned up the day before and asked about a cable to plug in inside to take power from next door's genny. I explained that these are called widowmakers for a reason and absolutely illegal - followed by "but it's easy to have a power inlet & changeover switch fitted".
Next day, get a phone call - could I go after all ! Sparky had fitted an inlet, but when they plugged in, it tripped the genny. By the time I got there, all sorted. Sparky had corrected the error, and next door had made a siamese cable (two cables sharing one plug and one socket).
The setup ? 80kVA diesel genny which they used to power electric mini diggers if no site power available. All 5 pin connectors. All 4 core cables

Neighbour had a widowmaker and for their own use has just been opening the main switch in the DB and plugging in.
The guy was actually quite nice to chat with, and I think they did fit an inlet properly afterwards.
But I didn't get through to him as to why 5pin connectors weren't appropriate. They could easily have used 4 pin throughout and carried a 5pin plug to 4 pin socket adapter. His attitude was that only they used these cables, and their machines didn't need a neutral.

however I also know where a premises is in the control of an ordinary person you should use type tested equipment, however again not found anything in BS7671 to tell me that.

It's there, tell you to use a CU to BS EN <something>-3, the -3 being the important bit - the part on specs for distribution boards intended to be operated by ordinary persons.

and the consumer unit is always single phase and never over 125 amp rated

Actually no and no - they can be polyphase and the limit is no 125A. When I find a few minutes, I'll start a new thread on that since it comes up from time to time. Won't say more as it'll spoil the quiz

JohnW2 · 8 Aug 2020

SUNRAY said:
A sub board is considered to be a 3ph load and may be fed by 3 fuses or 3ph MCB and a common neutral. ... At that point onwards single phase circuits with their own phase and neutral wires are not considered to be sharing a neutral as in this:

Indeed - and that's what I thought Simon was talking about (hence my comments/question) when he wrote ...

SimonH2 said:
You could not have spearate protection (i.e. 3off single phase breakers) because you are not allowed shared neutrals like that.

In in other words, I assumed he was talking about each circuit (corresponding to each of te single-phase breakers) having both L & N going back to the DB

SUNRAY said:
This is not permitted unless the OCD is a 3p MCB:...

Agreed, but I don't think that's peculiar to 3-phase setups. Even if it's a single-phase board one cannot have a neutral shared in that way by multiple final circuits - and, 'allowed' aside, it won't even work if one has RCBOs or some of the circuits sharing a common RCD.

Kind Regards, John

JohnW2 · 8 Aug 2020

SimonH2 said:
Still allowed IF sharing protection (fuse).

As I've just written/implied, is this not just a specific instance of a general truth/fact that anything on 'one circuit' (i.e. with a common OPD) can (and does) have a 'shared neutral', but that neutrals of different circuits must not be 'shared' (or connected) - and that that applies just as much to single-phase as to 3-phase situations?

Kind Regards, John

SUNRAY · 8 Aug 2020

SimonH2 said:
Still allowed IF sharing protection (fuse). But since most dimmers I can recall tend to have a fuse on each output, that's not the case. There's also the factor of these not being in scope of BS7671
And weren't they 8 pin - 7 plus a centre earth ?

Yes of course, thanks.
I always struggled to find 8 core cable so used 2 types of 7C which which IIRC were 6C of 0.5 or 1mm² and one of 1.5 or 2.5mm², and called them anateur or pro, similar to trailer cable but 300V rated. the plugs were rated at 5A[?] so I linked both neutral pins [there were standards but far too many didn't follow them!].
I know I haven't touched any of these since moving to current house in 1994 so some details like pin numbering are not in the head any more.

However every output was individually fused so yes there was/is contravention of 7671. It's easy to say it doesn't apply however what is the situation when systems like these are permanently installed in pubs, clubs, hotels etc?

SUNRAY · 8 Aug 2020

JohnW2 said:
As I've just written/implied, is this not just a specific instance of a general truth/fact that anything on 'one circuit' (i.e. with a common OPD) can (and does) have a 'shared neutral', but that neutrals of different circuits must not be 'shared' (or connected) - and that that applies just as much to single-phase as to 3-phase situations?

Kind Regards, John

I had an interesting situation a while back correcting following an EICR. I was sent to site with a drum of 2.5mm² blue and had to make phone calls to find out what the problem was.
3ph board, 32A MCB, 6mm² singles in 25mm steel tube looping in and out to 2 small steel enclosures containing 16A MCB, 4P RCD and contactor plus a smaller sp mcb for thermostat. load side of each being 3x flexible 20mm steel tubes with 2.5mm² singles to an immersion heater, six in total in 2 chillers to stop freezing.

I could see nothing wrong with the set-up and I certainly wouldn't have started installing blue neutral wires with the existing old colours.

The SP MCB may have come from the live supply

SimonH2 · 8 Aug 2020

JohnW2 said:
As I've just written/implied, is this not just a specific instance of a general truth/fact that ...

Yes, it is indeed. The twist here is having 3 phases and (we assume) 3 poles on one MCB. Some might be tempted to swap out the 3pole MCB for 3 single pole ones - which as already mentioned would be non compliant.

On USA practice ...
AIUI it's common to wire a pair of sockets with 3 core (plus CPC) cable. The neutral is shared, while the lines come from opposite sides of the DB (so opposite ends of the split phase supply). Thecworst case neutral current is with only one socket loaded.
I gather they also have 2 pole MCBs that can plug across both sides of the board for high power appliances running on 240V.
Do they use RCBOs over there ? If so, then I guess they either have to run 2 neutrals, or do they have an RCBO specially for these paired sockets ?

plugwash · 9 Aug 2020

The American term of what we call a RCBO is "GFCI breaker" and yes they do exist in 2 pole (and even 3 pole, though that is less common) versions.

Over on this side of the pond I notice schnider do an "earth leakage add on block" for their acti9 3 phase MCBs to effectively turn them into a RCBO.

JohnW2 · 9 Aug 2020

SimonH2 said:
Yes, it is indeed. The twist here is having 3 phases and (we assume) 3 poles on one MCB. Some might be tempted to swap out the 3pole MCB for 3 single pole ones - which as already mentioned would be non compliant.

As I said, it's really just a specific instance of the fact that (even with just one phase, let alone three), one can't have two 'circuits' (suppled by two different OPDs) sharing one neutral.

I'm not convinced that the situation you are envisioning would be at all likely to arise in practice. If one started off with 3 single pole MCBs (hence '3 circuits'), one would naturally have separate L & N for each circuit running back to the DB. If one started with a 3-pole MCB, supplying all the loads (maybe with a 'common neutral'), each with one of the three phases, I really don't see why one would be 'tempted to' change that the 3 single-pole MCBs, possibly creating the 'non compliant' situation you mention - and, even if one did, for some reason, succumb to that temptation, one would hope that consdieration would be given to what changes might be needed to the neutral wiring!.

Kind Regards, John

SUNRAY · 9 Aug 2020

JohnW2 said:
If one started with a 3-pole MCB, supplying all the loads (maybe with a 'common neutral'), each with one of the three phases, I really don't see why one would be 'tempted to' change that the 3 single-pole MCBs, possibly creating the 'non compliant' situation you mention - and, even if one did, for some reason, succumb to that temptation, one would hope that consdieration would be given to what changes might be needed to the neutral wiring!.

Kind Regards, John

Oh how I wish you are correct. I have come across a common neutral in 3phase installations on a number of ocassions wher, clearly, alterations have taken place to create the non compliance. EVEN IN OUR OWN WORKSHOP and to make it worse I only found it because we had 'long term' RCD tripping problems. On my second day there I was shown the correct sequence for powering up. Previousely it had been used for car repair with 3ph lifts, compressor etc and most of the wiring had been repurposed by our own guys.

JohnW2 · 9 Aug 2020

SUNRAY said:
Oh how I wish you are correct. I have come across a common neutral in 3phase installations on a number of ocassions wher, clearly, alterations have taken place to create the non compliance. EVEN IN OUR OWN WORKSHOP...

Fair enough. I was merely observing that, particularly if they were lazy, it would seem pretty unlikely that anyone would bother to 'go to the trouble' of changing a 3-pole MCB (which was protecting three single phase circuits, with a 'common neutral') into three separate single-pole MCBs, since I can't really see what would have been gained (other than an attempt at 'separation of circuits') - but I certainly accept that 'nothing is impossible' and that some poeple are capable of doing anything!

Kind Regards, John

SUNRAY · 9 Aug 2020

JohnW2 said:
Fair enough. I was merely observing that, particularly if they were lazy, it would seem pretty unlikely that anyone would bother to 'go to the trouble' of changing a 3-pole MCB (which was protecting three single phase circuits, with a 'common neutral') into three separate single-pole MCBs, since I can't really see what would have been gained (other than an attempt at 'separation of circuits') - but I certainly accept that 'nothing is impossible' and that some poeple are capable of doing anything!

Kind Regards, John

In the above workshop the original 3p MCBs were 10A and the IR wall heater having 4 bars tripped it. Replaced with 16A 1p + 2 others and another replaced with 32A 1P [for RFC] + 2 others. but a lot of the original neutral wiring was original, ie original 3ph isolator replaced with DSSO and 2 additional neutral run from there to wall heaterisolator and 3KW water heater isolator. Reds extended with choc bloc in trunking.

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Converting 3 phase to single phase

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