DIY 3 phase power distro

[SimonH2]

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, ...

I know of an outfit that does large electrically powered machines - all fitted with 5 pin plugs. Because the equipment doesn't need a neutral, all their cables are only 4 core - but fitted with 5 pin plugs & sockets
I'd have suggested that 4 pin plugs/sockets would have been more appropriate, and a few 5pin plug to 4 pin socket adapters to suit common supplies (and their genny).

They are next door to one of our customers, and have allowed our customer to use their genny during a few notified power outages. The first time, with 4 core cable and incorrectly wired power inlet was interesting I gather

 

[RF Lighting]

I regularly fit true three phase machines with 5 pin plugs and 4 core cable, but I will not fit 5 pin sockets if there is no neutral present.

 

[JohnW2]

I regularly fit true three phase machines with 5 pin plugs and 4 core cable, but I will not fit 5 pin sockets if there is no neutral present.

That sounds very reasonable. No harm will come from the former, whether the supply has a neutral or not, but (as has been discussed in this thread) the latter clearly has the potential to dangerously mislead, if the load requires a neutral but the supply does not have one. I'm actually surprised that it is 'allowed', for that very reason.

Kind Regards, John

 

[plugwash]

One thing that really got me when reading plug and socket standards (both BS1363 and IEC 60309) was that they seemed to go into excruciating detail about how the plugs and sockets should be made but gave basically no information about how they should be used.

 

[SimonH2]

I'm actually surprised that it is 'allowed', for that very reason.

Wouldn't that come under the catch all requirements for a safe design ? I'm pretty sure that if anything did happen, in court they wouldn't accept a defence that "the regs don't prohibit it".
Is there nothing in BS7671 about it ?

 

[Lectrician]

I regularly fit true three phase machines with 5 pin plugs and 4 core cable, but I will not fit 5 pin sockets if there is no neutral present.

I agree. And any new circuit should allow for 5pin regardless of use IMO.

 

[JohnW2]

I'm actually surprised that it is 'allowed', for that very reason.

Wouldn't that come under the catch all requirements for a safe design ?

Yes, I would have thought so, given the potential danger. What I really meant was that I was surprised that it is not explicitly forbidden (if it isn't - I'm not sure).

Kind Regards, John

 

[JohnW2]

And any new circuit should allow for 5pin regardless of use IMO.

That clearly makes sense. However (I haven't a clue) is it necessarily the case that (particularly in industrial settings), there will inevitably be a neutral available?

Kind Regards, John

 

[SimonH2]

I would have thought it would be rare - but not unknown - in the UK. Certainly I've never worked on a site without a neutral.
But if you did have a 3 wire system, then I'd suggest that the use of 5 pin sockets would be a complete no-no. If you need a neutral and don't have it, it's not hard to make your own with a 3 phase transformer.

However, on a ship a relative once crewed on they had a three wire 240V system. Ie, 240V phase-phase, three phase, no neutral. So all AC circuits had two "hot" sides. It did cause occasional problems when someone turned up with some "4 wire" equipment as the American head electrician called it.

They also had 240V DC, and some odd colour cores. If you think the debate about Brown/Black/Grey and single pahse is "interesting" - what do you use for what when the cores are white/red/green and you are on DC ?
Actually it was 240V floating (so nominally +120/-120 relative to the hull, and int the battery room were a pair of incandescent lamps which normally glow dimply - but if one side gets earthed, then you get one bright, the other out. They had this for a while (indicating a wiring fault) but couldn't find the fault. Eventually when someone investigated a DC powered flouro not working, they found two people had different ideas over core colours when they connected the two ends of a new bit of cable

Actually, the ship had been built as 240V DC, and the DC had been retained for just emergency lighting. All the hotel loads had been converted to AC - fortunately all the breakers (not fuses !) were DC/AC so they just had to put a distribution board across 2 phases instead of the DC supply. The ship even had DC operated flouros - before the days of electronic starters - with rotary switches that reversed the polarity each time they were used to even up the tube end wear.

PS - and I was fortunate to have seen some of the old switchboards. The wiring on the back of them was a work of art in itself. Really something to behold, with neatly arranged stacks of single cores, with each clip made to fit just the number of cores in the stack at that point.

 

[JohnW2]

I would have thought it would be rare - but not unknown - in the UK. Certainly I've never worked on a site without a neutral. ... But if you did have a 3 wire system, then I'd suggest that the use of 5 pin sockets would be a complete no-no.

Quite so - as I said, AFAICS, it certainly should be a 'non-no'.

Kind Regards, John

 

[RF Lighting]

I've worked in older properties where they have a single phase submains for lighting and general sockets, and a separate 3 phase 3 wire submains for machines / busbar trunkings.

I've also seen 3 phase 4 wire supples fed from this setup using the neutral from the single phase supply. This gets interesting when the single phase supply needs working on

 

[JohnW2]

I've also seen 3 phase 4 wire supples fed from this setup using the neutral from the single phase supply. This gets interesting when the single phase supply needs working on

That's obviously a potential problem if the 3-phase supply gets its neutral from 'the wrong side' of the single-phase supply's isolator but, otherwise, I presume that 'neutral is neutral'.

Kind Regards, John

 

[RF Lighting]

The real danger comes when the SP supply is interrupted upstream of the borrowed neutral. The neutral feedback makes the entire SP installation remain live, whilst the TP&N machine goes wonky.

I've also seen the n&e pins linked together in 5 pin sockets.

 

[JohnW2]

The real danger comes when the SP supply is interrupted upstream of the borrowed neutral. The neutral feedback makes the entire SP installation remain live, whilst the TP&N machine goes wonky.

That's what I meant. If the 3-phase neutral is derived from a point upstream of the single-phase supply's isolator (e.g. a main switch/incomer in CU/DB), then (neutral supply-side faults aside) there should be no problem.

If, exceptionally, there were a "neutral supply-side fault" then I think you would have a potential problem, regardless of how the neutral wiring within the installation was arranged.

Kind Regards, John