3 Phase question

It would work, however if the neutral is associated with an RCB further up the chain, it might trip :(

Andrew

The borrowing of a neutral is seriously frowned upon.

I believe the IEE 17th edition says that a circuit must electrically separate from other circuits.

I suppose I would be happy for a temporary way of proving the compressor works, but no self respecting electrician would permanently install it using a neutral from elsewhere. If need be, he could just run a neutral alongside the existing wiring.

If not practical, you could use a 3-phase transformer to generate a local neutral.

Many Thanks to you and Andrew. I forgot to explain that it was purely a temporary fix until, as commented, an electrician runs a neutral from further up the yard where the supply comes from. We have an emergency compressor on loan which needs to go back now the new one has been delivered. The company we bought the new compressor from let us have the loan one as delivery of the new one took nearly 2 months. This loan one doesn't have an in built drier. Our single phase circuits do run on breakers, so I guess it will trip as Andrew suggested. Not sure about the transformer idea, could you explain that one to me please. Bob

Bob,

If you connect a three phase transformer, that has the 'star point' available, that star point can be used as the local neutral. The star point is where the three individual windings join. So as an example, we have an 11 kV overhead line crossing the farm, with a transformer up a couple of poles generating a local 415v 3 phase. The 11 kV is distributed as 3 wires so no neutral. On the 415 volt secondary, the star point is not only used as the neutral, at the poles it is connected to earth via huge copper plates in the ground. Now the further you get away from the transformer down the 415 v distribution, the more your neutral will differ from true earth, but depending on local loads will always be within a volt or two.

Andrew

Many Thanks Andrew for the explanation. I think this is a tad beyond me. My dealings with Star points are minimal and only with electric motors, Star Delta etc. Probably safest for me to either go buy a length of suitable cable to extend the yard neutral source, as the so called "Electrician" seems to have mysteriously gone on holiday, which I guess means he has no idea, and is not what he says he is ! Bob

If he regularly works with any industrial electrical wiring which is generally 3-phase, he really ought to know.

I'm sure running a neutral cable is your best bet.

I think he is the odd job man who dabbles in simple repairs, the odd switch etc by the sounds of it.

Anyway, I located a fusebox 3 workshops down, and inside it is the Neutral bar at the bottom of the box. I should state this is a very old box, with the porcelain holder rewritable fuses in. A quick check with the meter has shown me its 3 phase, and 1 phase to the neutral bar gives 240v. I'm pretty certain the bar is Neutral, not Earth, as its insulated each end where it mounts to the box, forgot to check for a resistance between the bar and box, to confirm not connected to each other, will do that tomorrow. So far, seemingly so good. Now the worrying bit.... the phase cables in the box look very thin, at a guess 2.5, maybe 4mm at a push, as is the neutral. I think it safe to say I'm the heaviest user of 3 phase in the yard, and my supply is not from this box. So my dilema is this, can I run this neutral to my shop, as in theory its not my supply, and what size cable for a 25 metre run. I assume a neutral would only carry any 240v stuff ? So would something like a single 6mm cable do. Before you ask, I did ask a professional electrician about extending the main feed from the supply, but he declined to do it as the whole yard setup is very old and he said he would want to rewire the whole lot. Megga bucks !! Bob

I'm starting to get worried where you are having difficulty in identifying the neutral and earth.

The cable size will depend on the likely current draw and voltage drop. You would need to look at the regulations and tables.

I don't know the cable you currently have and it's rating. You say

2.5mm, but is that the diameter or cross-sectional area? Cables are defined in area.

I don't know how complicated or how old your yard is, so can't comment on any quote. Something suggests that a re-wire is recommended, though some electricians don't like small jobs and will convince the customer that a complete re-wire is in order. Perhaps a second or third opinion/quote is in order?

Bob,

Back at the supply companies transformer, neutral and earth will be tied together so at similar voltages, however as some current will be flowing in the neutral line, and hopefully none in the earth line, there will be a voltage drop of maybe a volt to several volts. (*) This will be at a VERY low impedance so will give huge currents if shorted to earth. Be very careful.

Andrew

(*) back in the days of DC supplies people were known to use this voltage to charge their radio accumulators as the current didn't register on the meter :)

Thanks all I think its time for me to take a step back and go back to basics. I'll hook the compressor up without any neutral and see what happens. If the drier part doesn't go, a lesson learnt. I think the sales guy threw me first, saying I might need to run a single phase supply for the drier, then on getting the machine, the main power cable has 5 wires, L1, L2, L3, Earth and neutral. It might just be possible that the neutral is there to satisfy the dreaded "Edition" wallers. The loan compressor has a terminal for said neutral, but I didn't put anything in it, and works like a dream. Both are Atlas Copco, loan one, no drier and model before the new one which is a GX7 FF bob

As far as I know, where a neutral isn't, required 17th Edition regs don't require a neutral in the wiring. BICBW

The problem you might have, is that when the compressor is running the drier will work, but as soon as the compressor cuts out, the neutral may be lost and the drier stop functioning. Also the compressor motor currents will be asymmetric, limiting start-up torque and increase resistive heating in the motor.

If this is an Atlas Copco GX7 with a 10hp compressor, you need more than

2.5mm2 cable.

If you can afford this new compressor, I can't see why you can't afford proper and safe wiring. I wouldn't offer any warranty on the compressor unless it was wired as spec.

Ok I can afford it, and willing to pay, but not willing to pay for the whole yard, after all, that's the landlords responsibility. I got let down by one so called electrician, and am looking for a quick fix for before the loan machine going back and getting the wiring done properly.

Further investigation today of the electrics in the yard revealed all sorts of things. It looks like the neutral bar in the fuseboxes in several workshops is used as an earth. There are several very old 1-2 way junction boxes from the main supply in 2 workshops. Looks like they split the main feed for some reason. Its hardly surprising that the first guy shied away from this. The only reliable neutral I can see is the on meter tails for my shop, which is much further up the yard. Anyway, hooked compressor up, and it runs fine. Having looked at the wiring diagram supplied with the machine, its evident that the neutral is solely for the drier, diagram shows clearly L2 to the drier, and the neutral. So obvious its not running that part of the machine.

Knowing now that the only need for said neutral is the drier, and I admit to limited electrical knowledge, don't really see why using the single phase neutral would cause a problem, unless its to do with balance. Isn't it common to use one phase from a 3 phase circuit to get single phase ? I recall the in my youth some workshops used to take one phase and earth to get the machine light to work ???

One reason why each circuit should be kept separate from any other is that is important to be able to isolate that circuit simply and easily by removing it's source sometimes using a double pole switch depending on earthing system.

Consider an electrician now working on your single phase circuit and your compressor kicks in. Said neutral will now become live, and I hope your insurance is up to scratch if the electrician's family sue you with the backing of NIC. Hence why is isn't recommended.

What isolator do you have for the compressor? Does the installation manual mention what level of isolation is required for maintenance etc? I can only see manuals dating from 2007 and 2008.

You've now told us where the source for the workshop neutral is, why not use that instead, and use a 4 pole isolator?

Ok, point taken. Still seems a simple fix to me with another isolator for the one cable, but I wont go down that route, and will get it done properly. Compressor is currently wired into a 3 pole isolator, but no problem to change that to a 4 pole when we get the neutral sorted. Seems I'll be buying 50 meters of substantial cable next week, the meter tails look like 16mm probably equivalent to the old imperial size, as they are the red, yellow, blue and Black

ed today, and the question raised before buying it was our lack of a neutra l in our 3 phase box on the wall. I recall a neutral appears about 4 worksh ops up from us in one of the workshops near the main supply. Never been a p roblem before for our machines, but this compressor has a built in drier th at I think uses single phase, so I assume it takes one of the 3 phases, and neutral for this supply. Now, there is a (Wink, Wink ) a neutral lurking i n the single phase ring main circuit nearby. Without getting into all the h eadache stuff of wiring regs and the like, would using this neutral be a po ssible solution to the problem ?

If you need to calculate the size of cable you need it might be worth tryin g this Voltage Drop Calculator on TLC's site.

John H

It sounds like the entire supply has been bodged by innumerable handless poltroons; in the short term getting a 3ph generator to run your compressor is the safest bet, while twisting the landlord's arm to get a proper rewire done in the near future.