A new mystery - 3-phase leccie

[john.r.davies]

Thanks, Roger!  Reading that makes me ask, three conductors one for each phase?  As in power lines?   But when it goes into a business or even a home workshop, can it then go down one three conductor cable? ( live, negative, earth?)

[RobH]

The supply comes as three  live conductors, one  for each phase plus a common neutral return conductor, which goes back to the central common connection in the substation transformer.  It is not usual for a separate earth conductor to be part of the incomer. The neutral and earth are the same conductor, neutral being earthed at the substation to form a PEN (protective earth and neutral). 

A premises with three phase connection gets all four connections in the incomer (3 phase + PEN). Within the premises the PEN is connected to an earth block and separate earth conductors for each circuit are run from this.  Single-phase connections may be taken from any of the phases and the cable will have three conductors, live+neutral+earth as usual.  Three-phase connections have five conductors 3 phases+neutral + earth. In both cases the neutral and protective earth conductors are all taken back to the PEN at the incomer. 

 

 

[john.r.davies]

Thank you, Rob!      

Last year, the Electricity dug a bloody great hole in my verge - no. they didn't ask first, but the Gas had already dug a trench along it to replace the gas pipes, so it was a bit academic.       I asked the workmen what was occuring, and they pointed to the upright cast iron box that forms part of my garden wall - their hole and the precast concrete box they sank in it was to replace the box.  So what was in the box?    Bloody great fuses!! 

The box help about nine of them in three racks.

They offered to removed the box if I wished, but I said no, I'd only have to rebuild the wall if they did.     Instead, I retrieved the original Town Crest from another, did it up and attached it to mine, now repainted.     

 

The underground box now has three  cables running into each end of it, thicker than your arm, and joined end to end by cigar-shaped objects.    I have no idea if the original box is stll connected but the fuses are still there.

Rob, does this mean that the domestic supply to a housing estate is 3-phase, but that each house will only get power from one main cable,  single phase?     So in theory, I, or anyone else could get 3-phase at home?      Not that I want to, as I'm sure the cost, of running the cable from these real mains cables would be astronomic!

JOhn

 

Edited January 5, 2022 by john.r.davies

[RobH]

Usually all three phases are run up the street. It is normal for the phases to be split so that ideally house 1 gets phase1, house2 phase2, house 3 phase 3 and then repeat. That is to even out the load.  Of course that is only the 'ideal' and with in-fill building and other pragmatic changes over the years  it might not be quite like that. In theory it ought to be reasonably easy to get a three-phase supply but the cost may be large. 

[Charlie D]

John.

I would guess that those ceramic things are more circuit breakers rather than fuses.  A way to isolate part of a circuit to work on it.

I’m sure tbutI think I was once told that all streets had 3 phase running down them, and that each house was fed in turn from each phase.

Next time there is a power cut look out the window and see if every other 2 houses still has the lights on. Then you know it’s your unlucky day because only your phase has gone down.

 

Charlie.

Ahhh... Rob beat me to it... So I must have remembered correctly.

Edited January 5, 2022 by Charlie D

[Lebro]

My 1930's house (semi detatched) was at some stage divided into flats - 1 downstairs, & 1 upstairs.  Not so since before I moved in, but I still have two phases entering the house. 1 goes through a 100A fuse, & onto my meter, the other goes into the fuse box with a blank fuse in it. I have measured 400V across the two at a time  when the fuses were not security wired in.

Bob

 

[Andy Moltu]

Our area is a bit prone to power cuts - I was told it was when they did generator testing on the industrial site that was once GEC but is now one of the remains.

Apparently they link the output to the mains but if they get it fractionally out of phase the locality gets tripped.

In the end the mains electricity could be likened to the output from a car's alternator but without the regulator/diode pack that converts it to DC. The phases are the indvidual outputs as the generator rotates and the different coils are energised.