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3phase to single phase
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soundguymatt View Drop Down
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Joined: 25 August 2009
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    Posted: 27 May 2014 at 8:36pm
So my mate has a cable to go from a 32/3 to a 32/1, where he uses one of the three phases. The other two are not connected. (system is 3p star). council bod seems to think this is illegal. Am i right in thinking that as long as there's a distro with appropriate breakers there's nothing wrong with doing this? 

I think it's fine and i cannot find anything anywhere saying i can't do this. Does anyone know if i can't do it, and can point me to a document saying this. 
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Matt
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ceharden View Drop Down
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Points: 11776 		Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 27 May 2014 at 8:42pm
It's fairly normal (and I don't think there is anything wrong with it) to own a cable with a 32/3 plug and a 32/1 socket which just takes one phase from the supply.  Never heard anyone say you can't do it.
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shagnasty View Drop Down
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Might want to ask the council what his qualifications in this arena are, as long as the cable has the right rating (3g6) and the ends are properly made off, I can see no issues there.
 
What the hell does he think happens in a 3PNE DB when you connect a ring main to it???
 
I think the guy needs to wind his limited knowledge back in a bit.
 
Conversely if the thing looks like a death trap...
 
 
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soundguymatt View Drop Down
Young Croc
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Originally posted by ceharden ceharden wrote:

a cable with a 32/3 plug and a 32/1 socket which just takes one phase from the supply.  

That's what i was thinking :) 
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Matt
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soundguymatt View Drop Down
Young Croc
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Points: 688 		Post Options Post Options   Thanks (0) Thanks(0)   Quote soundguymatt Quote  Post ReplyReply Direct Link To This Post Posted: 27 May 2014 at 9:18pm
Thanks :) i'll tell him that as i thought there isn't anything wrong with his cable (it is 3g6 lineax), and to ask the council people to write down exactly why they think that is isn't legal and to show the relevant rules. 

The DB point is the one i would have made. You just take a live, the neutral and the earth for each circuit. 
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Matt
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djeddie View Drop Down
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Originally posted by shagnasty shagnasty wrote:

I think the guy needs to wind his limited knowledge back in a bit.

Give him a break Shag. It probably took him all day to do the PAT course.

Anyway, I had a similar problem once but it turned out the bloke thought ALL phases had been connected to the single phase! So took the plug apart and showed him... even though he had no idea what he was looking at.
Chas n Dave : it's like Drum and Bass but with beards.             E=mc² ±3dB
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shagnasty View Drop Down
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Points: 7685 		Post Options Post Options   Thanks (0) Thanks(0)   Quote shagnasty Quote  Post ReplyReply Direct Link To This Post Posted: 27 May 2014 at 10:31pm
Originally posted by djeddie djeddie wrote:

Originally posted by shagnasty shagnasty wrote:

I think the guy needs to wind his limited knowledge back in a bit.

Give him a break Shag. It probably took him all day to do the PAT course.

Anyway, I had a similar problem once but it turned out the bloke thought ALL phases had been connected to the single phase! So took the plug apart and showed him... even though he had no idea what he was looking at.
 
There Kinda sits my point, you connect all 3phases and you need a new breaker as a 415V delta short tends to burst the MCB!!!
 
Patience with people who are responsible for safety but do not have the knowledge to discharge their remit, from me, is Zero.
 
Smile
 
 
 
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SamV View Drop Down
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O.oh that mean that someone can make me a 32/3 to 3x 32/1 cable? Cos I have 3x 32/1 to 6x16a distros.
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James Tengo View Drop Down
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Yes. We have them in our hire stock, I am using one on the Rudimental festival floor package as we speak, 32a3ph outlet on my dimmer rack, then a 32a3ph plug to a rubber box with 3x 32a1ph sockets with no breakers on it, to our u type distros which are 32a1ph plug to a rubber box with 4x 16a1ph outlets each with a 16a d curve mcb.
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SamV View Drop Down
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3 phase makes my head hurt. On a 1ph supply the Earth is a bigger pin than the others but then on a 3ph it's the same and shared with the others, along with the neutral. So why isn't it noticeably bigger as a ratio? And why no 1to1 ratio between live and neutral?
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audiomik View Drop Down
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Points: 2962 		Post Options Post Options   Thanks (1) Thanks(1)   Quote audiomik Quote  Post ReplyReply Direct Link To This Post Posted: 15 July 2014 at 7:38am
Sam
not very easy to explain, but a 3 phase balanced system ends up with a zero neutral current....

More info on Wikipedia here:
https://en.wikipedia.org/wiki/Three-phase_electric_power

A 3 phase 3 terminal connection with only a 1 phase load is a real no-no unless you really want 415 Volts instead of 240

Mik

Edited by audiomik - 15 July 2014 at 7:42am
Warning! May contain Nuts
plus springs, washers, screws, etc, etc.
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James Tengo View Drop Down
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Originally posted by SamV SamV wrote:

  On a 1ph supply the Earth is a bigger pin than the others but then on a 3ph it's the same and shared with the others, along with the neutral. So why isn't it noticeably bigger as a ratio? And why no 1to1 ratio between live and neutral?


In certain cases there is an argument for an over sized neutral, especially where loads that create harmonics are concerned. I've been shouted down on here on this point before, the "fact" that the current in the neutral "can never rise above that of any of the one phase due to cancellation"

Well here is a real world example for my case. The photos attached are from the MDU at Brixton Academy, the rig was a good mix of discharge, LED, moles and strobes


For most of the gig the neutral current was lower than any of the individual phases, but with dimmer harmonics adding up the neutral current can be higher than you might expect.
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