2 Phase and neutral supply ??

[Buzz1980]

Hi All

Question regarding 2 phase supplies. Came across this at a farm house, supply is in 3 phase and neutral to the main dist board, then from the 3 phase board at the farm house out to the garage is a 4 core SWA. They have taken 2 phases and neutral from a 63amp double pole MCB at the farm house to then split them Into 2 single phase dist board, but used the same neutral of the SWA. Is this common practice? Just haven’t seen this before ? Advice will be much appreciated

 

[timhoward]

I’ve had one of these with added fun of an underground joint and swapping to 2 core SWA for each consumer unit thrown in too.
The concerns are the sizing of the N conductor and whether the isolation arrangements can lead you to believe the N is safely isolated when in fact it is still carrying return current from the other consumer unit.
It’s very unlikely to meet the regs and be safe in my opinion.

 

[timhoward]

Reading it again - can you clarify this bit "They have taken 2 phases and neutral from a 63amp double pole MCB at the farm house"
Do you mean 2 phases from a 3 phase MCB?
Can you end up with one phase energised without the other one?

 

[Buzz1980]

Hi thanks for the reply. Double pole MCB installed, both will be isolated when the MCB is switched off as there is only one switch. It’s not 2 single pole MCBs installed. I thought it would be ok as it would be the same as supplying a three phase dist board, you have 3 phases and a neutral from a three pole MCB. It’s just I haven’t seen this before where they only take 2 phases and neutral out. Always just seen 3 phases and neutral ?

 

[brianmoooore]

Sounds fine to me. It's just a 3 phase submain, feeding single phase loads only. Only required 2 of the 3 phases, so the third phase was left out.
No different from if the third phase was wired, but then nothing connected to it.

 

[timhoward]

Agree, sounds ok. Sorry for confusion.
The one I had was two single pole MCBs and N leaving in 3 core, and ending up at two separate houses in 2 core, with location of joint unknown at first. Different kettle of fish really!

 

[littlespark]

I don’t think it’s right sharing a neutral.
They should be treated as 2x single phase and neutral supplies, a neutral each.

A 3ph and neutral supply is different as the the load on all 3 phases should be balanced, the neutral will carry hardly anything.
With only 2, it’s completely unbalanced in that section.

Have you checked it is a 3 phase supply, and not just 1 phase linked across L1, L2 and L3

 

[brianmoooore]

No different from if the third phase was wired, but then nothing connected to it.

 

[buzzlightyear]

Came across this at a farm house, supply is in 3 phase and neutral to the main dist board, then from the 3 phase board at the farm house out to the garage is a 4 core SWA. They have taken 2 phases and neutral from a 63amp double pole MCB at the farm house to then split them Into 2 single phase dist board, but used the same neutral of the SWA. Is this common practice
QUOTE ABOVE
this what should say the three phase to farm house .
they have 63 amp double pole mcb in the farmhouse feeding the cu in the garage used 4 core swa feeding two single phase boards
and are using the same neutral of the SWA .

 

[timhoward]

A 3ph and neutral supply is different as the the load on all 3 phases should be balanced, the neutral will carry hardly anything.

Agreed. But it isn't as bad as it sounds.
Assuming an equally sized N conductor the N current will not exceed L1 or L2's current in any circumstances.
If the two phases are both drawing 30amps, we'd see 30 amps on the N.
If L1 was drawing 30 amps and L2 was drawing 50 amps, we'd see 43 amps on the N.
The greatest imbalance would be zero on L1 and design current on L2 which would result in design current on N.

(It's been a while but N current is Square Root of ( L1^2 + L2^2 + L3^2 - L1*L2 - L1*L3 - L2*L3) )

 

[buzzlightyear]

Agreed. But it isn't as bad as it sounds.
Assuming an equally sized N conductor the N current will not exceed L1 or L2's current in any circumstances.
If the two phases are both drawing 30amps, we'd see 30 amps on the N.
If L1 was drawing 30 amps and L2 was drawing 50 amps, we'd see 43 amps on the N.
The greatest imbalance would be zero on L1 and design current on L2 which would result in design current on N.

using 4 core SWA using one for 1 sub board and 1for another
and using the neutral is not great design ,should have its own sub main cable to each one ,but hay ho.

 

[brianmoooore]

It would look more 'conventional' is a 3 phase board had been used at the garage end, fitted with single phase MCBs, but the net result is the same with two single phase boards.
Would have been better to include all three phases on the submain and split the garage load between them, but there's still nothing wrong with it.

 

[buzzlightyear]

Can you imagine two faults on two boards, stand back and watch the fire works all I can say.

 

[James]

In any multiphase system the N current can’t be higher than the current on any Live phase.

 

[Buzz1980]

Thanks for all replies. Yeah I am just trying to think what issue would occur if there was fault on both boards. But can’t see this being any different if fault occurs on a 3 phase dist board all single pole devices. If there is a short circuit on each phase then they are all going to use the same neutral, plus if for instance they use 2 phases more on a three phase supply then it’s not really balanced then across the 3 phases ?

 

[buzzlightyear]

. If there is a short circuit on each phase then they are all going to use the same neutral, plus if for instance they use 2 phases more on a three phase supply then it’s not really balanced then across the 3 phases ?

Are you doing a test on this system or just curious about it.

 

[Buzz1980]

Just curious as never seen before. Not testing installation. Was doing another job at the farm building none electrical related job I was doing.

 

[buzzlightyear]

Just curious as never seen before. Not testing installation. Was doing another job at the farm building none electrical related job I was doing

There is no doubt you are curious about it, I would be asking how may eggs do a chickens lay in a day walking round the farm, and asking how many pints sucked out of the cows.

 

[R-fur]

Are you sure the supply is 3 phase? In the past many farms had a split phase supply which was a 500V supply with a centre tap giving two 250V supplies 180 degrees apart. Large motors could be connected across the outers at 500V.

 

[Lucien Nunes]

Even if the supply is 3-phase now, it sounds like the submain and two single-phase boards were put in at a time when the supply was split-phase. That would have made complete sense, there would have been no purpose running SWA with a 5th core when only two line conductors were present on the site, and the common neutral feeding two single-phase sub-DBs would not have looked at all out of place. Then, when the supply was upgraded to 3-phase, the two existing lines were put onto two of the three new phases, which work exactly the same as far as the neutral current and sub boards are concerned despite the difference in line-line voltage.