3 sp circuits off a tp isolator

[ackbarthestar]

I think i would agree with that, DSkelton,

559.6.2.3 allows a 3-phase circuit to be split across three single phases sharing the same neutral as long as the three single phase circuits are isolated and protected by one 3-phase MCB/MCCB. Therefore treating the whole as a single three phase circuit.

However, it only specifically mentions lighting and not sockets. But then it doesn't condemn the use of other single phase outlets. So it might be a case of reducing ambiguity and applying common sense and waying up the probabilities of a possible loss of neutral where portable appliances are used.
I believe 'they' call it 'Risk Assessment'.....

 

[D Skelton]

I believe 'they' call it 'Risk Assessment'.....

Or as others have rightly pointed out, common sense! Lol

 

[Rockingit]

Or as others have rightly pointed out, common sense! Lol

Oi you....we'll have none of that fancy talk in here!

 

[Engineer54]

Can't quite believe what i'm reading here!! Am i to understand that qualified electricians here, would actually apply a 3 pole/phase MCB to supply 3 single phase circuits using a single neutral conductor?? Then start talking about commonsense and risk assessment!! i've heard it all now!! lol!!

 

[D Skelton]

I think you need to read the WHOLE 10 pages again Eng! Lol. No one here, regardless of our opposing views on certain matters have condoned this practice at all!

Edit: socket circuits we're talking about here.

Lighting circuits are a different matter, I have and will continue to apply this method of installation where suitable to certain types of lighting circuits.

 

[Engineer54]

I think you need to read the WHOLE 10 pages again Eng! Lol. No one here, regardless of our opposing views on certain matters have condoned this practice at all!

Edit: socket circuits we're talking about here.

Lighting circuits are a different matter, I have and will continue to apply this method of installation where suitable to certain types of lighting circuits.

Not on any of my projects you wouldn't!! You would actually wire all the 3 phase lighting loads, with the same single neutral conductor?? ...Wow!!

 

[Darkwood]

As i said on the first page i have come across 3ph and n track lighting which by its nature can only have 1 neutral... in the correct set-up its ok an no-one i believe has even indicated its a good method or practice regarding the OP's set-up but at most in some views that its not really something to get too concerned about although it has a finite chance that harmonics etc could lead to over-current on the N conductor now it would be a strange loading set-up on the sockets to even bring this in as an argumentative angle although its good for the debate to realise the potential exists even if very slim.

As for design the OP set-up shouldn't be designed as such as most would agree here but it is the case the OP has found the set-up as is.

Its in my understanding that the BS7671 is partly designed with probability in mind and this does show up in various regulations and of course will lead to the lengthy debates we see in this thread.... if probability wasn't at the core of some reg's then we wouldn't be working and installing systems at the voltage we do.

 

[D Skelton]

Not on any of my projects you wouldn't!! You would actually wire all the 3 phase lighting loads, with the same single neutral conductor?? ...Wow!!

That's your perogative, but I do so myself safe in the knowledge that given the right set up it is a perfectly safe and well recognised method of installation (although not hugely common) that is permitted by regulation.

 

[Darkwood]

Ive come across this method on more than a few occasions and installed it twice over 25yrs on lighting systems, designed right it is perfectly safe and is an economical method. Various ways exist if you want to go the extra mile like N monitoring where loss or overloading of N would drop out all 3 phases this is really only needed when existing install make it impractical to rewire and can open up the field of utilising a shared N.

I personally don't design my installs this way but thats not to say its not an option, its a method has been practised for decades and i would have thought if it was such a disaster waiting to happen then im sure the IET would have addressed it in the Regs.

 

[Engineer54]

As i said on the first page i have come across 3ph and n track lighting which by its nature can only have 1 neutral... in the correct set-up its ok an no-one i believe has even indicated its a good method or practice regarding the OP's set-up but at most in some views that its not really something to get too concerned about although it has a finite chance that harmonics etc could lead to over-current on the N conductor now it would be a strange loading set-up on the sockets to even bring this in as an argumentative angle although its good for the debate to realise the potential exists even if very slim.

As for design the OP set-up shouldn't be designed as such as most would agree here but it is the case the OP has found the set-up as is.

Its in my understanding that the BS7671 is partly designed with probability in mind and this does show up in various regulations and of course will lead to the lengthy debates we see in this thread.... if probability wasn't at the core of some reg's then we wouldn't be working and installing systems at the voltage we do.

If your talking about 3 phase bus bar type affairs with plug-in connections, then fine your never going to loose the neutral along it's length are you. But conventional lighting wiring methods no way, that neutral single conductor can end up three times the length of the phase conductors if wired in banks per phase!! Breaks in that single neutral conductor could add more than a little interest too!! Now why would you not run a separate neutral conductor with each phase leg, and if you are going to use a 3 pole breaker why not a four pole, so that if fault did occur, the breaker would isolate ALL live conductors!!

 

[D Skelton]

If your talking about 3 phase bus bar type affairs with plug-in connections

This (and other similar types of set up) is exactly what we are reffering to.

You can rest easy Eng!

 

[Darkwood]

If your talking about 3 phase bus bar type affairs with plug-in connections, then fine your never going to loose the neutral along it's length are you. But conventional lighting wiring methods no way, that neutral single conductor can end up three times the length of the phase conductors if wired in banks per phase!! Breaks in that single neutral conductor could add more than a little interest too!! Now why would you not run a separate neutral conductor with each phase leg, and if you are going to use a 3 pole breaker why not a four pole, so that if fault did occur, the breaker would isolate ALL live conductors!!

As i have said before i said a carefully designed set-up with added monitoring or pre-designed system like track lighting, we all seem to be arguing the same side of the coin from a different angle its just the case that its not the most common method in most members experience hence it throws up a lot of interest if ever mentioned and yes 4pole devices if they could fit your board as long as it early make / late break on the n cos we wouldn't want to be zapping the lighting with a voltage spike as you star point the N by accident.

 

[Knobhead]

In = √((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

 

[johnboy6083]

I did try and offer that calc as a way of calculating neutral currents earlier. We have raised this formula before, and I proved through a power analyser (screen shots I posted) that it doesn't work where harmonics are involved.

however in the instance I used, the harmonics were large enough that an active filter was used. The OP discussed sockets, in which only a crystal ball can predict future usage.

 

[Knobhead]

A couple of weeks back 4 transformers failed in Nottingham, from what I can gather it was the neutrals that failed. I live in student land so lots of SMPS’s. It’s a wonder the leccy board don’t put a zip fastener along the cable runs. No sooner have they filled one hole in their digging another, neutrals failing.

The neutrals in Victoria Centre are near melting! It’s only going to get worse. The transformers taking the brunt of it as they are the main filter for harmonics.

 

[Outspoken]

I think everyone knows why it's a bad idea on three socket circuits, there are various reasons, however it's common practice on lighting circuits so that might be one of many reasons why there is nothing specific in the regs about it.

Just my thoughts?

We will have to agree to disagree on that but I hold your position to be questionable.

Running three single phase lighting circuits in one location to load spread is indeed common practice, but the only time they should be on a common 3P+N MCB/Isolator etc is when contractor controlled so they are effectively a 3 phase circuit and should be treated as such. Neutrals should not be looped though and may come back to a common point but should be separate for each on the lighting legs.

If you are talking about standard lighting circuits with conventional switching etc then they should be on separate MCB's and not a 3 pole. They should not have shared neutrals and to do so would be extremely bad in my humble opinion.

Oh by the way, just got off the Phone with the NICEIC who agree with me on this, as do NAPIT and ELECSA. I have also conferred with the IET who have condemned the notion of running single phase circuits, except as stated where controlled via a contactor, via a 3 pole MCB/Isolator whether the neutral is shared or not.

I will admit that no-one could specifically point to a regulation to prevent someone doing this, but all stated they would not consider this good practice or good design.

 

[D Skelton]

When I refer to common practice I am only referring to the pre wired, modular trunking systems and busbar lighting track which are common in larger commercial installations. I agree and when I have installed these they have always been contactor controlled, I wouldn't do it any other way. With regards to wiring standard lighting circuits in this way, I am fully aware that this is a big no-no. I should have made it clearer and said something like this:

I think everyone knows why it's a bad idea on three socket circuits, there are various reasons, however it's common practice on CERTAIN TYPES of lighting circuits so that might be one of many reasons why there is nothing specific in the regs about it.

Apologies for the confusion.

 

[D Skelton]

When I refer to common practice I am only referring to the pre wired, modular trunking systems and busbar lighting track which are common in larger commercial installations. I agree and when I have installed these they have always been contactor controlled, I wouldn't do it any other way.

With regards to wiring standard lighting circuits in this way, I am fully aware that this is a big no-no. I should have made it clearer and said something like this:

I think everyone knows why it's a bad idea on three socket circuits, there are various reasons, however it's common practice on CERTAIN TYPES of lighting circuits so that might be one of many reasons why there is nothing specific in the regs about it.

Apologies for the confusion.

 

[Engineer54]

When I refer to common practice I am only referring to the pre wired, modular trunking systems and busbar lighting track which are common in larger commercial installations. I agree and when I have installed these they have always been contactor controlled, I wouldn't do it any other way. With regards to wiring standard lighting circuits in this way, I am fully aware that this is a big no-no. I should have made it clearer and said something like this: Apologies for the confusion.

Well that's cleared that one up then...!! lol

To be honest, i was more than a little surprised you were in favour of installing such a system, while the confusion was still reigning!!

I have seen such systems in use, and seen the consequences when things go wrong too!! It was on a NEC spec'd project and can still see clearly in my mind how the single neutral was branched off to supply light fittings. ....Yep you guessed it WING NUTS!! A good 30 or so of them!!

Haven't seen that guy about much here lately, that reckoned the Yanks multi branch circuits for both sockets and lighting, were going to be the way of the future in the UK!! He really was a classic with some of his ideas!! lol!!

 

[Engineer54]

As i have said before i said a carefully designed set-up with added monitoring or pre-designed system like track lighting, we all seem to be arguing the same side of the coin from a different angle its just the case that its not the most common method in most members experience hence it throws up a lot of interest if ever mentioned and yes 4pole devices if they could fit your board as long as it early make / late break on the n cos we wouldn't want to be zapping the lighting with a voltage spike as you star point the N by accident.

As far as i'm aware, most 2 and 4 pole breakers, (certainly those supplied to us by Schneider (MG)) have a designated Neutral pole for that very reason.