3 sp circuits off a tp isolator

[Silly Sausage]

Draw it out, two ways of doing it, it's an unbalanced star!!!
OP best go back and make sure all his Neutral connections are secure, floating Star point is another box of frogs!

 

[johnboy6083]

I'm aware that the DNO abide by the ESQCR regs (or should do) and not BS7671.
however I don't see how a standard cutout with three BS88 fuses is any different in its theory to what we have in the OP.
i can see where I think you are coming from.
are you along the lines of the neutral current from say the far sockets will sum with the neutral currents of the middle sockets and then the near sockets.
Current always sums in series, (the way the neutrals are wired) but only if they are in phase. I understand that they will not always be 120 degrees exactly out of phase because of load characteristics, but the regs have a built in safety margin to allow for the unknowns. It's not always wise to rely on them, but we are talking about assessing an existing install, not installing from new.

 

[netblindpaul]

At last Archy!
Also there are other issues.
I was not going to say this, but, I did suggest this when I said that the currents could not balance without affecting the other 1ph circuits.
I can't believe how difficult it can be to get people to understand?
Also from the star point to the protective device there could be other issues...
Are there not?...

 

[D Skelton]

Paul mate, no one is arguing the point that the circuit is badly designed! We all agree, just not on the neutral overload point.

 

[netblindpaul]

I still maintain that the circuit can be overloaded on the N conductor, as no-one has proven otherwise with 3 off socket outlet circuits as the point of use.
I'm not talking about PF, here either, we are at least talking harmonics & distorted N currents, which you cannot ignore.
Whilst the simplification I posted earlier I agree may have been a little "too" simple at 60A due to the 3 x 20A phase currents, no one has proven as yet that this cannot happen, & I don't believe that anyone can, when the final loads are unknown!

 

[Silly Sausage]

Point 1: I still maintain that the circuit can be overloaded on the N conductor, as no-one has proven otherwise with 3 off socket outlet circuits as the point of use.
Point 2: I'm not talking about PF, here either, we are at least talking harmonics & distorted N currents, which you cannot ignore.
Whilst the simplification I posted earlier I agree may have been a little "too" simple at 60A due to the 3 x 20A phase currents, no one has proven as yet that this cannot happen, & I don't believe that anyone can, when the final loads are unknown!

Point 1:
Without posting my sketches, I'm convinced that the max current that Neutral will carry is one phase loaded and the other two unloaded, as many have already said.

Point 2:
On a few 13A SOs, would that really come into play?

 

[netblindpaul]

Well Archy, I'm not and no one has convinced me that the N cannot carry more than 20A without affecting the other circuits.
Also no one seems to be prepared to consider that the N current & voltage may not be in phase, nor that there may be no harmonics in the N current.
On a few 13A socket outlets yes I do believe it could come into play.

I'm not convinced & I still stand by the fact that the N currents can sum, and no one yet has proven otherwise, it is actually easy to prove they can, but I'm not here to do that.

 

[Engineer54]

Harmonics have got all the DNO's very worried these days, and for very good reason too!! Those 3 1/2 core cables they installed just about everywhere, are coming back to bite them in the arse!! Not quite the money saving solution they once thought them to be!! lol!!

 

[D Skelton]

Ok Paul, three phases (120deg out), three sockets (one on each phase), one common neutral. A 2.99kW light bulb (13A) is plugged into each socket. Will the neutral current sum to 39A?

Edit: LAMPS! I mean lamps, before you pedants jump on board! Lol.

 

[netblindpaul]

So you are drawing 52A per phase so it will sum to more than 39A.
Remembering that you have 2x2x13A socket outlets per phase & you are now drawing 13A per outlet, that is 26A per pair, thus the circuit is non compliant, however, you would hope that the circuit protective device would operate, to protect the conductors.

However, at 26A per phase how long would the MCB take to trip, if you were to only connect a single lamp to each twin socket outlet, and would the 2.5 be rated at this for a single phase, in its installed configuration, you would hope it was, and would there be no instances where it could be possible for the N to be overloaded, possibly not in a perfectly balanced loading scenario.

However, you cannot guarantee that this is all that will be connected.

I'm not going to do the maths at this time of night, sorry, I can see where you are coming from and it is possible for the N, not to be overloaded as the circuits could be perfectly balanced. though I doubt it.

Also, I'm now getting very tired and I'm getting to the end of the computer repair I'm doing tonight so it's time for bed as I'm up @ 06:00 in the morning for work.

This will have to continue the next time I am free to debate things.

However, I'm still maintain that the N CAN be overloaded.

 

[D Skelton]

No, three 13A lamps in total. A 13A load per phase in simple.

- - - Updated - - -

No, three 13A lamps in total. A 13A load per phase simply put.

 

[netblindpaul]

OK, sorry missed the one per phase bit, read it as one per socket!
So currents will be reduced as per load reduction, the loads are now perfectly balanced (within reason) so the current drawn per phase will be say for arguments sake 13A

 

[Silly Sausage]

I'm not convinced & I still stand by the fact that the N currents can sum, and no one yet has proven otherwise, it is actually easy to prove they can, but I'm not here to do that.

You're pulling our chains now, bog standard 3 phase theory.

 

[netblindpaul]

No I'm not pulling chains, look at what can be connected, in the way of non-linear loads.

 

[Silly Sausage]

Fancy talk like non linear loads, reactance, harmonics and fancy things like that...I'm off down the pub.

 

[D Skelton]

OK, sorry missed the one per phase bit, read it as one per socket!
So currents will be reduced as per load reduction, the loads are now perfectly balanced (within reason) so the current drawn per phase will be say for arguments sake 13A

Forgive me if I'm wrong, but it does appear to me that you're being a tad evasive with your answers now. I know what the current per phase will be, what I want to know is what you believe the current in the common neutral will be at any given moment?

Would you be so kind as to answer the question with a simple A, B or C please:

A. The current in the neutral will sum to 39A.
B. The current in the neutral will be exactly the same as the current per phase (13A).
C. The current will be somewhere between 13A and 39A.


Secondly, what I would like to know is what you think the current in the common neutral would be if ONE of the 13A loads was then disconnected from one phase?

A. The current in the neutral will sum to 26A.
B. The current in the neutral will be exactly the same as the current per phase (13A).
C. The current will be somewhere between 13A and 26A.

 

[ackbarthestar]

Harmonics have got all the DNO's very worried these days, and for very good reason too!! Those 3 1/2 core cables they installed just about everywhere, are coming back to bite them in the arse!! Not quite the money saving solution they once thought them to be!! lol!!

Well it serves them right in not investing in the distribution system when they saw how the power requirements were changing, which will be borne, no doubt, by the tax payer. Every installation now relies on SMPs and almost all industrial sites uses VSD control.

I believe they call it CPD for their designers and workers.

 

[Rockingit]

Muesli, btw.

 

[Rockingit]

which I believe to be a linear load. Had it been eggs, well those currents can hang around for days.

 

[Tidy Max]

Again just going to point out that this isn't actually my work! Didn't expect to see 8 pages of posts!

 

[D Skelton]

there seems to be a significant lack of understanding of what I am trying to put across

I'm sorry Paul, but as you haven't answered my two questions, I can only summise that the only lack of understanding is on your part. The following quotes clearly demonstrate this:

each phase of the 20A TP mcb will carry 20A without "flagging" a fault, as each phase will be independent and all connected loads will be 1ph, all of the current WILL return down the N, resulting on 60A of N current, why is this so hard?

The N will have to take the sum of the single phase currents.

Then you have 1x2.5 N conductor carrying ALL the return current.

It is clear to me that at least towards the start of this discussion you had a clear and fundamental lack of understanding of three phase theory. It was only later when your argument started to fall on its backside that you had a rethink, realised that you were mistaken and then rather than just holding your hands up you proceeded to talk about non linear loads and harmonics in order to try and justify your already failing point. Even still, the effects caused by possible non linear loads on a 20A socket circuit have been proven to be negligable so even that part of your argument falls on its bum.

I can't calm down this is basic stuff!

I operate mostly as an Engineer, & consultant thus I am paid for my opinions

Yes, I agree, this is basic stuff, and YOU don't get it. If you are truly giving people this information and charging for it then this is where my respect for you starts to wane. Just because you charge people for consultation and have some sort of engineering background doesn't mean that you're perfect. We all get things wrong from time to time and believe it or not I even know 5WWs who know more than some of the CONSULTANTS at the IEE! And these guys with their completely flawed understanding of electrical theory are writing regulations!

What I respect more than anything, more than knowledge itself is integrity. You would have my ultimate admiration if you could sit there now, hold your hands up and say "you know what, at the start I got it wrong, my understanding was flawed and now it has been corrected. Thanks for pointing me in the right direction". Call me a cynic but I somehow doubt that is going to happen.

Back one final time to the OP, in this situation it is not impossible, but HIGHLY HIGHLY HIGHLY improbable that the neutral conductor would ever be overloaded, and even then, the maximum possible overload that could occur (caused by 1x20A worth of PURE inductance on one phase) on the next sequential phase would be so small that it would easily be coped with considering that the cable is rated higher than 20A.

I like you Paul and I'm not going to sit here and slag you off, but you can't start belittling others (I appreciate you've already apologised for getting nasty earlier in the thread) for their lack of 'basic knowledge' on a subject when you have clearly demonstrated that you are prone to the same blonde moments as everyone else.

 

[Sharpend]

Blinding post chaps, taken the whole of me lunch break to read but well entertaining. The reverse double strike by mr Skelton late on, superb!! Upto that point I thought Paul had it by a hair, however seems it all balanced itself in the end and no I'll feelings.

Well done to all contributors.

Although got lost with the introduction of muesli?

 

[Tidy Max]

Muesli came in to it when one of the members said the load is about as relevant as what he is having for breakfast i think. Then he kindly informed us what it was that he had haha. Made me laugh! But so did "why did mrs ohm marry mr ohm".

Found it,

Not sure what I'll be having for breakfast, either! You can over-think things sometimes.

 

[Outspoken]

The maximum neutral current will be equal to the maximum line current on one phase at any one time. Maximum LINE current = 20A then maximum neutral current = 20A

My answer without reading everything said after this post is...

Voltage Distortion % = [(V[SUB]3[/SUB][SUP]2[/SUP]+ V[SUB]5[/SUB][SUP]2[/SUP]+ V[SUB]7[/SUB][SUP]2[/SUP])[SUP]1/2[/SUP]/V1] x 100

Where
V[SUB]3[/SUB] = Harmonic voltage on L1
V[SUB]5 [/SUB]= Harmonic Voltage on L2
V[SUB]7[/SUB] = Harmonic Voltage on L3
V[SUB]1[/SUB] = Line Voltage

Now I would say that both sides of this debate are right.

Paul is correct to bring harmonics into the equation, but only as far that these are the very reason the set up in the OP is not allowed and would fail any inspection by a competent inspector. However within this setup the harmonics would be so small, unless you had seriously odd loads attached, say pure inductive to one, pure capacitive to another and load that was seriously pf bad on the third, that the harmonics can largely be ignored for this circuit.

Sharing a neutral across 3 single phase circuits is a 100% taboo.
Having 3 single phase circuits fed via a 3 Pole MCB or Isolator is also 100% taboo

The debate went on far too long and should have amounted to only a few posts. The OP setup should be changed to either a single phase circuit as a ring, or 3 single phase circuits across the phases with independent MCB's and circuit independent neutrals and cpc's all fed via the appropriate DB capable of supporting single phase circuits.

 

[44ALLAN]

Again I have to say..............fantastic stuff guys and every one of you is a winner in my eyes:detective::smart::yesnod:
Riveting thread ...............

"Outspoken".
The debate went on far too long and should have amounted to only a few posts.

I really like reading your posts and with all due respect mate.............the debate is not there for people like yourself who know these things as second nature.
The debate 'happened' and it is good that it was allowed to continue to a conclusion(there is one isn't there Paul lol?) for 'plebs' like myself to get educated in these things that we take for granted in our normal jobs..............my college days are long gone but great to see this debate and get the mind going again...................

I was wrong every time mind you so do your shed re-wire yourself lmao

 

[netblindpaul]

I had to go to bed last night as I was up early this morning.
I've been out all day, just finished, and no way can I respond to these posts at this time of night now as I am away early again tomorrow.
I will respond and put my arguments forward, but, it will take time.
I was working last night in front of another pc doing programming works, so I had another along side to come on the forum.
No such luxury tonight, probably not tomorrow, or for the rest of the week, as I am on long hours, and away working the weekend and into next week by the look of things.
So, from now on my replies will be bitty and in relation to a single post and, will probably have some time between them.
Sorry, but I have to work unfortunately.

 

[44ALLAN]

Yup.................same as all of us mate !! lol

 

[Sharpend]

So to conclude with the input of Outspoken,

basically the circuit design was incorrect and of no fault of the OP who only discovered it by chance,
A full bodied debate takes place involving several members over several pages, highlighting that individually almost all had a correct part answer however no one had an absolute answer, which points too if we all worked together then the worlds electrics would be perfect!!!!!!(providing we all knew which part of our answers were correct)

I still don't see the relevance of the muesli???

Great thread.

 

[yellowvanman]

Sharing a neutral across 3 single phase circuits is a 100% taboo.
Having 3 single phase circuits fed via a 3 Pole MCB or Isolator is also 100% taboo

Not disagreeing with you but can you quote regs these 2 situations contravene?

 

[Engineer54]

Not disagreeing with you but can you quote regs these 2 situations contravene?

3 single phase circuits fed/protected via a 3 pole breaker/MCB is in the States is called, ''A Multi Branch'' circuit. It comes with many restrictions within the NEC, but can still represent a number of risks when a neutral connection that is common across all three phases is lost, or becomes a high resistance connection etc!!

They are often used to supply both lighting and power on the same MCB, or banks of lighting in office areas and the like. Remember the single neutral conductor is common for all three poles/phases. So think a little about that scenario and it will become clear why you would never install such a system....

 

[yellowvanman]

So think a little about that scenario and it will become clear why you would never install such a system....

I understand the issue, but as i said is there a reg that you contravene with this? I don't think there is.

I haven't done this myself but thinking further if it was seen doing an inspection and test how does it contravene regs?

 

[MarkKruger]

Common chaps! Overloading the neutral? Think back to your phase angles.......... What we technically have here is a small 3way distribution board...ableit in its most basic and crude manor! I would at an absolute Minimum run a separate neutral to each pair of sockets from the isolator,that completely eliminates any risk of shared neutral shock being incurred by any poor sod in the future performing any tests/maintenance.Ensure all the bonding on the SWA is in fine shape,check you have a good ZS figure and that would just about comply!However lets not forget,depending on the circumstances- it may be necessary to have an RCD installed,which opens up another can of worms,I had a case very similar to this after a recent EICR test,Due to switch gear & accessibilty constraints it was actually easier to Instal RCD socket outlets! In you case I would probably suggest this.

 

[Outspoken]

Not disagreeing with you but can you quote regs these 2 situations contravene?

Don't have a Regs book to hand, but certainly section 53(?) Selection and Erection would cover this, but also any reg that deals with using the correct equipment for the correct job would be appropriate..further, the design of an installation is covered by other regulations, such as CDM and also would such an installation pass any "proper design" guides from the ECA, NICEIC or any Industry body such as the IET or CIBSE

I will look up the regs later if no-one has done so, it amazes me how many here seem to have a problem with looking up a regs book themselves!

 

[D Skelton]

I would at an absolute Minimum run a separate neutral to each pair of sockets from the isolator,that completely eliminates any risk of shared neutral shock being incurred by any poor sod in the future performing any tests/maintenance.

How would this happen? The three circuits are protected by a TP breaker.

 

[Outspoken]

537.2.1 - 537.2.2.6

Though not specific it give a good guidance

Then

523.6.3

for starters.

 

[Rockingit]

537.2.1 - 537.2.2.6

Though not specific it give a good guidance

Then

523.6.3

for starters.

Sorry mate, but 'straws' and 'clutching' are the only two words I see when I look those references up.

Stick with the muesli; you know where you are with a decent bowl of muesli.

 

[Outspoken]

Sorry mate, but 'straws' and 'clutching' are the only two words I see when I look those references up.

Stick with the muesli; you know where you are with a decent bowl of muesli.

If you consider that a helpful answer to a question then I feel sorry for your customers or clients.

The simple fact is there is nothing specific in BS7671:2008 that states you cannot do what the OP has witnessed, the reasoning being is that no sensible and decent engineer or electrician would do it so it has not come onto the radar of those writing the Regs. That is why we need to fall back on the Regs I listed as GUIDANCE...in other words, apply ruddy common sense and if you do not understand why you should not do this then your a shiite electrician who should be a bus driver!

 

[D Skelton]

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?

 

[Rockingit]

Don't have a Regs book to hand, but certainly section 53(?) Selection and Erection would cover this, but also any reg that deals with using the correct equipment for the correct job would be appropriate..further, the design of an installation is covered by other regulations, such as CDM and also would such an installation pass any "proper design" guides from the ECA, NICEIC or any Industry body such as the IET or CIBSE

I will look up the regs later if no-one has done so, it amazes me how many here seem to have a problem with looking up a regs book themselves!

If you consider that a helpful answer to a question then I feel sorry for your customers or clients.

The simple fact is there is nothing specific in BS7671:2008 that states you cannot do what the OP has witnessed, the reasoning being is that no sensible and decent engineer or electrician would do it so it has not come onto the radar of those writing the Regs. That is why we need to fall back on the Regs I listed as GUIDANCE...in other words, apply ruddy common sense and if you do not understand why you should not do this then your a shiite electrician who should be a bus driver!

Now.... I'm almost starting to enjoy the sport in this! Which post is it, exactly?

Look, when in a hole stop digging. No-one has yet to suggest that this set-up is a good idea, far from it. No-one here is a shiite sparky - those that are went all glazy eyed and went to the pub several pages ago not having a clue about the finer-ish points of 3ph theory and harmonics.

But the original OP was asking the question of 'is it a code'? And as you have just so succinctly pointed out, no it isn't as you can't justify holding that opinion [of it being so] based on the rules we abide by.

 

[D Skelton]

What he said! ^^^^^

 

[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!!