3 sp circuits off a tp isolator

[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.