3 sp circuits off a tp isolator

Unless I am wrong stan, and if I am, then I apologise, as I have misread the OP. it seems everyone missed this!
This is the CRUX of my argument!
It is NOT acceptable to share a 1x2.5mm sq N with 3x2.5 phase conductors on 3 x 1ph circuits.
 
I'm not convinced, surly the neutral currents would be cancelled out at the point where the neutrals are joined together? This may be a isolator etc, they must be linked together somewhere if all the neutrals have not been brought back to the db?
Anyway, I've not read though the whole thread so I apologise if if missed something obvious.
 
I'm gona re read the OP. if the supply to the switch fuse is 3ph, and the loads from the fuse are single phase, then I don't see a problem. It's not conventional to wire seperate circuits for the same switchfuse, but it isn't dangerous.

As long as the cables are suitable for the protective devices, and they are suitable for the load, then the neutral only needs to equal the size of the largest phase conductor (not that of ever size them differently anyway).

Each circuit from the switchfuse will have to have it's own neutral, but the 3ph submain would only need one neutral. So in other words, the switchfuse becomes an unconventional DB.
 
johnboy6083 said:
I'm gona re read the OP. if the supply to the switch fuse is 3ph, and the loads from the fuse are single phase, then I don't see a problem. It's not conventional to wire seperate circuits for the same switchfuse, but it isn't dangerous.

As long as the cables are suitable for the protective devices, and they are suitable for the load, then the neutral only needs to equal the size of the largest phase conductor (not that of ever size them differently anyway).

Each circuit from the switchfuse will have to have it's own neutral, but the 3ph submain would only need one neutral. So in other words, the switchfuse becomes an unconventional DB.
Click to expand...

johnny
Up to the switch fuse the N is 1x2.5 the phases are 1x2.5 each, all of which are protected by a 3 pole 20A mcb, i.e. 20A/ph, which is the crux of my argument.
Unless I have misread the OP.
 
I've re-read the OP.
so he's wired the neutral as a single radial for all six sockets, and fed groups of two from each phase.
Now this is bad practice at best.

Using maths for say the worst case scenario of two fully loaded phases, and one unloaded phase (I'm going to use 1amp, as anything x 0 = 0 and will fudge the calc)

neutral current = square root((a squared + B squared + C squared)-((AB)+(AC)+(BC)))

so it's 19A I make it in my example.
So I wouldn't code it, but I would make a note of it.
The only reason it's not unsafe is because they all have common isolation.
 
johnny,
That assumes balanced & linear loads, that cannot be guaranteed with socket outlets.

Oh & what happens if the N between the switch fuse & the DB fails?...
 
Sorry to say Paul but if I understand the OP correctly then you're wrong. Let's say for example that there is a 20A load plugged into each of the three phases (60 total current). The three line conductors are out of phase with each other, therefore the resulting current in the neutral from any one of these lines will be out of phase also. In very simple terms; at the point in which L1 is drawing its 20A, L2 and L3 aren't drawing, therefore the neutral current is 20A, 120 degrees (150th of a second) later L2 is drawing 20A whilst L3 and L1 are drawing nowt. Neutral current = 20A. And so on...
 
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Then you have big issues anyway, even on a standard 3ph submain, as the voltage will rise with respect to earth.
as I've stated, I'd definately not install like this, and I would whinge if I found it, however my opinions don't count on a report. It's facts, experience, science and regulations that matter.

i agree you can't guarantee the loads on socket outlets, however we aren't in the realms of data suites ect here, just general socket outlets.
 
D Skelton said:
Sorry to say Paul but if I understand the OP correctly then you're wrong. Let's say for example that there is a 20A load plugged into each of the three phases (60 total current). The three line conductors are out of phase with each other, therefore the resulting current in the neutral from any one of these lines will be out of phase also. In very simple terms; at the point in which L1 is drawing its 20A, L2 and L3 aren't drawing, therefore the neutral current is 20A, 120 degrees (150th of a second) later L2 is drawing 20A whilst L3 and L1 are drawing nowt. Neutral current = 20A. And so on...
Click to expand...


No they are not,
They are 120 degrees out of phase in the line conductors, we have no idea where they are in the N.
You cannot make simple assumptions like this.
 
Thing is johnny, you don't know where the sockets are, or what is going to be plugged into them.

I stand by my arguments and no one has put anything forward to change my mind yet.

You have a single 1x2.5mm sq carrying the return current from 3 x 20A single phase circuits, they are NOT 3 ph at the point of use, if they were then I would never have commented.
This is not acceptable, show me a reg that says it is, or give me some REAL science that proves that the N can never be overloaded.
 
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
 
Can you please explain that in fundamental terms on 3 separate single phase circuits please?
Remember these circuits ONLY share an mcb they are otherwise not linked on the phase side, only the N side, so each can take 20A, yes, if they are perfectly balanced then you "could" argue a cancelling affect, though how would this cancelling current affect each individual single phase load?
To cancel the current has to go elsewhere other than down the N conductor, thus it has to flow back down the other two phases, how will this return current affect the electronic single phase loads connected to these single phase socket outlets?

You cannot flow N current from a single phase circuit back down a different single phase circuit to cancel out N current without affecting the characteristics of both separate single phase circuits.
 
D Skelton said:
If you didn't have blinkers on you'd understand mate.
Click to expand...


I don't have blinkers on, you cannot flow N return current back down a separate phase conductor without affecting the separate single phase circuit, we are NOT talking about a 3ph circuit here.
We are discussing 3 single phase circuits with a shared N, the only way the N current can balance is by affecting the other 2 1ph circuits.
This cannot be allowed, each 1ph circuit MUST be independent.
 
netblindpaul said:
We are discussing 3 single phase circuits with a shared N
Click to expand...

No, we are discussing 3 OUT OF PHASE single phase circuits with a shared N.

Again I say, if there is a 20A load plugged in on each phase, when L1 is drawing 20A, L2 and L3 are drawing NOTHING, therefore the neutral current is 20A.

150th of a second later 20A will be drawn down L2 whilst L3 and L1 draw NOTHING.

At any one time only one phase will be drawing 20A therefore the neutral current can ONLY be 20A.
 
i agree with pauls & jonnys sentiments that a shared neutral is bad practice / design

if single phase & unbalanced loads ( the sockets ) are connected to a 400v supply then i'd expect seperate neutrals for each circuit back to the origin.
whether the devices are 1 or 3 pole is neither here nor there in my mind , it would be a C3 defect if i was inspecting it.
 
OK,
I can see where those of you who say that the N current will be delayed by 120 degrees, this would be the case with balanced 3 phase loads of known characteristics.
It cannot be guaranteed to be the case with 3 off single phase socket outlet circuits spread across 3 phases, you have no control over what the loads are or what they an "do" to the N return current, you cannot assume that the N return current will be phase balanced.
There are too many non-linear loads in current installs.
If the loads are perfectly balanced then why do we not use 1/2 size N conductors for 3ph boards all the time now?
 
Forget balancing, it is completely irrelevant to this discussion.

Even still, take a TP+N load, L1 = 100A, L2 = 0A, L3 = 0A then N = 100A.

The neutral current can never exceed the line current!

If you can guarantee a specific Ib to stay at a certain value per phase then it is completely permitted to calculate the size of the neutral conductor required and install that.
 
It doesn't matter, the L current per phase will be out of phase with the others, and so will the neutral current.

Bad design for sure, but absolutely no danger.
 
Yes it could be out of phase, but by how much, and, why are all 3 phases bound to be loaded at the same time and by the same amount and with a suitable phase shift to ensure that the N cannot be overloaded, you cannot guarantee that they will, you can get sufficient phase shift to overload the N, remember there are so many non-linear loads around now and the OP does not define what is connected.
There are too many unknowns to give this an OK.
 
You're not going to get any plug in non-linear loads without suitable PF correction fitted within. Even still, any negative effects caused by slight inductance or capacitance will be negligable.
 
So you are categorically stating that if 2 off 20A single phase loads were plugged into 2 separate single phase circuits they could NEVER overload the 2.5mm sq N conductor?
 
D Skelton said:
You're not going to get any plug in non-linear loads without suitable PF correction fitted within. Even still, any negative effects caused by slight inductance or capacitance will be negligable.
Click to expand...

I can understand where Paul is coming from, with a purely theoretical view, but as a practical application I have to go with this ^^^ ..... We're talking about some sockets in a dado trunk, not industrial connections.
 
netblindpaul said:
So you are categorically stating that if 2 off 20A single phase loads were plugged into 2 separate single phase circuits they could NEVER overload the 2.5mm sq N conductor?
Click to expand...

No, I'm just saying it's HIGHLY improbable.

Think of TP+N lighting power track systems. These share a common neutral and are protected by TP OCPDs. Commonly these are also fluorescent fittings, an assumption can be made that a standard rate of cap faliure within the fittings will equal itself out over time across the three phases thus causing very little cause for concern. Still, installations like these need regular maintenance where things like this SHOULD be picked up if the guys are doing their job properly.

Like I said, the circuit brought to discussion in the OP is badly designed with it's use in mind, however it would cause me no concern if I came across the same thing on an install.
 
netblindpaul said:
Thing is you don't know what is going to be plugged in.
Click to expand...

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

Have you ever analysed the non-linearity of power loads in even a simple domestic install these days?

Think SMPSU's, your fluoro fittings, motor loads, vsd's in washing machines, yes, there will be pfc, & each should individually meet the requirements of the LVD & the EMCD, but cumulatively this can have a significant affect on the connected systems, so i stand by my points that the loads can sum as they can be way out of phase.
Take an accurate power analyser and check a few installs and see what you get at the tails, then guarantee me that you can never overload the N in this scenario.
 
What local distribution network, all of these things can be found on a small domestic install these days, I'm not going to back down and no one has yet given proof that the N cannot be overloaded in the scenario given by the OP.
So until this is done I stand by my arguments, I DESIGN, I DO NOT GUESS values, so the N can be overloaded, & if I was being paid for my consultancy then I would be bothered to calculate things, as I am not, it;s down to you lot to prove otherwise, which, as yet, you have failed to do.
 

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