3 sp circuits off a tp isolator

[D Skelton]

Because they will be, they are single phase circuits!

 

[netblindpaul]

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?

 

[D Skelton]

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.

 

[netblindpaul]

It's not a TP+N load though!
Where did you get it is?

 

[D Skelton]

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.

 

[netblindpaul]

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.

 

[D Skelton]

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.

 

[netblindpaul]

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?

 

[johnboy6083]

The calc I posted proved it. Harmonics notwithstanding!

 

[netblindpaul]

So you can't then, end of argument.

 

[Rockingit]

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.

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]

Thing is you don't know what is going to be plugged in.

 

[D Skelton]

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?

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.

 

[Rockingit]

Thing is you don't know what is going to be plugged in.

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

 

[netblindpaul]

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.

 

[44ALLAN]

Fantastic thread guys.............well done to all.

Great stuff

 

[D Skelton]

You're definitely over thinking things now!

We were talking about three 20A circuits, not a local distribution network lol!

 

[netblindpaul]

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.

 

[tom79bmth]

Well how come the supply distributors don't have a bigger neutral than the phases,

 

[netblindpaul]

Because it is not needed in that scenario, because it is not what is being discussed here, this is a very different situation.