3 sp circuits off a tp isolator

[netblindpaul]

Yes the N currents will balance at the point of linking, that is one of the points of my argument, however until then they will sum.

 

[netblindpaul]

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.

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.

 

[johnboy6083]

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.

 

[netblindpaul]

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?...

 

[D Skelton]

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...

 

[johnboy6083]

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.

 

[netblindpaul]

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...

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.

 

[netblindpaul]

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.

 

[D Skelton]

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

 

[D Skelton]

If you didn't have blinkers on you'd understand mate.

 

[netblindpaul]

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.

 

[netblindpaul]

If you didn't have blinkers on you'd understand mate.

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.

 

[D Skelton]

We are discussing 3 single phase circuits with a shared N

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.

 

[netblindpaul]

No you area assuming that the N return currents are in phase with the phase currents.

 

[Guest55]

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.