3 sp circuits off a tp isolator

[yellowvanman]

OK, it took me some time to write the last post and further ones came in.

If you're talking about harmonics then yes that could be an issue but that's always an issue, doesn't get worse with 3 x 1ph loads as opposed with 1 x 3ph load.

 

[snowhead]

Where did you get 3 N conductors from the OP said there is ONLY one hence my issue, there is ONE N conductor for all 3 off single phase loads, when are you lot going to get this.

Yes, misread that, couldn't believe it actually meant that.
So the installation is potentially dangerous if the neutral fails at the switch as 2 phases will then appear across the sockets.

If the neutral runs to all sockets on 1 phase first, then 2nd and then 3rd phase the neutral to the switch will be overloaded
If it crosses between sockets on different phases it will balance to some extent.

 

[netblindpaul]

Where can the 3ph currents balance out in 3 off 1ph circuits then?
In a 3ph circuit YES no bother.
How can you guarantee that the return current from 3 off 1ph circuits will balance out so as not to over load a 2.5mm N conductor which is common to all 3 off 1ph circuits each of which can carry 20A independently and you have NO control over what is connected to them, or how the connected loads will affect the power factor, that is current/voltage relationship?

No one has so far given a satisfactory explanation, you are all talking about 3ph circuits, which I would wholeheartedly agree, but we are NOT discussing a 3ph circuit.

We are talking about 3 off 1ph circuits sharing an N, and they are feeding socket outlets, so you have NO control over what is connected, so non-linear loads etc. etc. etc.

The only thing you can rely on is the over current device protecting each 1ph circuit, which in this case is 20A per cct, that is 60A all flowing down a 2.5mm N.

If these 3x2.5 were feeding something other than 3 off single phase socket circuits sharing a common N conductor then this would not be such an issue.

Forget the phase balancing, forget your root 3's forget your 3ph loads, there are none.

You don't know what is being connected, you have 3 independent single phase circuits, that can have anything connected to them.
Yes there could be situations where they would cancel, but if you have 3 off single phase utilisations with each carrying 20A how can you guarantee that the N return currents will all be suitably phased so that they do not overload the common N conductor when you have no control or no idea of what the connected loads will be!

- - - Updated - - -

Yes, misread that, couldn't believe it actually meant that.
So the installation is potentially dangerous if the neutral fails at the switch as 2 phases will then appear across the sockets.

If the neutral runs to all sockets on 1 phase first, then 2nd and then 3rd phase the neutral to the switch will be overloaded
If it crosses between sockets on different phases it will balance to some extent.

Hallelujah!

 

[Deleted member 72666]

Whoever wired up this arrangement in the OP did a crap job! Amongst other things why did they skimp on the neutrals anyway? I don't think we would be talking about this if all 3 circuits had their own neutrals.

 

[netblindpaul]

Look, we are not discussing 3ph loads here.
How can you even think about it.

The OP has stated, 1 off 3 pole mcb.
1 off 5 core cable feeding 3 separate 1ph circuts.
Each 1 ph circuit is protected at 20A.
There is at no point a connection between the phases.
All of the return current up to the point of balance has to be carried by the N conductor as you have no idea what it can be.
These are socket outlet circuits, thus you have no idea of what is connected.
I'm not even considering the back feeding in the event of a lost N connection.

Once you get back to the "real" 3ph circuits then you can look at balancing, in general 3ph circuits are generally balanced, in the same way we normally try to balance 1ph loads across a 3ph supply.

Look at this another way.
L1 carries 20A, L2 carries 20A, L3 carries 0A, how will this balance out in the SINGLE N conductor feeding BOTH circuits, i.e. 20A down L1 PLUS 20A down L2, when you have NO idea of the linearity of the load?

- - - Updated - - -

Whoever wired up this arrangement in the OP did a crap job! Amongst other things why did they skimp on the neutrals anyway? I don't think we would be talking about this if all 3 circuits had their own neutrals.

stan,
If each line had its own N, then I doubt I would have even commented!

 

[Deleted member 72666]

Paul
Just before I wrote that I re read the original post and to be honest I'd either missed or misread the bit with the shared neutral, so in light of that I've backtracked a little and can see your point.

 

[johnboy6083]

I think your having a "senior moment" Paul. I know what type of work ect you do, and you have impressive credentials from what I remember, but in this instance you have the wrong end of the stick.

when you feed a TPN DB for general building services from a BILL fuse board, You would use a 4 core or a 5 core cable. You would have no idea of exact loading of the phases. The maximum loading on a neutral on a conventional three phase system (not including harmonics) is equal to one phase.

What this thread is discussing is no different really to a standard small three phase service cutout. The neutral size is equal to the phase conductor.

 

[netblindpaul]

stan,
No hard feelings, however, I stand by my stance that 3 off 1ph circuits fed with 3x2.5 phase from a 3ph MCB, and a common 1x2.5 N conductor for all 3 circuits is unsafe and could result in an overload of the N conductor.

 

[netblindpaul]

I think your having a "senior moment" Paul. I know what type of work ect you do, and you have impressive credentials from what I remember, but in this instance you have the wrong end of the stick.

when you feed a TPN DB for general building services from a BILL fuse board, You would use a 4 core or a 5 core cable. You would have no idea of exact loading of the phases. The maximum loading on a neutral on a conventional three phase system (not including harmonics) is equal to one phase.

What this thread is discussing is no different really to a standard small three phase service cutout. The neutral size is equal to the phase conductor.

johnny,
No it is not quite the same.
The N is common from the board to ALL 3 single phase circuits fed from a 3ph MCB.
In a 3ph install the N conductor would be sized to carry the current of the circuit protective device at the origin.
In this case you have 3x20A single phase circuits sharing the same 2.5N conductor.
In your case you would have an N conductor matched to the circuit protective device.
So no issue.

Just re-read your post johnny, in this case the N conductors is 1/3 the size of the combined phase conductors, so this is NOT safe.

 

[Deleted member 72666]

stan,
No hard feelings, however, I stand by my stance that 3 off 1ph circuits fed with 3x2.5 phase from a 3ph MCB, and a common 1x2.5 N conductor for all 3 circuits is unsafe and could result in an overload of the N conductor.

You are absolutely correct! The text in bold is the bit I missed / overlooked! My apologies Paul!

 

[netblindpaul]

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.

 

[Deleted member 72666]

Paul I bet it feels like you've done 10 rounds with Mike Tyson!

 

[netblindpaul]

10,
Well not rounds in the ring with Tyson, probably cans of beer trying to calm down!

 

[electric will]

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.

 

[johnboy6083]

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.