Shared Neutral (not borrowed) 3 phase

Treading the line, 314.4 of BS7671 states all circuits must be separate,

I look at the following on paper and looks good?

3 pole breaker feeding 3 x single RCD protected (built in) sockets each on a different phase but sharing the neutral (one per machine),

cables are routed via steel conduit,

I am an engineer not a qualifiued electrician ,

I would rather not have to rewire our current setup as cant fit more cable in the steel conduit !

Any takers on the legitamacy of this, as I said it seems to look good on paper as the sockets are connected via steel conduit essentially making them an isolated single 3 phase outlet !

Nope, that's not right.

If the 3P+N ran to a sub board first then it would be fine.

If it ran to a machine control cabinet it could be fine (depending on how the machine was wired).

If it is literally just 3 sockets on a wall on the end of effectively a 4 core cable then it isn't actually correct. Electrically it will work, but it is not the correct way of doing it.

shared / borrowed are the same thing. the neutral belongs to 1 circuit, the other 2 are borrowing it. its also potentially dangerous if someone isolates 1 circuit and then the neutral is suddenly live one disconnected

Do as Lurch said........TPN to sub-board, use rcbos for final circuits & bin the 7288's

I would argue it is a single circuit with multiple outlets and therefore I can’t see a problem.

Tony S said:

I would argue it is a single circuit with multiple outlets and therefore I can’t see a problem.

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I think the fact it is clearly 3 different single phase BS1363 outlets that is the issue here.

From the one end it is a single circuit because of the TP MCB, but then the socket outlet connected to L1 and N would be on a circuit, which means that L2 & L3 are 2 other circuits.

I wouldn't be surprised if you could actually argue this either way, I'd have to check the book to see what it specifically says.

The three pole breaker makes it a single circuit as far as I’m concerned.

One out, all out!

Tony S said:

The three pole breaker makes it a single circuit as far as I’m concerned.

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The single phase sockets make it 3 circuits for me.

All these years I have been running in 3 neutrals for every 3 radial circuits. You're telling me I should have just been putting them all on a TP MCB and saving myself some cable?

Two thoughts:

If anything happened to that shared neutral, (bad connection, etc), any connected loads could be seriously damaged by over-voltage.

If the loads have switching PSUs problems could occur due to third harmonic neutral currents.

Tony S said:

The three pole breaker makes it a single circuit as far as I’m concerned.

One out, all out!

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good point, i missed that part and thought he was using 3x SP breakers as 3 individual circuits. but yes, a TP breaker would make it 1 circuit

If the neutral  going back to the board was lost  , cut ,  loose connection  ,   yanked out,  the three connected loads would become star  configeration .   

Evans Electric said:

If the neutral  going back to the board was lost  , cut ,  loose connection  ,   yanked out,  the three connected loads would become star  configeration .   

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that would apply to everything. loose neutral at the DB, loose neutral at the service head, loose neutral on a DNO cable

Anyone else here old enough to remember when it was common for a 3P&N circuit to have a smaller neutral core?

just Deke

I know Andy, but final circuits ,  say 3 individual  industrial  sockets   or 3  rows of  lighting  , one on each phase , would need  it's own neutral  .    Thats as I see it anyway , so  although it works , its not technically correct .     Or am I missing something here?

As you say with supply cables ,  we've seen it on a row of houses where the Neutral failed ,  stuff in the houses fried ,  and because snow was on the ground  , you could follow the cable route by the melted snow . 

Andy™ said:

just Deke

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Sometimes I hate Geordies   :B        

Yes I remember coming across SWA's with a reduced neutral .   Could have been a saving on materials issue during the war  .

Stepps will remember that. :innocent  

OK lets put this in to another context.

Say I have a 3Ø+N busbar trunking system and I want 3x1Ø+N circuits at various points along its length do I have to install 3 neutral bars?

Tony S said:

OK lets put this in to another context.

Say I have a 3Ø+N busbar trunking system and I want 3x1Ø+N circuits at various points along its length do I have to install 3 neutral bars?

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As I said, I know it makes no odds and it works fine. It's all about what the regs says though.

Lurch said:

As I said, I know it makes no odds and it works fine. It's all about what the regs says though.

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if its on 1 MCB then its one circuit, so one neutral

if it was on 3x MCBs then 3x circuits so would require a neutral each

The issue you will have I have brought up before if your memory has not yet failed that badly Tony, is harmonics from the potential loads.

This is why 3.5 core cables have gone away, even on the DNO side.

Harmonics especially triplen harmonics could seriously overload the N, whilst the vector sum of currents may be 0 if the 3 loads are equal, if they are not then the N current will be somewhere other than 0, once you start injecting harmonics then you open a whole new can of worms!