3 phase and borrowed neutrals?

[Philip Saunders]

Hi everyone,

I want to repurpose an old cable that was previously used to power a 3-phase machine. The cable is 4L1.5 micc, and I don't want to replace it unless I really have to, as the cable run is convoluted/inaccessible to say the least. It was obviously installed before many other things (such as floor joists, walls etc) were built - maybe that's why they used MICC as it's all but indestructible - the builders knew it would be next to impossible to repair should it ever fail.

In any case, I want to use this cable as a feed to a small consumer unit that will power 2 immersion heaters and provide power to some control valves, via 2x16a and 1x6a B-curve RCBO. The cable is fed by a 3-pole 20amp C-curve MCB, with neutral being taken straight from the neutral bar in the DB unswitched. I'm aware this might not provide much selectivity, but if the main MCB trips, it won't be the end of the world. They're only backup heaters anyway.

After reading some other posts it would appear that I can't do this due to the fact that the neutral would be shared by all 3 final circuits.

Given that the main DB for the property only has 1 neutral that is shared by all 29 circuits (mostly single phase but some 3-phase), I'm just curious as to why my 'sub main' would be considered a borrowed neutral and the rest of the circuits in the house aren't?

 

[westward10]

Parallel the cores into a single phase supply, the consumer unit will be single phase.

 

[Philip Saunders]

Parallel the cores into a single phase supply, the consumer unit will be single phase.

I could - the cable as is has all 3 phases but I could combine them and change the feed to a single phase MCB ... I'd have to parallel the neutral too tho. Hasn't thought of that option.

 

[James]

A distribution circuit, 3 phases and a N from a single source to a distribution board is not a “shared N”
although obviously all the circuits being fed from the distribution board will share it, it is considered a single distribution circuit as it is fed from a single source.

the term shared N would relate to more than 1 single phase or multiphase circuits using the same N cable.

 

[Lucien Nunes]

Yes it's all about the definition of a circuit. If you were to put one 3-pole MCB in the main board and connect the MICC to that and neutral, you have created one 3-phase 4-wire circuit of which all the conductors are separate from any other circuits. But if you fit three single-pole MCBs instead, you have by definition created three single-phase 2-wire circuits, which require a total of six conductors for them all to be separate from each other.

If the three single-phase circuits are all on different phases, the common neutral would be large enough for safe operation, but would not allow independent isolation or testing. There would be a risk for example of someone locking off one MCB to work on one immersion heater, but leaving the other one energised. Disconnecting the common neutral could then result in 230V appearing on the supposedly isolated circuit. This would not occur on a single 3-phase circuit as the TP MCB would ensure that all lines associated with that neutral were isolated together.

 

[James]

Yes it's all about the definition of a circuit. If you were to put one 3-pole MCB in the main board and connect the MICC to that and neutral, you have created one 3-phase 4-wire circuit of which all the conductors are separate from any other circuits. But if you fit three single-pole MCBs instead, you have by definition created three single-phase 2-wire circuits, which require a total of six conductors for them all to be separate from each other.

If the three single-phase circuits are all on different phases, the common neutral would be large enough for safe operation, but would not allow independent isolation or testing. There would be a risk for example of someone locking off one MCB to work on one immersion heater, but leaving the other one energised. Disconnecting the common neutral could then result in 230V appearing on the supposedly isolated circuit. This would not occur on a single 3-phase circuit as the TP MCB would ensure that all lines associated with that neutral were isolated together.

As always, @Lucien Nunes explains what I am trying to say in a way that others can understand.
just one of this guys many talents.

 

[Philip Saunders]

Yes it's all about the definition of a circuit. If you were to put one 3-pole MCB in the main board and connect the MICC to that and neutral, you have created one 3-phase 4-wire circuit of which all the conductors are separate from any other circuits. But if you fit three single-pole MCBs instead, you have by definition created three single-phase 2-wire circuits, which require a total of six conductors for them all to be separate from each other.

If the three single-phase circuits are all on different phases, the common neutral would be large enough for safe operation, but would not allow independent isolation or testing. There would be a risk for example of someone locking off one MCB to work on one immersion heater, but leaving the other one energised. Disconnecting the common neutral could then result in 230V appearing on the supposedly isolated circuit. This would not occur on a single 3-phase circuit as the TP MCB would ensure that all lines associated with that neutral were isolated together.

Thanks for that explanation - the source of the circuit is a 3-phase breaker (but not neutral - I could change that that to a TP+N if it makes a difference) and switching that off to isolate the board I want to install wouldn't be an issue.. The board would have it's own TP+N incomer anyway. Only at that point would it become 3 separate single phase circuits.

If I can't do it, then that's a pain but I'll deal with it. I just like to understand the reasons. I won't be doing the actual connections as I need a certificate to certify it, just the mechanical stuff like fixing the CU to the wall and drilling holes and such. Just want to know what to ask the sparky for - they'll be much more cost involved if I/they have to run new cables.

 

[Lucien Nunes]

It's fine, if you are launching with a TP MCB; that makes it one 3-phase 4-wire distribution circuit which only needs one neutral. It might be the smallest distribution circuit on the planet and as you say, you have to ignore the selectivity issue, but it's fine as far as the neutral is concerned.

Don't switch the neutral (assuming TN supply) as it's always worth keeping it as solid and reliable as possible.