3ph Q

[Rick Hughes]

Question raised by neighbor ...

He has 3ph & Neutral supply (temp supply) on a new build.

He wants to have one 3 ph circuit in garage, to a couple of outlets ........and a single phase supply to house.

He asked me what way should he be preparing the wiring for final connections ...

My assumption would be .. TPN to main isolator. (A) The output of that providing two feeds;

1) One feed to a 3ph distribution board ... fitted with a TPN isolator, and then a TPN mcb for each circuit.

2) Second feed would be SPN ... 'tails' to his consumer unit.

The main isolator (A) providing full isolation of the supply, the TPN isolator on dist board allowing isolating of 3PH circuits The standard DP switch on Consumer unit providing isolation of the house install.

If the above is OK .... a second Q ... In the garage which is the only part of house to have 3ph ... there will also be outlets on a standard 220V ring final circuit.... can these be fed from the main consumer unit ? ... or do they have to come from the 3ph dist board.?

Just wondered if there was any regulation that required SPN circuits to be isolated at same time the TPN is isolated .... you could turn off distb board isolator and assume all was 'safe' whereas in fact the SPN outlets would still be live.

Full isolation is provided by the main 'incoming; isolator (A)

[Andy Wade]

All with the proviso that you must provide fusing within 2 or 3 m of the meter, to satisfy the local DNO's requirements. Hence if either or both of the dis-boards is/are any distance from the origin, (A) will need to be a switch-fuse and not merely an isolator. Also, since this is a domestic installation, the main isolator must break the neutral, i.e. it must be 4-pole. The normal industrial type of TP&N isolator or switch-fuse with 3-pole switch and bolted neutral won't be acceptable.

Also note the usual 17th ed. requirements re, RCD protection.

In general there are no hard and fast rules about things like this. Clear labelling should always make it clear what is fed from where. Isolation (means of) which is remote must be lock-off-able. That said, in this case I'd definitely feed all the garage final circuits from the

3-ph board since that will give a clear divide between "house" and "garage". It's perfectly normal for 3-ph boards to feed a mixture of single- and 3-ph loads, the former being spread between the phases to balance the load.

[Rick Hughes]

Cheers Andy .. seems my logic was sound ... thanks for pointer on the isolator type, I have just looked in TLC catalogue .. can't find any 4 pole ? ..... I see that they mark some as TPN and others as TP&N .... is the TPN designation what I need ?

On the consumer unit side .. all is wired as it should be including RCD requirements.

Are there any specific RCD requirements or 3Ph side ... obviously if we run ring cct for 220V outlets form dis board an RCD will be used for that ... do you need to provide RCD for all 3PH outlets .

I have worked on 3PH in factories but never domestic .. hence my questions.

[Andy Wade]

4-pole types are certainly less common. Most 3-ph installations are non-domestic and don't require a switched neutral at the main switch (unless TT earthed). TPN and TP&N are the same thing so far as I know - TPSN (Three-Phase Switched Neutral) is what you need for all-pole isolation.

Actually I might have misled you a bit here. Checking the regs, the relevant text is "A main switch intended for operation by ordinary persons, e.g. of a household or similar installation, shall interrupt both live conductors of a single-phase supply" [537.1.4, 2nd para.] So if the supply is 3-phase, only the line conductors need to be switched?

RCD requirements are no different between single- and three-phase. 30 mA RCD additional protection is required for:

- all socket-outlets of up to 20 A rating intended for general use;

- mobile equipment of rating up to 32 A for use outdoors;

- circuits where cables are buried in walls at

[Rick Hughes]

Cheers ... all points noted

[Rick Hughes]

The supply is 3Phase plus supplied N

So does that mean just the 3 phases need to be interrupted at incoming ?