Single phase load 3 phase supply

[gazdkw82]

been helping out at a local industrial unit today and something got me thinking. There was an old 3 phase isolation switch that was still live from 3 phase DB2 however there was no load from the switch. The owner wants some power sockets and lighting on the upstairs mezzanine area so the 3 phases/neutral and earth from that isolating switch was pulled and re-routed up to the mezzanine and made safe ready for some sockets on another day.

I can't seem to work out how they will wire single phase sockets with a 3 phase supply.

What's the normal way of practice? Terminate all 3 phases into a 3 phase box and then take a feed for sockets? A feed for some lighting?

All 3 phases are fed from the same 3 phase mcb

 

[Marvo]

Yes, mixed single phase loads such as sockets and light can be supplied from a 3-phase DB. All 3 phases should be terminated into a main triple pole MCB. From there the phases can supply single phase RCD's for socket circuits and single pole MCB's for lighting circuits etc. The single phase loads should be 'ballanced' as well as possible to prevent overloading any of the phases.

 

[gazdkw82]

So from 3 phase db2 the 3 phases would then terminate into another distribution board and supply single loads from there? What would this distribution board be? A 3 phase board? Not sure I understand how the phases would be terminated into a board and then be used as single phases

 

[gazdkw82]

This may help (or not) ha

 

[Marvo]

I hope I'm understanding what your question is but I'm worried I might not be. Here's a 3-phase DB with mixed single phase final circuits as well as a 3-phase final circuit on the right hand side, you can kinda see the wiring in the video. The 3 incoming phases would go into the t/pole MCB. ;

 

[gazdkw82]

Ok I can see that but how do the single phases actually get the power? Is that 3 phase supply on a din rail that all the single phase and 3 phase mcb's operate from?

What's the options if another board is not possible? At the moment the phases/neutral and earth are just wound up in a corner on the mezzanine floor. I didn't get the impression another board was gonna be used

 

[SparkyChick]

So in a three phase DB, you have the main isolator (typically 4 pole I believe), which feeds three sets of bus bars. Which when you look at the board provide an outlet position for phase 1, 2, 3, 1, 2, 3, 1, 2, 3 etc.

For a three phase circuit you bang a 3 phase protective device in which connects to all three phases. For a single phase circuit, you use a single phase protective device which connects to the phase provided in that slot.

 

[Ian1981]

Typically labelled 1L1 1L2 1L3 2L1 2L2 2L3
3L1 3L2 3L3 etc etc.

 

[gazdkw82]

Had a look at the board today. The phase which
Is being fed to the mezzanine is actually being fed from a single mcb. It's the only single mcb on the bkrd which is 3 phase.

The Other 2 phases have been blanked. Is this normal practice?

 

[Richard Burns]

Generally if you only need a single phase supply then it is not worth distributing a three phase supply, however if you are intending to expand later then a three phase supply may be a good idea. It is possible this is what you are describing.
However you should as a whole be intending to balance the loading across the total supply, but this may be fairly irrelevant if you are supplying some sockets and lights while the rest of the installation is using MW across the phases.
If the board being installed is three phase then it is possible to only use the MCB positions for a single phase but is a bit of a waste of money, space and time installing a three phase board.
If a board is mainly three phase circuits and is not full then fitting a single pole MCB and blanking off the other two positions may help to avoid mistakes as it can be possible to install a 3 pole MCB one row out of line so across say L2,L3,L1 rather than L1,L2,L3. Obviously the blanks could be removed if you wanted to supply other single phase loads from the other two phases.
This is a diagrammatic representation of single and three phase loads being supplied from a three phase board.