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Joined: Jul 2003
Posts: 18
Hi there fellow sparks,

I'm hoping some kind soul would refresh an old memory of any precautions I should bear in mind when rewiring a large open plan office (Primarily IT equipment);

All the wiring will be contained within skirting trunking, and there are around 180 DSSO's to be had, probably wired something like 10 doubles per MCB.

I'm proposing that this should be spread about the three phases, to balance loads etc, thus having 60 doubles per phase.

Now, the sockets are very evenly distributed around the skirting, and each socket will be within human reach of the next.

Are there any regulations, or special precautions I should be taking, knowing that in this situation, it would be possible for a monitor to be on red phase, and base unit on blue phase.

I always over-worry when working on three-phase - it's what you get for spending a long time on domestic work :-)

Any tips & pointers are welcome.

Have a nice day, y'all [Linked Image]


Joined: Oct 2002
Posts: 134
Hi Steve,
All that is required nowadays is to provide a warning notice of the maximum voltage present between adjacent, simultaneuosly touchable accessories on different phases.
See Reg 514-10-01 of the current edition of BS 7671.

Hope this helps.

Joined: Sep 2002
Posts: 1,498
Likes: 1
C-H Offline
The concern with having sockets on different phases seems limited to Britain. Neither the Americans nor us Continentals seem to bother. Should the electrical installation or the appliances lack earthing there would be a real danger, but that's not the case in modern installations. The protective earth will provide isolation between the phases and there is really no safety concern. It seems four failures have to occur for someone to recieve a 400V shock: Both the basic insulation and the earthing in two appliances need to have failed.

[This message has been edited by C-H (edited 01-19-2004).]

Joined: Aug 2001
Posts: 7,520
Yes, the IEE does always seem to have been rather obsessive about this compared to most other places.

You could group outlets around the trunking to an extent, e.g. ten on red phase, next ten on yellow, next ten on blue etc. Then you'd just need a warning label between groups where adjacent outlets are on different phases:

[Linked Image from]

[This message has been edited by pauluk (edited 01-20-2004).]

Joined: Jun 2002
Posts: 159
Interesting point,David. Would you be kind enough to read this Regulation again? I have just re-read it and it uses Uo, the nominal voltage to earth, not the voltage between lines. I therefore do not think that in a standard three phase and neutral installation in the UK that there is any need to concern ourselves.
Any way,Steve, you should perhaps have a wee look at the Regs in Section 607 relating to protective conductor currents, likely where IT equipment is to be installed. You might also need to consider the csa of your neutral given the likelihood of harmonics.


lyle dunn

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