Phases?

[The Medway Handyman]

Working for a regular customer in an industrial unit today & he asked me to check why a 230v socket wasn't working. Socket tester showed no live but all wires were properly in place. No mcb tripped on the panel.

Found another socket on an adjacent wall not working either, but sockets in between them work fine.

There is metal trunking running all around the unit just over head height & conduit running down to sockets. Since its a cash & carry type place I couldn't get to a lot of the sockets to check them because of the display shelving. Must be at least 20 double sockets though judging by the conduit drops.

Big scary cabinet on wall with stickers saying 415v and 100 amps - not going anywhere near that!

I'm guessing, but are industrial units wired up with some 230v sockets running from each of the three phases? Is this to spread the load out?

Seems logical to me if some sockets are fine & others not. Is every third socket from phase A, every forth socket from B etc?

He is getting a sparky in, but I'm curious.

[Steve Walker]

Yeah, it's something like that - we have problems at work sometimes where we lose a 'random' assortment of 13a sockets due to a phase going out.

[Chas]

Leave it to the sparks, what do you know. You is only a handyman.

[The Medway Handyman]

But a handyman who likes to know about these things..............

[nightjar]

I have accidentally worked on live 415v, but putting down an insulating floor mat first and automatically making sure I never touched two different metal objects at the same time, meant I didn't notice until I traced the cables later.

Maybe, maybe not. It was always considered good practice to keep at least 6 feet between sockets on different phases. If they are closer, they are likely to be wired in blocks, with several adjacent sockets on one phase. If they are widely spaced, they might be wired on alternate phases. Again, good practice would be to mark phase colours, either with phase disks by the socket or by sleeving the live with a phase colour.

Colin Bignell

[Chris J Dixon]

Isn't there a requirement for a warning sign if this is not the case?

Chris

[Andy Hall]

... but probably bigger than you.....

[nightjar]

I've not done any three phase since the latest regs came in, so I'm not up on them, but there never used to be. The requirement was for a warning sign where two or more phases are present inside equipment that might normally be expected to be single phase, such as light switch banks.

Colin Bignell

[David Hansen]

On Wed, 24 Oct 2007 21:19:18 GMT someone who may be "The Medway Handyman" wrote this:-

If the building is large enough for a three phase supply to be sensible.

It is to attempt to balance the load on each phase as much as possible. If they are perfectly balanced then there is no current in the neutral, the more they are out of balance the higher the current in the neutral. Obviously it is not possible to perfectly balance a random collection of single phase loads, but they can balance reasonably well.

Large single phase loads can cause problems within the electricity system, which is why they are discouraged. An exception is AC trains where it is not really possible to make them three phase within the constraints of picking up electricity on a moving train. While the electricity system is generally robust enough not to have a problem with them there are places where the system is not robust enough. In such places there may be restrictions on the number and speed of electric trains, especially when some railway sub-stations are not in use due to maintenance and trains are being fed from other railway sub-stations.

Best practice is to have areas of the building on one phase. The designer will try and split the building into suitably arranged circuits to keep phases separate where the public encounter electricity.

[The Medway Handyman]

Colin Bignell wrote;

Thanks for that Colin, makes sense.

This is an industrial estate built in the late 1960's. I guess the sockets are 8 to 10' apart, alas no markings on the two I looked at.

Looks like they could be wired alternately.

[The Medway Handyman]

I'm bigger than most people :-)

[Andy Champ]

You do realise that 415V is just the difference between two 230V phases?

Andy

[Steve]

He does and he also has the common sense to ask questions rather than just poke about in a system that is unusual for his normal sphere of work.

Steve

[Andrew Gabriel]

Actually, it's the difference between two 240V phases ;-)

[Bob Eager]

And, possibly, the difference between life and death! TMH was right to ask...

[The Medway Handyman]

No. I just know is scares the sh*t out of me so I stay well away :-)

[Dorothy Bradbury]

Sit watching wisps of oil smoke rise from a loop-in single phase lead-sheathed-oiled-paper service cable.

You sit wondering... nah... just my imagination... sip... its coffee steaming... no... whereupon two eyeballs are all that is left hanging momentarily in mid-air as you run.

Neighbours with electric shower, cooker and a welder all combine with your load to play Diversity Roulette.

Never assume anything about 415V installation, leave it to lazy electricians who switch off RCBO by shorting N-E only to find out someone got even lazier with colours.

You would think electricians would figure it out when we cover the floor with rubber & plastic. Get it wrong and we just roll you up in it and turn you into CEF counter staff.

[Andy Hall]

.. and you were telling us that you were a "big guy". Is it a size

28 blouse? ;-)

[Si]

In message , Andrew Gabriel writes

can someone please explain these statements

230 x 2 != 415 & 240 x 2 even more so != 415!

[Bob Eager]

It's (as I recall, rustily) the vector sum of the two 240v components, out of phase by 120 degrees.