Phases?

[Si]

Oh Fsck! - sounds vaguely like something I learned for A-Level Physics in the 70s! I also will stay well away from such boxes. :)

Thanks Bob,

[The Natural Philosopher]

IIRC there is a legal requirement for something like that distance.

[dom]

The UK (and many other countries) use 3 phases equally spaced at 120 degrees (I'll explain the phase angle thing in a mo).

First, what's a phase?

Electricity is generated and distributed in the UK on a 3 wire system, each wire carries a *phase*.

Each phase is an alternating current - it goes 0V, +325V, 0V, -325V,

0V etc, and completes 50 of those cycles per second.

Now for the purposes of power calculations, it's much more useful to know the *average* voltage than the peak voltage. However if we just took the average of the numbers above the answer would be 0V. So we take the RMS (root mean square) which in our case comes out at 230V (and gives identical results in simple power calculations as if it was the same d.c. voltage). We find this by dividing the peak voltage by

1.414 (see for a detailed explanation)

Now this alternating voltage has a shape known as a sine wave. And 3 phase is a set of 3 sine waves, each on it's own wire. Each of the 3 phases is identical in voltage and frequency - the only difference between them is they *peak* at slightly different times. The 3 phases are identified by colours, red, yellow, blue under the old colour code or brown, black, grey under the new colours.

So red peaks, then yellow peaks, then blue, then red again. A useful way of picturing this in many electrical calculations, is to imagine 3 points equally spaced around a circle. 360 degrees in a circle, so they're 120 degrees apart.

Now the answer to your question, because there's a time difference between the phases, but one phase is just a delayed version of another, NOT the exact opposite of another - it's not as simple as adding the numbers together to get the difference in voltage between them. (Again we're interested in the RMS voltage, as the exact instantaneous voltage between them varies all the time - what we're interested in is how much power we can draw by connecting between the phases).

For 3 equally spaced sine waves, we need to multiply the RMS phase to neutral voltage by 1.732 to get the phase to phase voltage. So 230V phase to neutral comes out as 398V phase to phase.

(see detailed explanation here

for industrial premises drawing lots of power, they're usually supplied with all 3 phases and large machines are connected directly to these (and not necessarily to the neutral wire). The main reason being that it's much more practical both to generate electricity and to use it to power large motors with 3 phase.

For the domestic consumer, usually only 1 of these 3 phases goes into your house (along with a neutral wire, nominally at 0V) and you get what you're familiar with - 230V a.c. at 50Hz.

Finally for the sake of completeness, although the phases are nominally sine waves of 230V, 50Hz and their phase difference 120 degrees, in real-world operating condition all of these can change and the calculations get far more complicated.

[nightjar]

I used to think that, but it was discussed at length some time ago, possibly on another group, and nobody could come up with any piece of legislation.

Colin Bignell

[The Natural Philosopher]

Definitely back in e 90's our eletricians fresh out of HNC's said there was..

Or at least 'good practice guidelines'.

Mm. My building reg book ignores electricity except in respect of fire issues.

[Frank Erskine]

AFAICS, there is no mention in the 16th edition of segregation, as there was in earlier editions.

I'm a volunteer at an industrial museum which, until a good few years ago, had night storage heaters. The heating side of these was fed from three-phase, distributed about the building, but the fans were all fed from one phase. Where the phases didn't coincide there were stickers on the double switches feeding each heater, warning of >250V inside.

[Andy Hall]

Perhaps the point was that it would then be impossible to stick your fingers in two sockets on different phases at the same time. 400v across the chest at suitable current would probably spoil the day for most people.

[jim]

/snip//

The most likely explanation is that the fuse (or perhaps a contactor) on one of the phases has blown (or tripped), though there's other possibilities.

There are several reasons for using 3 phase in industrial & high load installations, amongst them being economy of generation and distribution and better control of electrically driven machinery. Downside, if it is one, is that single phase loads need to be equally balanced between phases so far as possible, plus the higher voltage between adjacent single phase fittings when on different phases.

IIRC there was once a 6ft distance between accessible_by_touch single phase fittings requirement (IEE 14th Ed???) but that has long since disappeared from the IEE regs.

However there is one other important reason for using 3 phase derived lighting (ie lighting shared between phases over one area) when rotating electrical machinery is in use and that is to avoid the strobe effect of a motor appearing to be standing still when in fact it is going at full tilt. It is in the back of my head that it is or was a legal requirement under one or other of the Factories Acts or similar, but cannot vouch for it.

Flourescent tubes, including CFLs, flicker much more than ordinary GLS, so is this a looming problem if CFLs become common place?

Lastly, whilst 3 phase is rare in homes in the UK unless used for storage heaters, it is far more common on the continent.

BTW you need a waste transport licence (around =A3340 for 3 years from Environment Agency IIRC) for even thinking about taking away virtuially anything from a job site where you've been working. Even your Mars bar wrapper is deemed scrap by the Blairite shock troops ;-) And yes, even the filaments in the scrap flourescent tubes;-)))

HTH

[Tim Lamb]

In message , jim writes

Quite correct 14th. edition reg. A.20

*All socket outlets in any one room shall be connected to the same phase (or pole of a 3-wire system).

Exemption. -In non-domestic premises, if it is clearly impracticable to comply with regulation A. 20, more than one phase (or pole) of the supply may be utilized provided that all socket outlets on one phase (or pole) are grouped together and are not intermingled with socket-outlets connected to a different phase (or pole) ; and provided that in no circumstances may a socket-outlet be installed at a distance less than 2m from any socket -outlet connected to a different phase (or pole).*

regards

[manatbandq]

Anyone doing that sort of thing is unlikely to be missed from the Gene pool.

MBQ

[The Medway Handyman]

When it comes to 415v 100a its brown trousers :-)

[The Medway Handyman]

Thanks for that matey - I am a wiser man. Still not going anywhere near it though :-)

[The Medway Handyman]

Aha. That sounds right.

This particular unit is now a hair & nail supplies house, full of pink girlie stuff. It was obviously once a real industrial unit doing manly stuff. Loads of sockets, two what look like forklift charger sockets visible behind racks of emery boards, big FO cables now hidden by racks of shampoo.

Everything seems to run off a mcb board which is marked up wih various areas. I do recall once moving a socket for them that wouldn't turn off when the mcb was tripped, the main power switch had to be turned off.

Scrap flourescent tubes seem one up from Anthrax if you read the ads by the recycling companies :-)

[Owain]

It only hurts more if you touch two different phases at once. Which, as you shouldn't be doing live work anyway, shouldn't happen.

Oh, and you have to remember to connect blue to black and black to blue....

Owain

[Frank Erskine]

Black to black Green to the pipes, and Blue to f$ck.

[<me9]

An open switchboard I worked on 30odd years ago had 3 phase knife switches to isolate 415v, and carried about 300A, and had a rubber mat 1/2 inch thick on the floor in front of it.

A routine test about 40 years earlier (when it was fairly new) was to feel the switch blades to see if they were heating up. Someone new decided to check two at the same time to speed up the process. Shortly afterwards it ceased to be a routine check!

[The Medway Handyman]

I cut through a 415v cable once - it was just a phase I was going through...........

[Owain]

Were you feeling blue at the time, or did you see red?

Owain

[Andy Hall]

These days he would have caused a black out.

[cj]

now Andy you know that could be a Grey area it could be called a Brown out . Either way he was probably White with fear.

CJ