can someone please explain these statements

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

--
Ai

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 http://en.wikipedia.org/wiki/Root_mean_square 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 http://en.wikipedia.org/wiki/Three-phase_electric_power)

Now 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.