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Distribution
Fields from distribution wiring
Distribution wiring is the main source of magnetic
fields in homes in the UK. The way in which distribution wiring
produces magnetic fields is quite complicated. This page explains
how.
Distribution voltages
The electricity that goes into a normal home is
at a voltage of 230 V. But for technical reasons to do with something
called “three phase electricity”, distribution circuits
supplying electricity to several houses at once are usually referred
to as “400 V” circuits.
Overhead and underground distribution
A minority of homes have 400 V distribution with
separated-phase overhead wiring – the individual conductors
are separated, usually by 0.3 m or so. With separated phases, magnetic
fields can arise from the load currents on the conductors, just
as with transmission lines.
However, most homes
have underground distribution, where the individual conductors
are very much closer together within a single sheath. Figure
1 represents a simple circuit of this type, where the load current
drawn by a house passes out along the live or “phase”
conductor and back along the neutral conductor. The currents
are exactly balanced. Each conductor produces a magnetic field,
but because the conductors are extremely close together, the
magnetic fields cancel, and there is negligible external field. |
A simple distribution circuit: only one
earth
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Net currents
However in practice the situation is more complicated, because of
something called "net current".
Protective multiple earthing
Figure 2 shows, protective multiple earthing (pme). Pme has become
increasingly common on 400 V distribution circuits since it was
first introduced with pilot schemes in the 1930s, and is now applied
to about 85% of overhead circuits, 65% of underground circuits and
30% of supplies to individual consumers in England and Wales. Pme
involves earthing the neutral conductor at points along its length
and bonding it to other services. Even where homes do not officially
have pme, up to 20% may have accidental neutral-to-earth connections.
With pme, some
fraction of the neutral current in a circuit can divert out
of the neutral conductor and return to the substation through
water pipes, gas pipes, sewers, or the ground itself. The
currents are no longer balanced, and both cables have a "net
current". This produces net currents not only in the
distribution circuits but also in any conducting utilities,
all of which contribute to the background magnetic field in
homes. The magnetic field from net currents, varying as the
inverse first power of distance,
forms the background field in the majority of homes.
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A more complicated distribution circuit:
multiple earths
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Sizes of net currents
Virtually every distribution circuit in the country has a net current,
but its magnitude depends on the impedances of individual pme links
and interconnections between circuits, making it difficult to predict.
Studies undertaken by National Grid found the average net current
in a sample of underground 400 V distribution circuits in urban
areas to be 3.6 A, which on average was 15% of the neutral current.
The geometric-mean background field in a sample of homes throughout
the country (caused predominantly by net currents) was 0.036 microtesla
(µT). Background fields typically vary between homes from
below 0.01µT to above 0.1 µT (even in the absence of
higher-voltage lines ).
In any given home they also vary with time, broadly following the
daily and annual variations
of load on the relevant circuit.
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