We explain on our main page on low-voltage distribution that magnetic fields arise when the neutral is earthed in more than one place. This allows the current to divert out of the neutral conductor and creates a "net current". We explain here how this happens, specifically, in UK distribution systems.
Protective multiple earthing
Consider first a distribution system as it used to be wired before the introduction of multiple earthing. The neutral of each distribution circuit is earthed at the substation but nowhere else, so no net currents can arise. Each house has an earth either from the earth in the distribution circuit or from a local ground rod, but both of these are entirely separate from the neutral.
Now consider the other extreme, a circuit which is installed with protective multiple earthing (pme). The neutral is still earthed at the substation but it is also earthed at several other places including at the end of the circuit. There is no separate earth wire in the distribution cable. Each house takes its earth connection from this combined neutral and earth conductor. The earth terminal in each house is also bonded to any conducting services entering the house (such as water and gas pipes) to create an equipotential zone. (If you don't have pme, you still have to bond the earth terminal to the metal pipes, but with pme the requirements are more stringent.)
Because the neutral is now earthed at more than one place, net currents are created, which are a source of magnetic field. More on the sizes of net currents etc.
Finally, in practice, many circuits were installed before pme but later converted to pme. They still have separate-neutral-and-earth cables, but the neutral has been earthed at multiple places as required for pme. (It is possible to repair these circuits using combined-neutral-and-earth cables, so sometimes, the earth conductor may not be continuous all the way.) Individual houses may or may not have pme.
There is another way wiring practices can produce net currents. In urban areas, circuits from adjacent substations often meet each other at a link box. Normally, the neutral link is left in, but the phase links are left out, and inserted only if it is necessary to backfeed one circuit from the other substation. So the neutral current in one circuit can divert into the other circuit, creating a net current in both circuits. This applies whether the circuits have pme or not.
Interrupted neutrals
The presence of pme is enough to create a net current. But the effect is much bigger if it is combined with an interrupted neutral conductor.
Prevalance of pme
Protective multiple earthing (pme) has become increasingly common on 400 V distribution circuits in the UK 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. Even where homes do not officially have pme, up to 20% - probably more - may have accidental neutral-to-earth connections.
See also
