How earthing practices on UK distribution systems produce magnetic fields
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.
We explain above how the system of protective multiple earthing used in the UK produces net currents which are the main source of magnetic field in most homes. But the net current can be much bigger if pme is combined with an interrupted or broken neutral conductor.
Consider first the situation with an intact neutral conductor:
The live current has to flow in the distribution cable but some of the neutral current can divert out of the cable. This is what produces the net current. The amount of the neutral current which diverts is proportional to the impedances. Suppose the earth connection has an impedance (or resistance) 10 times bigger than the neutral current. Then 10% of the neutral current will divert and the net current is 10% of the neutral current.
Now consider what happens if the neutral conductor is broken for some reason:
None of the neutral current can return in the cable so it all has to divert into the earth (or through a link box into an adjacent circuit - see above). The net current is now 100% of the neutral current.
Accidental neutral-earth connections
Within there home, there shouldn't be any neutral-earth connections, but these do sometimes happen as we describe in our pages on house wiring. They then have a very similar effect to neutral-earth connections outside the home described above.
Sizes of net currents
We give statistics on the sizes of net currents, prevalence of pme, etc, on a separate page.
- the general principles of how distribution wiring produces magnetic fields
- what we know about the sizes of net currents in the UK
- magnetic fields from substations and from high-voltage power lines
- the equivalent wiring practices in the USA