We describe how the various features of UK distribution wiring produce net currents and hence magnetic fields on a separate page. Here, we give what information is known on the sizes and prevalences of these features.
Sizes of net currents
We explain on a different page how the system of protective multiple earthing on UK distribution systems produces net currents. It is these net currents which are the main source of magnetic fields in most UK homes.
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. Sometimes the net current even arises from accidental connections.
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 (at the point where they left the substation), 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.
Prevalence of protective multiple earthing
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.