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.
