Low-voltage distribution

Distribution wiring is the main source of magnetic fields in homes in most countries including the UK and USA. The way in which distribution wiring produces magnetic fields is quite complicated. This page explains how.  It deals with low-voltage wiring outside the home - see here for house wiring

 Overhead and underground distribution

thumbnail of 400 V separate phases wood poleSome homes (a minority in the UK but more in the USA) have 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 UK homes have underground distribution, where the individual conductors are very much closer together within a single sheath. This diagram shows 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.

 

 

 

 

diagram of circuits with no earths

Net currents

However in practice the situation is more complicated, because of something called "net currents".  These are produced when the neutral conductor is earthed or grounded in more than one place.

 diagram of circuit with earthing


With multiple earthing, 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 the circuit has a "net current". It 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.
 

Distribution wiring in specific countries

UK

In the UK, a system called "protective multiple earthing" has been progressively introduced and produces net currents.  See here for a full explanation of UK distribution practices.  See also what we know about the sizes of net currents in the UK.

USA

In the USA, both primary and secondary distribution circuits are multiply grounded.  See here for a full explanation of USA distribution practices.

See also:

Distribution voltages

In the UK and Europe, 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.  In the USA the voltages in use in homes are 120 V and 240 V.