According the the rules in the UK:
- you are allowed to have connections between the neutral and earth conductor in the distribution wiring outside the home, as part of a scheme of protective multiple earthing (pme).
- you are not allowed to have any connections between the neutral and earth inside the home. If the home has pme applied, the neutral and earth will be connected at the cutout position (ie at the electricity meter). But from there on, the neutral should be isolated from earth.
However, many homes have accidental connections between the neutral and earth. This is possible because such a connection can just sit there unnoticed. If you accidentally connect live to earth, you blow a fuse; if you accidentally connect neutral to earth, nothing happens.
Ways in which this can happen include:
- an earthed screw holding a socket outlet or light switch on the box behind it cuts through the insultaion of the neutral conductor
- in appliances involving heat and water (washing machines, water heaters), general corrosion can degrade the neutral insulation and the bare conductor can brush against the chassis
- sometimes, an appliance is just wired up wrongly
Measurements on samples of homes in the UK suggest that many homes have such accidental connections. In measurements made in a sample of homes "as found" the proportion showing evidence of neutral-earth connections was up to 20%. But when scientists actively looked for such connections they were found in 70%. In some (but not all) this results in the house wiring becoming a significant source of magnetic field: .