House wiring

Normal, correctly functioning house wiring should not be a source of magnetic field, because wherever there is a current, there should be an equal return current in the same cable, and the magnetic fields cancel each other out.

However, house wiring can become a significant source of magnetic field in a minority of homes if one of the following happens (more on the number of homes where house wiring is a source of field):

  • There is a connection somewhere in the house between neutral and earth, so that some of the neutral current can flow out of the cable into the earth.  Then the currents are no longer balanced - there is a net current - and a magnetic field is produced. more on net currents.  See more on this below.
  • There is an interruption (or even just a high-impedance joint) in a ring main.  Then the current can split in different proportions round the two arms of the ring and the whole ring main circuit can act as a large coil producing magnetic field.
  • A circuit is wired so that the current flows in different cables which aren't necessarily close to each other.  This happened with some old styles of underfloor heating (current systems avoid this and are not a source of field) and it can sometimes happen with two-way switching of lights - see below.

Two-way switching of lights

Two-way switching wired the wrong way: creates a loop

diagram showing wrong wiring of lights

Two-way switching wired the right way: live and neutral kept together, no loop.

diagram showing correct wiring of lights

SAGE made recommendations to eliminate these specific features

 

Accidental neutral-earth connections

Accidental neutral-earth connections

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.

Electric fields

For electric fields, the cancellation effect is not so effective, and house wiring is often the principal source of field in the home.