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
  
• There is an interuption (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 necessarilly 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.

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

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

SAGE made recommendations to eliminate these specific features

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