According to UK National Grid data there is an average ground current leakage of 15%. This leakage, otherwise known as net currents represents an average figure of 3.6 amperes at the source of distribution.
So what does this mean?
When an electrical current flows it is accompanied by a corresponding magnetic field. So these net currents are the main source of magnetic fields within our homes. Magnetic fields can have carcinogenic results and the WHO has classed them as such. If you suspect you are being exposes to magnetic fields it may be worth having your home professionally surveyed.
So what causes these net currents?
They are the result of the UK electrical distribution policy called Protective Multiple Earthing (PME) which requires the neutral line from the distribution transformer to be grounded multiple times (hence the “M” in PME) which allows different current strengths to enter the earth instead of return to the distribution transformer.
The amount of net current in the ground at your property will largely depend on how much current you are using. Theoretically there should be no leakage of current inside your property – except at the electric meter. However, scientists who actively look for accidental neutral–ground connections within properties have found up top 70% of properties do in fact leak current into the ground inside the property, giving rise to net currents inside the house.
Sources:
http://www.emfs.info/sources/distribution/uk/
http://www.emfs.info/sources/house-wiring/