When electricity flows through a wire, we can compare it to water flowing through a hosepipe; the current would be the volume of water, and the voltage would be the pressure pushing the water through the hosepipe. To transfer electricity, we always have a voltage and a current. 

Electric fields are produced by voltage and are independent of the current (load) in an overhead line. 

Electric field strengths are measured in :

• volts per metre (V/m)
• kilovolts per metre (kV/m)  1 kV/m = 1000 V/m

The atmospheric electric field at ground level is approximately 100 V/m in fine weather and may rise to many thousands of volts per metre during thunderstorms.

Electricity is supplied to UK homes at a voltage of 230 V but outside homes it is distributed at higher voltages - from 11,000 V (usually written 11 kV) up to 400,000 V (400 kV). Generally, the higher the voltage, the higher the electric field.

Anything which uses or carries electricity is potentially a source of power-frequency electric fields. They affect the Earth's steady natural fields. The strength of the electric field effect depends on the voltage of the equipment, which remains broadly constant when the equipment is energised.

Electric fields are readily screened by most materials including bricks, trees and hedges (unlike magnetic fields).