Sources of electric and magnetic fields include:
• house wiring
• electrical appliances (toasters, ovens, hair dryers, washing
machines etc)
• national electricity transmission and local distribution.
What determines the strength of electric and magnetic fields?
Electric fields
Electric field strength:
• is directly related to voltage.
• reduces rapidly with distance from the source of the field.
• can be screened effectively by objects (walls, trees, ground). This means that
within homes, electric fields from transmission lines are effectively blocked.
• is measured in units of volts per metre (V/m) and is normally given as kilovolts
per metre (kV/m) where 1 kV/m=1000 V/m.
Magnetic fields
Magnetic field strength:
• is directly related to current.
• reduces rapidly with distance from the source of the field.
• unlike electric fields, is not screened by objects.
• is measured in terms of the magnetic flux density – tesla (T). Measurements
are more frequently given in microtesla (µT), which is one millionth of a tesla.
What are some typical strengths of electric and magnetic fields?
New Zealand’s National Radiation Laboratory, part of the Ministry of Health,
has provided the typical field strengths shown below which are gained from
their experience in measuring fields from a range of sources. These values are
“typical” only in that values outside the range are possible. More detail on typical
field levels around Transpower’s transmission lines are also provided in this fact
sheet for comparison.
Electric and Magnetic
Field Strengths
Voltage is like water pressure –
it drives the electric charge through
a conductor such as a wire.
Current is like the flow of water in pipes,
it is the actual flow of electrical charge
on that conductor at any given time.
2009
FACT SHEET
1 2
4 5
The public exposure guidelines
(provided by ICNIRP and detailed
in Fact Sheet 4) recommended by
the NZ Ministry of Health are 100µT
and 5kV/m for magnetic and electric
field respectively.
Typical range of magnet