Electricity Substations and Power Lines

Local area substations are the components of the distribution system which convert electricity at 11 kilovolts to 415 volts and thereby provide the link with the domestic electricity supply. They are sources of magnetic and potentially electric fields.

NRPB has surveyed representative local area substations in order to characterise magnetic fields along enclosure boundaries. Magnetic field measurements taken at 1 m above ground level revealed an overall magnetic flux density of 1.1 microteslas, with individual substation flux density means ranging from 0.1 to 6.6 microteslas. Highest individual measurements of magnetic flux densities in the range 2 to 10 microteslas were encountered in close proximity to the feeder cables. In all cases, at distances varying between 5 and 10 m from the boundary fence, magnetic fields due to substations were undetectable above between 0.02 and 0.05 microteslas; levels typical of low household magnetic fields associated with the electricity supply system. Along the path of cables and lines, magnetic field strengths of up to 1 microtesla were measured.

A National Grid Company survey [1]of suburban substations, with measurements taken at 0.5 m above ground level within 1 m of enclosures, revealed mean magnetic flux densities of about 1.9 microteslas falling by a half over an average distance of 1.3 m, and in the vicinity of housing becoming indistinguishable from the background due to other domestic sources within 5 m.

Electric field strength measurements close to local area substations indicate that electric field strengths are often below 1 volt per metre and this is attributed to the shielding provided by the metallic casing on components and cables, and to the enclosure walls. Only where overhead feeder lines occur, are electric fields likely to exceed a few volts per metre. Up to a few tens of volts per metre have been measured beneath associated high voltage supply lines; circuit configurations which are generally rare.

The power frequency magnetic fields recorded around local area substations are much less than the ICNIRP reference levels for public exposure of 100 microteslas and 5 kilovolts per metre, which are based on preventing well established biological effects [2].

1. Renew, D C, Male, J C and Maddock, B J. Power-frequency magnetic fields: Measurement and exposure assessment. 36–105, CIGRé, 1990 Session, Paris (1990).

2. NRPB. Advice on limiting exposure to electromagnetic fields (0-300 GHz). Doc. NRPB, 15, No. 2, (2004). July 1996  Documents of the NRPB 15/2 (PDF, 500 KB)

July 1996

Updated: March 2004