Static field limits

This site concerns the 50 and 60 Hz fields that are produced by AC power systems.  In the UK up to now, DC has been used only really for the undersea interconnector to France.  But in the next few years it is quite likely that we will be building some DC connections within the UK.  See the Energy Networks Strategy Group report for more information.

DC electricity produces static fields.  We summarise here the exposure limits that apply to static fields.  See also other information on static fields on this site.

The UK public limits in summary

For static magnetic fields, the limit is 40 mT, where the time of exposure is significant, and with certain exceptions.

For static electric fields, there is no limit, but a suggestion that fields above 25 kV/m should be avoided.

The UK public limits in detail

As described here for AC fields, the policy for public exposure in the UK is that we apply the 1998 ICNIRP exposure guidelines in the terms of the 1999 EU Recommendation.  The 1998 ICNIRP Guidelines cover only AC fields, not DC fields.  For DC fields, the 1999 EU Recommendation uses the values from the earlier 1994 ICNIRP Guidelines for static magnetic fields.  The 1994 ICNIRP limit for static magnetic fields, included in the EU Recommendation, is 40 mT.  So that applies in the UK, though, in accordance with the EU Recommendation, only where the time of exposure is significant.

That is in line with the then NRPB's advice to Government in 2004:

"General public exposure
100 It is concluded that restricting time-weighted average magnetic flux density of 40 mT for whole-body exposure is appropriate for the general public.
101 Exposures in excess of 40 mT are appropriate for occasional access to special facilities under controlled conditions provided that the occupational exposure restrictions are not exceeded.

Recommendation
102 The ICNIRP exposure guidelines should be used for restricting occupational and general public exposure to static magnetic fields (see the appendix)."

In 2009 ICNIRP published new guidelines on static magnetic fields.  These raise the public limit to 400 mT.  But this probably does not change the situation in the UK which is governed by the 1999 EU Recommendation.

UK limits on electric field

The 1999 EU Recommendation does not contain any limits for static electric fields.  Instead, there is a statement:

"For most people, the annoying perception of surface electric charge will not occur at field strengths less than 25 kV/m.  Spark discharges causing stress or annoyance should be avoided."

Occupational limits

We set out here the occupational as well as the public limits for the two ICNIRP Guidelines.

 Public limitsOccupational limits
ICNIRP 1994

40 mT

Occasional access to special facilities allowed under appropriately controlled conditions up to occupational exposure limits

Time weighted average over the work day: 200 mT

Ceiling: 2 T

Limbs: 5 T

ICNIRP 2009400 mT

2 T

With control of movement: 8 T

Limbs: 8 T

The EU Directive 2013 on occupational exposure uses the ICNIRP 2009 values.