Compliance with public limits
Statement of compliance with public exposure limits
This page provides a formal statement of compliance by the UK electricity industry for all equipment used on UK electricity networks with the EMF public exposure limits in force in the UK.
The exposure limits are detailed here. In summary, for public exposure, the UK complies with the 1998 ICNIRP Guidelines in the terms of the 1999 EU Recommendation.
There is a Code of Practice agreed between the Energy Networks Association and the Government, which specifies how compliance will be determined. Among many other details, it sets out how to demonstrate compliance with the public exposure limits. It splits electricity equipment into two categories:
- Equipment that is always compliant with public exposure limits through design
Evidence of compliance is given on this page along with generic Certificates of Compliance for each equipment type in this category. - Equipment where compliance must be demonstrated
The Code of Practice specifies that compliance is demonstrated on a case-by-case basis. The details of equipment that fall into this category are provided along with the principles to demonstrate compliance.
Equipment that is always compliant with public exposure limits through design
Each equipment type that is inherently compliant with public limits is noted below, including evidence and Certificates of Compliance.
Overhead lines which operate at 132 kV and below are inherently compliant as even the largest capacity lines cannot exceed the public exposure limit by design. The largest fields produced by lines at 132 kV and below are those produced by 132 kV overhead power lines with the largest design of pylon, operating at maximum load and minimum clearance. The largest 132 kV design in current use in the UK is the “L7” pylon, which can carry a maximum of 1400 Amps per circuit. For this highest field-case line, the maximum calculated electric and magnetic fields are:
| Electric field | Magnetic field | |
|---|---|---|
|
Worst-case design criteria: 132 kV, “L7” pylon 7 metre clearance of conductors to ground 1400 Amps per circuit |
3.6 kV/m | 40 µT |
| Public exposure limits | 9 kV/m | 360 µT |
| Conclusion | COMPLIANT | COMPLIANT |
These calculations are for the conditions specified in the Code of Practice below, which also explains the limit values.
All current designs of 132 kV overhead lines produce lower fields than those included in the table above. All current designs of overhead lines at voltages below 132 kV are smaller and/or carry lower loads, so produce lower fields. See more calculations for different lines and different conditions here.
Underground cables do not produce external electric fields because they are surrounded by a metal sheath which screens the electric field.
The largest magnetic fields produced by an underground cable are produced by the design of cable where the individual cores are furthest apart, and which carry the largest currents.
Underground cables are not constructed to specific designs as overhead lines are; each one is potentially slightly different. We therefore take a hypothetical highest field-case design that has the cores separated by more than any physical cable would, and which carries a larger load than any physical cable would. If this hypothetical design is compliant, then any physical design, which will produce lower fields, will also be compliant. This hypothetical design could be operated at any voltage, as the magnetic field depends only on the current and geometry, and not on the voltage.
The hypothetical design has cores separated by 1 metre, buried 1 metre below ground, and carries a load of 1000 Amps per phase. For this highest field-case design, the maximum calculated electric and magnetic fields are:
| Electric field | Magnetic field | |
|---|---|---|
|
Hypothetical highest field-case design criteria: Cable cores separated by 1 metre 1000 Amps per phase |
0.0 kV/m | 72 µT |
| Public exposure limits | 9 kV/m | 360 µT |
| Conclusion | COMPLIANT | COMPLIANT |
Fields from substations are usually measured rather than calculated. Calculations are not usually feasible because of the complex geometry of the current paths within a substation.
The electricity industry has performed extensive measurements around existing substations at all voltages from the highest (400 kV) to the lowest (11 kV). Fields reduce with distance, so the highest fields are found at the perimeter fence or wall.
Large, high-voltage substations: outdoors and indoors
At the perimeter fence of a large, high-voltage substation, the highest fields are produced by overhead lines or underground cables entering the substation.
Electric fields are extremely low because of the screening effect of the perimeter fence or, for indoor substations, the building.
Smaller, lower-voltage substations: outdoors and indoors
Substation designs are such that, the largest fields are produced by the cables entering and leaving them. These lower-voltage substations produce even smaller fields than higher voltage substations and will always comply with the exposure limits.
The most common of these lower-voltage substations are local distribution substations and there are many hundreds of thousands of these throughout the UK, each typically supplying up to a few hundred houses. These substations are an essential and integral part of the electricity distribution system and have to be located close to the area they supply.
Sealing end compounds
A sealing end compound is where the transition is made from an underground cable to an overhead line.
The field produced by a sealing end compound is determined by the underground cable and overhead line. The compound itself contains no components that produce significant fields.
At 275 kV and 400 kV, the underground cable and overhead line will be subject (when necessary) to individual assessments of compliance (see tabs above). Assuming these comply, the sealing end compound will also automatically comply, and no separate assessment is required.
Equipment where compliance must be demonstrated
The following equipment normally complies with the public exposure limits. But the Code of Practice specifies that this will be demonstrated on a case-by-case basis when required, for example when applying for consent or an easement for a line or cable.
- Overhead lines at 275 kV and 400 kV
- Underground cables at 275 kV and 400 kV
- Substations containing air-cored reactors
How do you demonstrate compliance?
In the UK, the Code of Practice on compliance specifies that calculations should be carried out using the following conditions:
- For electric fields: for nominal voltage and design minimum clearance for overhead lines (excluding reduced clearances that occur only during exceptional ice loading).
- For magnetic fields: for the highest rating that can be applied continuously in an intact system (i.e., including ratings which apply only in cold weather, but not including short-term ratings or ratings which apply only for the duration of a fault elsewhere in the electricity system) and, for overhead lines, design minimum clearance.
- For both electric and magnetic fields: for 1 m above ground level on a plain, level surface.
- For both electric and magnetic fields: for the 50 Hz field only, ignoring harmonics.
The reason given for these is:
"Government policy is that the ICNIRP guidelines for the general public will be observed in areas where the land use is such that exposure might be for a significant period of time. Therefore, it is not appropriate to assess compliance for extreme, rare, or unlikely situations."
Where do the public limits apply?
In the UK, the public limits apply where the time of exposure is significant, rather than anywhere and everywhere where the public have access. This specifically refers to residential properties, and to properties where members of the public spend an appreciable proportion of their time.