This section covers lines at 132 kV and below - compliance of lines at 275 kV and 400 kV is demonstrated on a case-by-case basis.

The largest fields produced by lines at 132 kV and below are those produced by 132 kV overhead power lines with the physically largest design of pylon, operating at maximum load and minimum clearance.  The largest design in current use in the UK is the L7.  For this worst case line:

These calculations are for the conditions specified in the Code of Practice, which also explains the limit values.

All other current designs of 132 kV overhead line produce lower fields.  All current designs of overhead line at lower voltages are smaller and/or carry lower loads so produce lower fields.  See more calculations for different lines and different conditions.  If any future larger design is introduced we will assess it and add it here if appropriate.

This section covers underground cables at 132 kV and below - compliance with 275 and 400 kV underground cables is demonstrated on a case-by-case basis.

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 physically furthest apart and which carry the largest currents.

Underground cable are not constructed to specific designs as overhead lines are; each one is potentially slightly different.  We therefore take a hypothetical design that has the cores separated by more than any practical cable would, and which carries a larger load than any practical cable would.  If this hypothetical design is compliant, then any practical 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 the geometry and not on the voltage. 

The design chosen has cores separated by 1 m, buried 1 m below ground, and carries a load of 1000 A per phase.

This cable would produce 72 µT magnetic field and zero electric field, calculated for the conditions specified in the Code of Practice.  The magnetic field limit is 360 µT as explained in the Code of Practice.  Therefore this cable, and all practical cables at 132 kV and below, are compliant.

See more calculations for different underground cables and different conditions.

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 round existing substations at all voltages from the highest - 400 kV - to the lowest - 11 kV.  Fields fall with distance, so the highest fields are found at the closest approach, at the perimeter fence or wall.  Based on these measurements:

Large, high-voltage substations: outdoors and indoors

At the perimeter fence of a large, high-voltage substation, the highest fields are invariably produced by overhead lines or underground cables entering the substation. 

Away from these lines and cables, the field would normally be below 1 µT.

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

Lower-voltage substations produce smaller fields.

Smaller, lower-voltage substations: indoors

Modern designs of substation are such that, even when placed indoors, the largest fields are produced by the cables entering and leaving them.

Exceptionally, an older design of indoor substation can involve the "low voltage board" or the "transformer tails" being fastened to one of the exterior walls of the substation, and therefore producing an elevated magnetic field in the room on the other side of the wall (or ceiling).  Even in these circumstances the fields remain compliant.

Substations containing air-cored reactors

There is one type of equipment - large air-cored reactors - that can produce higher magnetic fields.  Air-cored reactors are found in, for example, Static Var Compensators (SVCs) and Manually Switched Capacitor banks (MSC).  If these are located unusually close to the perimeter fence, the elevated field they produce may extend beyond the perimeter fence.  The highest field known to have occurred in such circumstances is less than 20 µT.  It is possible that fields could be as high as 100 µT though this has never been observed in practice.  Although it is believed such fields would never exceed the public exposure limits, evidence of compliance with the limits will never the less be supplied on a case-by-case basis for installations including air-cored reactors, rather than generically as for all other substations.

Conclusion

All fields produced by substations are below the limit value of 360 µT.  All present designs of substation are therefore compliant.  If any radically different design were introduced, we will reassess this conclusion.  For substations containing air-cored reactors, compliance will be demonstrated on a case-by-case basis.

See more information on fields from substations.

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 effectively determined solely by the underground cable and overhead line concerned.  The compound itself contains no components that produce significant field.

At 275 kV and 400 kV, the underground cable and overhead line will be subject (when necessary) to individual assessments of compliance (see above).  But assuming these comply, the sealing end compound will also automatically comply, and no separate assessment is required.