At BTS we talk a lot about overhead electricity lines and how they can affect the successful delivery of a development. The decision about whether to divert apparatus underground (to free up more development land) or settle for compensation (in return for that loss), is rarely straightforward and can depend on a number of different factors:
- the operating capacity of the line.
- whether any wayleaves and easements apply.
- the cost of diverting the apparatus – would this outweigh the cost of compensating the landowner?
- whether planning permission is in place, and if so, does the permission depend on removal of the apparatus?
Overhead transmission and distribution lines operate at different voltage levels, each serving a specific purpose:
11kV (supply)
Primarily used to supply electricity locally to residential, commercial, and industrial areas. These lines are less intrusive than higher voltage lines but can be more susceptible to faults due to lower insulation levels and exposure to environmental factors.
District Network Operators (DNOs) may resist paying the full cost of diverting an 11kV if it is likely to be used as a point of connection for the future development.
33kV, 66kV and 132kV (distribution)
These lines carry power from substations to regional distribution networks. They require more robust infrastructure, with taller H Poles, steel lattice towers, and wider spacing between conductors. As such, they are considered to have a significant environmental impact, due to their presence in urban and semi-rural areas.
If less than three spans of a 132kV is oversailing a development site, the DNO will resist grounding it due to the added technical complexity of doing so, and the increasing likelihood of faults occurring in the network.
Outages are required for any works to be carried out on these lines, and no works are possible between October and March, due to the increased demand for electricity during these months. It can take 2 to 2.5 years to ground 33kV and 66kV OHLs and in excess of 3 years to deliver the diversion of a 132kV. A developer would need to take all of this into account when setting out their development programme.
275kV and 400kV (transmission)
These lines are owned and operated by National Grid and have much greater capacities, to carry electricity over long distances from generation plants to regional substations. They have larger tower structures to support the heavier conductors and withstand greater electrical stress. They are highly efficient in terms of large-scale power transmission but have a significant visual and environmental impact on the land around them.
These lines are almost never diverted underground unless the project concerned is of national significance (e.g. for HS2 or for works on an airport).
What are the main benefits of undergrounding overhead electricity apparatus?
- Release of additional net developable area – this adds value to the overall scheme.
- Reduced visual impact – underground cables eliminate the need for large towers and overhead wires, preserving rural landscapes and improving the appearance of urban areas.
- Improved safety – removing overhead electricity lines reduces the danger of accidental contact (for people, animals and aircraft).
Despite these advantages, underground transmission comes with challenges such as high costs, complex installation, and maintenance difficulties. The final decision will come down to a trade-off between practicality, profitability and aesthetics.
