43. High voltage overhead lines and substations can generate noise, the level of which depends mainly on the voltage of the overhead line or substation.

44. Noise from energised overhead lines is produced by a phenomenon known as “corona discharge” (a limited electrical breakdown of the air). While conductors are designed and constructed to minimise corona, surface irregularities caused by damage, insects, raindrops or pollution may locally enhance the electric field strength sufficiently for corona discharges to occur. This can be audible in certain conditions as a “crackling” sound, occasionally accompanied by a low frequency hum. The noise level generated by a high voltage overhead line is weather-related, with highest noise levels occurring during damp conditions. Overhead lines are normally quiet during dry weather, except during long, dry spells when airborne debris adheres to the conductors. Any noise disappears when sufficient rain falls to wash the debris away.

45. Transformers are installed at many substations, and generate low frequency hum. Whether the noise can be heard outside a substation depends on a number of factors, including transformer type and the level of noise attenuation present (either engineered intentionally or provided by other structures).

46. National Grid is able to provide information and advice on noise from high voltage plant to both planning authorities and developers. It is possible for the developer to mitigate significantly the effects of noise from an existing overhead line by attention to site layout and design of new developments, for example by including landscaping or by placing the noise-sensitive elements away from the high voltage plant.

47. The Department for Communities and Local Government Planning Policy Guidance 24 (PPG24, Planning and Noise), and Technical Advice Note (Wales) 11, guide local authorities on the use of their planning powers to minimise the adverse impact of noise. They outline the considerations to be taken into account in determining planning applications both for noise-sensitive developments and for those activities which generate noise. They also advise on the use of conditions to minimise the impact of noise.