53. High voltage equipment produces electric fields which can cause nearby conductive objects to acquire a charge. When discharged to earth through a person touching the object, a small microshock may be experienced. For instance, a car parked under an overhead line can pick up a voltage and when a person touches it, a small spark may occur between the car and that person. Microshocks may sometimes be annoying, but are not normally regarded as dangerous or a health risk.
54. Metal-clad buildings and metal fences under overhead lines can similarly pick up a voltage. These should be appropriately earthed to reduce the effect of such voltages.
55. Magnetic fields from power cables and overhead lines can also induce voltages on conductive services, such as pipelines or telecommunication cables, that run parallel and close by. These voltages can be significant if the length of parallelism is considerable. In such cases, an assessment of the impacts of induced voltages will be required and National Grid should be consulted for further advice.
56. At petrol filling stations and other sites where flammable materials are stored, where spark discharges can be a safety hazard, appropriate electrical screening and earthing of the site may be required if it is located under a high voltage overhead power line. A safety assessment should be carried out with the effects of the nearby power line taken into account, and National Grid consulted for further advice.
Computer screen interference
57. Some monitors or display screens used with computers suffer a distortion of the displayed image, usually a “flicker” or “wobble,” in the presence of 50 Hz magnetic fields above about 0.5 microteslas. Such magnetic fields can be found around most electrical equipment, including high voltage overhead power lines. The magnitude of interference will be dependent on the proximity and orientation of the display screen to the overhead line and the magnitude of current flowing in the line. Flat Screen Displays are not affected.
58. Distribution wiring in buildings and adjacent equipment can also generate a magnetic field of sufficient level to interact with computer screens.
59. For existing equipment that is being affected, there are techniques available that can reduce the interference. National Grid can provide information and advice to minimise interference to computer screens in the design and layout of new buildings. If Flat Screen Displays are specified for new installations, the need for any such mitigation will be removed.
Electromagnetic compatibility (EMC) issues
60. Some electronic and radio communications equipment may be susceptible to the electromagnetic fields and low level radio noise produced by high voltage equipment. Generally, it is easier and less costly to design and plan to avoid EMC issues than it is to correct the problems after they have arisen. As such, it would be prudent for the electromagnetic environment to be taken into consideration when new electronic equipment is being specified.
61. The locations of TV and radio aerials relative to high voltage electricity transmission lines or substations can sometimes result in poor reception. The careful siting of such aerials can usually resolve this issue.