High voltage power cables, particularly those close to airports, need to be visible day and night. During the day, brightly coloured balls positioned along the length of the cables are sufficient, but during the night, lighting is necessary. These beacons which emit a red glow (the standard colour used in aviation for warning beacons) are used to provide this lighting.
The system is extremely simple, efficient and reliable and is considered the most economically viable solution. Its working principle is attractive because it uses a phenomenon usually considered as a weakness. However a large disadvantage is that it does not work when the line is switched-off. It cannot also be used on HVDC-powerlines, but similar devices may be also used on mast radiators.
The interest of the system lies in the way it obtains a power supply directly from the single cable on which it is mounted. It is difficult to obtain a very small amount of energy from a line which transports an enormous amount.
Each high voltage cable creates an electric field around itself, in the same way as any live electrical conductor. When the electric potential of such a cable is sufficiently high, its electric field causes a significant voltage difference between the cable and its immediate neighbour. (It is the strength of the electric field that makes it difficult use low powers.)
As a result, a second conductor a few metres long, insulated from but parallel to the high voltage cable, will have a potential different from that of the main cable. Together the conductors make a capacitor charged across an air gap (dielectric).
Under certain conditions, the accumulated charge (and hence the potential difference) is sufficient to trigger a discharge lamp. This what is used in practice, giving a reliable and robust beacon.
- David F. Rider Jane's airport equipment fifth edition, Jane's Pub. Co., 1986 page 92