The article (the Sunday Times, May 2) on tornadoes and lightning alerts us to the need to protect ourselves from lightning damage, particularly with respect to TV antennas and electricity service connections.
The advice given is to disconnect the antenna from the TV and the electrical appliances from the mains supply, but this is not always possible. However, there are measures we can take to minimise risks without disconnecting either antenna or electrical appliances.
In Sri Lanka, the majority of TV antennas in use are of the Yagi type. These comprise an active element, which is a folded dipole and a few passive elements, one of which serves as a reflector while the others serve as directors. In products sold in the market here, all these elements are connected to a common metal boom using insulators. This need not be so, particularly with the folded dipole, the active element.
The mid-point of the dipole is a zero potential point, and therefore it can be grounded at this point without affecting the signal strength or its quality in any way. The folded dipole should be mounted directly onto the metal boom without any insulation in-between. The cable to the TV receiver or the booster could be connected to the two open ends of the loop.
When a lightning bolt occurs in the vicinity, any high voltage that may develop in the antenna will get grounded directly through the boom and the mast that carries the boom, rather than travelling to the receiver through the cable, as the latter is a high impedence path than the mast.
However, you must make sure the antenna is fitted to a grounded mast, and not to a wall bracket or a similar contraption.
In the case of high voltage along power lines as a result of lightning in the vicinity, there is a simple way to prevent it from entering the building’s wiring system. Instal a surge arrestor at the point of entrance of the service connection to the building. This is not anything new. The telecommunication authorities instal such devices in all their landline connections, as a rule. Surge arrestors are sold in the market, but they can be expensive.
A simple device can be manufactured locally for about Rs. 500. (The components of a suitable device are shown in the attached diagram.) It comprises three strips of copper of a width and thickness that can carry the maximum rated current, with the two outer strips connected to the live and the neutral of the supply line, respectively. The centre strip is connected to the earth.
Across each pair, there are spark gaps with the ends tapering to a point and the gap across the points not more than 1 mm wide. Across the two outer strips and the centre strip are connected two varistors. This device under normal voltage has high resistance, but at very high voltage offers a very low resistance path.
Along each strip are also connected two inductors, which are simply coils of wire of sufficient diameter to carry the rated maximum current and with a few turns. Under normal conditions, it offers no resistance to the current path, but when lightning causes a sudden surge of current, offers very high impedance, diverting the current to the ground along the low resistance path offered by the varistor or causing a spark across the gap. The entire circuit can be enclosed in a box, with only the terminals exposed.
It should be mandatory with electricity utilities, such as telecommunications, to have such devices installed in every service connection provided. An entrepreneur could supply these to the utilities at a nominal cost.
Dr. Janaka Ratnasiri,
Nawala |
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