- The earthling screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against traveling waves which may reach the terminal apparatus.
- The lightning arresters or surge diverters provide protection against such surges.
- A lightning arrester or a surge diverter is a protective device which conducts the high voltage surges on the power system to the ground.
- Figure shows the basic form of a surge diverter. It consists of a spark gap in series with a non-linear resistor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded.
- The length of the gap is so set that normal line voltage is not enough to cause an arc across the gap but a dangerously high voltage will break down the air insulation and form an arc.
- The property of the non-linear resistance is that its resistance decreases as the voltage (or current) increases and vice-versa. This is clear from the volt/amp characteristic of the resistor shown in Figure.
Action: The action of the lightning arrester or surge diverter is as under:
(i) Under normal operation, the lightning arrester is off the line i.e., it conducts r’ no current to earth or the gap is non-conducting.
(ii) On the occurrence of over voltage, the air insulation across the gap breaks down and an arc is formed, providing a low resistance path for the surge to the ground.
(iii) In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line.
(iv) It is worthwhile to mention the function of non-linear resistor in the operation of arrester.
(v) As the gap sparks over due to over voltage, the arc would be a short circuit on the power system and may cause power-follow current in the arrester.
Since the characteristic of the resistor is to offer high resistance to high voltage (or current), it prevents the effect of a short circuit. After the surge is over, the resistor offers high resistance to make the gap non-conducting.
- Two things must be taken care of in the design of a lightning arrester.
- Firstly, when the surge is over, the arc in gap should cease. If the arc does not go out, the current would continue to flow through the resistor and both resistor and gap may be destroyed.
- Secondly, IR drop (where I is the surge current) across the arrester when carrying surge current should not exceed the breakdown strength of the insulation of the equipment to be protected.