Lightening Arresters:
• Protection against direct lightening strokes is provided by using the earthing screen and ground wires. But these two apparatus fail to provide protection against travelling waves; which may reach the terminal apparatus.
• The lightening arresters or surge diverters provide protection against such surges.
 |
| (A) Construction of Simple L.A |
• A lightening arrester or a surge diverter is a protective device, which diverts the high
voltage surges on the power system to the ground.
 |
| (B) Characteristics of non- linear resistance |
• Fig. 4.39.2 (a) shows the basic construction of a lightening arrester. It mainly consists of a spark gap and a non-linear resistor in series with it.
• One end of the surge diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded.
• The air gap between the spheres is so adjusted such that under normal line
voltage this gap does not produce any are. But a dangerously high voltage will breake
down the air insulation and form an arc.
• The non-linear resistance is the resistance which having the non-linear characteristics i.e. the magnitude of resistance decreases as the voltage (or current) increases and vice-versa. Fig. 4.39.2 (b) shows the volt /Amp characteristic of the non-linear resistor.
Working:
The action of the spark gap lightening arrester or surge diverter is explained
follows:
(i) Since the spark gap and non-linear resistance provides very high resistance to normal operating voltage, the lightening arrester conducts no current to earth or the gap is non- conducting.
(ii) When the over voltage occurs on the system, the air insulation across the gap
breaks down and an arc is formed.
(iii) Now the non-linear resistance provides a low resistance path for the surge to the ground. In this way, the excess charge is diverted towards the earth and hence equipment is protected against failure.
(ii) It should be noted that while designing a lightening arrester that when the surge is over the are in gap should cease. Otherwise if the arc does not go out, the current would continue to flow through the resistor and both resistor and gap may be destroyed.
0 comments: