Horn Gap Arrester

Lightning arrestor is connected between the line and earth. It is a protective device which conducts the high voltage surges (due to both direct and indirect stroke) on the power system to the ground and protects the equipments connected to it.

Construction

It consists of two horn shaped metal rods A and B separated by a small air gap. The distance between them gradually increases towards the top as shown. The gap between the horns is so adjusted that normal supply voltage is not enough to cause an arc across the gap. The horns are mounted on porcelain insulators. One end of horn is connected to the line through a resistance R and choke coil L while the other end is effectively grounded. The resistance R helps in limiting the follow current to a small value. The choke coil offers small reactance at normal power frequency but a very high reactance at transient frequency. Thus the choke does not allow the transients to enter the apparatus to be protected.

Working:

Under normal conditions, the horn gap does not conduct (i.e. normal supply voltage is insufficient to initiate the arc between the gap). When an overvoltage occurs, spark-over takes place across the small gap G. The heated air around the arc and the magnetic effect of the arc cause the arc to travel up the gap. The arc moves progressively into positions 1, 2 and 3. At position 3, the distance may be too great for the voltage to maintain the arc. Consequently, the arc is extinguished. The excess charge on the line is thus conducted through the arrester to the ground.

Advantages

• The arc is self-clearing. Therefore, this type of arrester does not cause short-circuiting of the system after the surge is over as in the case of rod gap.
• Series resistance helps in limiting the follow current to a small value.

Limitations

• The bridging of gap by some external agency (e.g. birds) will create a short circuit.
• Corrosion or pitting will adversely affects the performance of the arrester.
• The time of operation is comparatively long (about 3 seconds) when compared to modern lightning arresters..