LIGHTENING ARRESTOR/SURGE ARRESTER

Q – What is lightning?

A –An electric discharge (also known as flash) between clouds or clouds and earth or between charge-centers of the same cloud is knows as lightning. Normally it’s of very short duration with very high voltage/potential accompanied by flash.

Q – Why do clouds get charged?

A –There are many theories exist which explain how clouds get charged but most accepted one is that asthe air rises, water vapour cools and forms a cloud.  Heavier, negatively charged particles sink to the bottom of the cloud. When the positive and negative charges grow large enough, a giant spark – lightning – occurs between the two charges within the clouds or with earth.

Q – What is stepped Leader?

A – When negative charge of clouds may become so great that it may discharge to another cloud or earth. When a cloud passes over the earth it induces equal & opposite charge on the earth below. As charge acquired by the cloud increases, the potential between the cloud & earth increases. When the potential gradient is sufficient to break down the surrounding area, the lightning strokes takes place.

As soon as air near the cloud breaks down, a streamer (carries charge) called pilot streamer or leader streamer starts from the cloud towards earth. This journey of leader streamer continues towards earth if there is sufficient charge to maintain gradient at the tip of the leader otherwise leader streamer stops & charge is dissipated. No lightning stroke takes-place in this case.

In many cases leader streamer continuous its journey towards earth & makes contact with earth or tree or some object on the earth. As the leader streamer moves towards earth, it is accompanied by points of luminescence which travel in jumps giving rise the stepped leader.The stepped leaders have sufficient luminosity & give rise to first visual phenomenon of discharge.

The path of leader streamer (negative charge) is a path of ionization that is nothing but complete break- down of insulation.

Q – What is Upward Streamers?

A – As the leader streamer reaches near the earth, a return streamer (with positive charge) shoots up from the earth to the cloud, following the same path as the main channel of downward leader. This return streamer is called the Upward Streamer.

This phenomenon of meeting leader stream & upward streamer is called lightning which neutralizes negative charge of clouds.

Some facts about Lightning –

  • The duration of lighting is very less i.e approx. – 10μS to 100μS,
  • The range of lighting current is approx. – 3000A to 2,00,000 A,
  • The cloud potential is approx. more than 100 MV,
  • Lighting takes place in steps by leader strokes stepping some tens of meters at a time. When last step brings the tip of leader close to earth & upward streamer leaves the earth to join the tip of downward leader.

 

Effects of Lightning Stroke –

When lightning occurs in an area, it affects that area in following way-

  1. Electrical effects – When high lightning current is discharged through the earth electrode (of LA), a temporary high voltage is produced around the earth conductor with respect to ground. Hence this high potential difference is dangerous to persons & animals,
  2. Side flashing – When high voltage is produced around the earth conductor, therefore a risk of flashover from earth electrode to any other nearby metal structure is there. If flash-over occurs, the lightning current will be discharged through the structure which may be a risk to occupants or equipment.
  3. Thermal effects- When lightning current passes through the conductor which causes a high temperature produced for a very short period (during lighting only), therefore the cross-sectional area of lightning conductor should be chosen carefully for its mechanical strength which means it should be large enough to keep the rise of temperature to 1ºC
  4. Mechanical effects – When lightning current is discharged through earth conductor, a mechanical force is produced in a parallel conductor with sharp bends, therefore, it is necessary to fix it properly.

LIGHTNING ARRESTER