- 1. It must not take any current at normal system voltage
- 2. Any transient wave with voltage peak exceeding the spark over voltage must cause it to break down.
- 3. After break down it must be capable of carrying the resulting discharge current without any damage to itself and without voltage across it exceeding the breakdown voltage.
- 4. The power frequency current following the breakdown must be interrupted as soon as the transient voltage has fallen below the breakdown value.
Location of Lightning Arrestor
Lightning Arrestor Ratings
- Normal or rated voltage: It is designated by the maximum permissible value of power frequency voltage which it can support across its line and earth terminal while still carrying effectively and without the automatic extinction of the follow-up current. The voltage rating of the arrestors should be greater than the maximum sound phase to ground voltage.
- Normal Discharge current: It is the surge current which flows through the LA after the spark over, expressed in crest value (peak value) for a specified wave shape. Example 10, 5, 2.5, 1.5, 1 kA rating.
- Power frequency spark over voltage: It is the RMS value of the power frequency voltage applied between the line and earth terminals of the arrestor and earth which causes spark over of the series gap. As per IS 3070, the recommended spark over voltage is 1.5 times the rated voltage.
Selection of Lightning Arrestor
For the protection of substation above 66KV an arrestor of 10kA rating is used.