# Lightning Arrester – Location, Rating and Selection

- Link: https://studyelectrical.com/2014/06/lightning-arrester-location-rating-and.html
- Published: 2014-06-23T16:21:00+00:00

A lightning arrester is connected to protect a piece of equipment from lightning
and switching surges.

Overvoltages may cause the burning of [insulation of substation](https://studyelectrical.com/2015/09/electrical-bushings-types-and-purpose-classification-construction.html)
equipment if not well protected. Lightning is one of the most serious causes of 
overvoltages. 

There are various types of lightning arrester construction. They are 

 1. **Rod gap LA**
 2. **Expulsion type LA**
 3. **Valve type LA**
 4. **Horn gap LA**
 5. **Pellet type LA**
 6. **Thyrite type LA, etc.**

 

An **ideal Lightning Arrester** should possess the following characteristics.

 * It  must not take any current at normal system voltage
 * Any transient wave with a voltage peak exceeding the spark over voltage must 
   cause it to break down.
 * After the breakdown, it must be capable of carrying the resulting discharge current
   without any damage to itself and without voltage across it exceeding the breakdown
   voltage.
 * 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 Arrester

**Lightning arrester should be located close to the equipment that it is expected
to protect. **

![Lightning Arrester](https://studyelectrical.com/wp-content/uploads/2014/06/Lightning-
Arrester.jpg)

Lightning Arrester on a Transformer

In large [substations](https://studyelectrical.com/2019/04/electrical-substation-equipment-types-components-functions.html),
arrestors should be installed at take-off points of the lines and of the terminal
apparatus.

Many factors like system voltages, basic impulse insulation level, arrestor rating,
station layout, number and arrangement of lines, the position of isolators, the 
distance between equipment, etc. have to be taken into account in fixing the location
of the arrestors. 

The length of the arrester lead should be as low as possible and should not exceed
10M.

The Arresters are installed both on the High Voltage and Low Voltage side of the
transformers. Junction of an OH line and the cable should be protected by LA. Separate
earth should be provided for each LAs. LA ground leads should not be connected to
the station earth bus.

## Lightning Arrester Ratings

The rating of a lightning arrester is given below,

 1. 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 arresters should be greater than the maximum
    sound phase to ground voltage.
 2. Normal Discharge current: It is the surge current that 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.
 3. Power frequency spark over voltage: It is the RMS value of the power frequency 
    voltage applied between the line and earth terminals of the arrester and earth 
    which causes spark over of the series gap. As per [IS 3070](https://archive.org/details/gov.in.is.3070.3.1993),
    the recommended spark overvoltage is 1.5 times the rated voltage.

There are also other ratings like maximum impulse spark over-voltage, residual or
discharge voltage, maximum discharge current, etc.

## Selection of LA

Here we are selecting an appropriate rating of lightning arresters for the substation.

For the protection of substation above 66kV, an arrester of 10kA rating is used.

Voltage rating of LA = Line to line voltage × 1.1 × coefficient of earthing.

Power frequency spark over voltage = 1.5 ×Voltage rating of LA

(Assuming coefficient of earthing equals 0.8 for the effectively earthed system)

**For 220KV side:**

Voltage rating = 1.1 × 220 × 0.8 = 193.6KV

Power frequency spark over voltage = 1.5 ×193.6 = 290.4KV

Rated discharge current = 10 kA

**For 110KV side:**

Voltage rating = 1.1 × 110×0.8 = 96.8KV

Power frequency spark over voltage = 1.5 × 96.8 = 145.2KV

Rated discharge current  = 10kA

**For 66kV Side **

Voltage rating = 1.1 × 66×0.8 = 58.08kV

Power frequency spark over voltage = 1.5 × 58.08 = 87.12kV

Rated discharge current = 10kA

**For 11 KV side:**

Voltage rating = 1.1× 11×0.8 = 9.68KV

Power frequency spark over voltage = 1.5×9.68 = 14.52KV

Nominal discharge current = 5kA

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