# Lightning Arrester

Lightning Arrester is the device used to provide protection against travelling waves by diverting the abnormal high voltages to ground without affecting the continuity of the supply.
It is connected between line and earth i.e. in parallel with equipment to be protected.
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## Working of Lightening Arrester

Lightning Arrester generally consists of a spark gap as shown in figure in series with a non-linear resistor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that normal voltage is not enough to cause an arc but a dangerously high voltage will break down the air insulation and form an arc. The property of the non-linear resistance is that its resistance decreases as the voltage (or current) increases and increase with the decrease in voltage.

The action of the lightning arrester or surge diverter is as under

1.   Under normal operating voltage, the lightning arrester does not conduct current to the ground because the spark gap is non-conducting.
2.   On the manifestation of over voltage i.e. voltage above the normal, the air insulation across the spark gap breaks down and an arc is setup, providing a low resistance path for the surge to the ground. In this way, the excess charge on the line due to the surge is safely conducted through the arrester to the ground instead of being sent back over the line.
3.   It is worthwhile to mention the function of non-linear resistor in the operation of arrester. As the gap sparks over due to over voltage, the arc would be a short-circuit on the power system and may cause power-follow current in the arrester. Since the characteristic of the resistor is to offer low resistance to high voltage (or current), it gives the effect of short-circuit. After the surge is over, the resistor offers high resistance to make the gap non-conducting.

An ideal lightning arrester should have the following characteristics

1.    It should not draw any current during normal operating condition, i.e., it sparks over voltage must be above the normal or abnormal power frequency that may occur in the system.
2.    Any transient abnormal voltage above the breakdown value must cause it to break down as quickly as possible so that it may provide a conducting path to ground.
3.    When the breakdown has taken place, it should be capable of carrying the resulting discharge current without getting damaged itself and without the voltage across it exceeding the breakdown value.
4.    The power frequency current following the breakdown must be interrupted as soon as the transient voltage has fallen below the breakdown value.

Maintenance of Lightning Arrester

1.   Cleaning the outside of the arrester housing.
2.   The line should be de-energized before handling the arrester.
3.   The earth connection should be checked periodically.
4.   To record the readings of the surge counter.
5.   The line lead is securely fastened to the line conductor and arrester.
6.   The ground lead is securely fastened to the arrester terminal and ground

Types of Lightning Arresters

1.   Horn Gap Arresters
2.   Multi-Gap Arresters
3.   Valve-Type Arresters
4.   Metal Oxide Surge diverter

Horn Gap Arresters

This arrestor has two metal rods in separated by a small air gap. The metal rods are located on ceramic insulators. The connection of the horn can be done by connecting it to two dissimilar wires. One side connected to the line throughout a resistance & choke coil whereas the other side is grounded.

Multi-Gap Arresters

These types of arresters are designed with a sequence of metal cylinders that are insulated from one another and separated by small intervals of air gaps. In the sequence of cylinders, the primary cylinder is connected to Power line whereas the remaining cylinders are connected to the ground by series resistance. By the inclusion odf series resistances, the degree of protection against travelling waves is reduced. In order to overcome this difficulty, some of gaps are shunted by resistance.

Valve-Type Arresters

These arrestors incorporate non-linear resistors and are extensively used on system operating at high voltages. It consists two assemblies
(a). Series spark gaps
(b) non-linear resistor discs made of material such as thyrite or metrosil.
The non-linear element is connected in series with the spark gaps. Both the assemblies are accommodated in a tight porcelain container.

Metal – Oxide Arrestors

These arrester uses zinc oxide semiconductor as a resistor material, such type of arrester is known as a metal oxide surge arrester or ZnO Diverter. This arrester provides protection against all types of AC and DC over voltages. It is mainly used for overvoltage protection at all voltage levels in a power system.

Rating of Lightning Arrester
Duty Cycle Rating or Voltage Rating
The Rating lightning arrester is generally below the system voltage. Suppose we have system voltage of 11KV (Line to line Voltage) the lightning arrester used for this system voltage is 9KV because we connect the lighting arrester between line and ground not between the line to line. The line to ground voltage of the 11KV (line to Line voltage) system is 11/√3 = 6.35 KV. 6.35KV is per phase voltage of 11KV system voltage. Peak Value of the system is √2 x 6.35 = 9KV
For system above 33KV the rating of lighting arrester is decide as follows
Line to line voltage = 33KV
Line to ground voltage = 33/√3= 19.05
Peak line to ground voltage = 19.05 x √2= 27
Take 10% of margin = 27 x 1.10 = 30KV
Thus rating of lightning arrester should be 30KV

MCOV (Maximum Continuous operating Voltage)

The another rating of lighting arrester is MCOV (Maximum Continuous operating Voltage). This Voltage taken 85% of lightning arrester rating e.g. 9KV lighting arrester has MCOV = 9 x .85 = 7.65KV

Current Rating

The nominal discharge current is the peak value of lighting current impulse with wave shape as 8/20µs, which is used to classify an arrester, It is also a basis for calculating lightning impulse protection level, LIPL of a surge arrester.
Standard ‘In’ values are 2.5 kA, 5kA, 10kA & 20kA but the value of the nominal discharge current alone does not give enough information about the performance of the arrester. Additional information about the application either Distribution or Station class and duty low, medium or high along with repetitive charge transfer ‘Qrs’ rating is required to be specified.