Lightning arresters are the most effective means of protecting an electrical apparatus against traveling voltage waves caused by lightning and switching. Lightning arresters are connected across and apparatus to provide a Lowresistance path to ground, thus limiting the transient voltages below the Basic Impulse Level of the apparatus. There are four different classes of arrester.

1. Station

2. Intermediate

3. Distribution

4. Secondary

The functions of a lightning arrester are

1. To act like an open circuit during the normal operation of the system i.e., to hold off the system voltage,

2. To limit the transient voltage to a safe level with the minimum delay and fitter, and

3. To bring the system back to its normal operation mode as soon as the transient voltage is suppressed, i.e., to interrupt the power-follow current and to reseal itself.

The normal operation or operational mode includes the system under faulted condition. Under several types of system faults, such as the single line-to-ground faults, the voltage to ground across the unfaulted phases will rise above the normal voltage level. The arrester must not go into conduction during this fault condition. It should also be able to interrupt the power-follow current and reseal itself under system fault conditions when the power-frequency voltage across it rises.

SELECTION OF LIGHTNING ARRESTER

The lightning arresters are designated by the crest magnitude of the discharge current having 10×20 A second wave shape which the arrester can safely pass without damage. The lightning arresters are designated as 8, 10, 20 KA. They can safely discharge these current crests. As the arrester is a protective device, if is a general impression that if should be rated for most severe conditions of discharge currents says 20 A. The discharge current from the arrester varies from a few hundred amperes to kilo-amperes and sometimes if is even 20 KA. Maximum discharge voltage and discharge factor for the arrester is defined the maximum value of voltage which appears across the arrester terminals at the time of discharging if rated current determines its impulse level of protection. The discharge factor if is,

In the early designs of arresters, this discharge factor was quite high (about 5.6) but due to the advent of better material and Modern research it has been reduced varying to 2.4 to 3.0. The above ratio for arresters manufactured by different firms varies from 1.5 to 1.7, so, the average value may be taken as 1.6 E is the rated arrester voltage KV (R.M.S) and LS is the minimum impulse insulation level in kV (crest valve) its value after allowing 10% as tolerance factor and 25% as margin factor can be obtained.

EARTHING SYSTEM

The frame of every generator, stationary motor, and so far as particable, portable motor, and metallic parts of all transformer and regulating and controlling apparatus connected with supply shall be earthed by the owner by separate and distinct connection with earth. Every conductor used on earthing shall be of stranded as solid copper or suitable copper alloy, and shall be protected wherever liable to mechanical damage and also, where necessary, against corrosion, particular attention being given in these respects to the earthing leads at its point of connection with the earth electrode. The coefficient of earthing is below 80 percent. On four wire distribution systems, with solidly ground transformer neutral at every voltage level, coefficient of earthing is generally less than 80%. On high voltage transmission systems the coefficient of earthing does not exceed 75%.

In resistance of Common Types of Earth-electrode, there are three types.

1. Plates

2. Pipes and rods

3. Strip or conductor electrodes

 

From: Internet