In 220 kV switchyard why lightning arrestor should be properly grounded?

In 220 kV switchyard why lightning arrestor should be properly grounded?
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1 Answer

Answer :

a. During lightning, surges should be discharged to ground, otherwise it will puncture the equipment insulation and it is possible only when lightning arrestor is grounded properly. 

b. In order to make it effective, the ground terminal of lighting arrestor should be connected direct to the tank of transformer. This will eliminate voltage build up due to earth resistance. For example for each ohm of earth resistance the voltage build up for 5000 Amps discharge current is 5 kV. Soil resistivity a should be minimum and may be it is 3.5 ohm per meter.  
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