Surge arrester is a device used to protect equipments against over voltages. MOS (Metal Oxide Surge Arrester) - Without gaps will be used in UKHP. It consists of ZnO element which has excellent non-linear voltage-current characteristic.
MAIN CAUSES OF OVER VOLTAGES
1. Lightning Surges - 8/20 current impulses.
2. Switching Surges - has a peak value of discharge current having a virtual front time greater than 30 μs but less than 100 μs and a virtual time to half-value on the tail of roughly twice the virtual front time.
3. Temporary over voltages - by line-to-ground faults, load rejection & continued for a few milliseconds to a few seconds.
Nominal discharge current - of an arrester is the peak value of lightning current impulse which is used to classify an arrester.