Surge Protection Devices (SPD)
How do Surge Protection devices work:
Surge protection devices (SPDs) can be thought of as safety pressure valves that prevent the build up of excessively dangerous voltage in power system. SPDs are installed across the power supply / signaling / data / communication lines before the equipment to be protected and are normally in a high impedance state having no effect on the circuit operation.

A Surge Current Pulse (waveform) is defined by the time taken by the surge to reach its maximum value & then the time taken by the surge to reach its half of the maximum value.For example this waveform addresses true lightning and time/ duration of stroke by producing 50,000 amps in 10 ìs. After 350 ìs, the waveform is still at 25,000 amps. In this example the waveform of the surge is designated as 10/350 ìs with the amplitude of 50 kAmp. Similarly the switching surges are addressed by a waveform of 8/20 microseconds, for example a surge current of 20,000 amps in 8 ìs, coming down to 10,000 amps at 20 ìs can be termed as the surge of 8/20 ìs, with 20 kAmp amplitude.

First stage: The arrester Class B type (as per VDE classification) - lightening current arresters are based on the spark gap technology that operates by arc chopping principle with a rating for arresting lightening currents of up to 100 kA for lightening current pulses of 10/350 ìs and is installed at the main distribution area of the power supply.

Second Stage: is the medium level protection. The arrester class C type (as per VDE classification) are single, high capacity varistor based surge voltage arresters that are available with at least one time surge arresting capacity of 40 kA, 8/20 ìs pulse and a repeated capacity for 20 kA, 8/20 ìs pulses. The technology used is that of MOV with an indication to check its proper functioning and are installed at sub distribution level.