Let us understand 3 components commonly used in SPD.

Transient voltage suppressors:They are based on avalanche diodes and zener diodes. Diodes are made of semiconductors usually silicon, at a junction between n and p doped regions. TVS devices provide protection with combination of forward bias and reverse bias breakdown conduction.

MOV (Metal Oxide varistor) : Varistors are usually made of ceramic of zinc oxide grains in a matrix of other oxides. The grains form diodes with surrounding matrix, creating a complex array of parallel and anti-parallel diodes. At higher voltages, the individual diodes begin to conduct and the resistance of the varistor drops dramatically.

Gas Discharge Tubes: They are usually formed with ceramic body filled with gas mixture containing neon and argon and two or more electrodes. When voltage across the electrodes exceeds a specified value an arc occurs within the tube which provides low current path.

Following table mentions difference between these components.