MOV Surge Energy

Formula

Description

Metal oxide varistors (MOVs) absorb surge energy by clamping the voltage across them and dissipating the energy as heat. The energy absorbed during a surge event is approximated by the capacitive energy difference between the clamping voltage and the normal supply voltage. This calculation helps select an MOV with sufficient energy rating for the expected surge environment. The MOV must absorb this energy without exceeding its single-pulse or cumulative energy rating, which degrades over multiple surge events.

Variables

• E — Energy absorbed by the MOV (J)
• C — Effective circuit capacitance during the surge (F)
• V_clamp — MOV clamping voltage (V)
• V_supply — Normal supply voltage (V)

Practical Notes

For AC mains protection, MOVs are rated by their maximum continuous operating voltage (MCOV), which must exceed the highest expected line voltage including tolerances (e.g., 275V MCOV for 230V mains). MOV energy ratings range from 1-10 J for signal-level protection to 100-1000+ J for mains surge protection. MOVs degrade with each surge event; GDTs (gas discharge tubes) and TVS diodes are preferred where longevity matters.