Surge Withstand

Surge withstand quantifies what happens to a resistor when struck by a brief, very-high-energy event. Standards model the surge with two waveforms: 8/20 μs (the IEC 61000-4-5 “combination wave”, representing indirect lightning and switching transients, rise time 8 μs, decay to half-peak 20 μs) and 10/350 μs (representing direct lightning current per IEC 61643). The peak current can reach kiloamps; the energy can reach hundreds of joules.

A resistor's surge rating describes the largest such pulse it can absorb while keeping its resistance within a stated drift band (usually ±1 % or ±5 %) afterward. Wirewound parts with large element mass excel — a 25 W wirewound surge-rated part can withstand 6 kA / 8/20 μs without measurable drift. Thick-film parts in HV cylindrical packages are tested per IEC 60384-14 for 1.2/50 μs voltage withstand; some achieve 50 kV impulse without flashover.

Surge-withstand is critical in lightning-protection circuits, where the resistor sits in series with MOVs to limit follow current; in pulse-discharge applications (defibrillators, capacitor banks, photoflash); in input stages of EV chargers and PV inverters where switching can create kV transients. Specifying for surge requires both the energy figure (single-event joules) and the residual drift after a defined number of pulses (often 10, 100 or 1000).