Estimating the Lifetime of a Varistor on the Mains – What an Electronic consultant should know!

Introduction

Varistors (typically MOVs) are widely used on mains lines for surge / transient protection. Under normal conditions they are passive, but when high‐voltage transients (switching surges, lightning induced, line over­voltage) occur, the varistor conducts, clamps the voltage and absorbs energy. Over time the repeated stress (especially large impulses) causes the varistor’s characteristics (e.g., breakdown/clamp voltage) to degrade. Eventually the device may fail (either catastrophically short or simply lose clamping ability).

This makes lifetime (or more precisely “service life” or “useful remaining life”) of a varistor a relevant design consideration, especially for mains protection devices expected to last many years with modest maintenance.

What happens during aging / lifetime degradation

Some of the mechanisms involved in varistor degradation include:

• Each surge/impulse event causes thermal and electrical stress in the MOV disk (or equivalent structure). Even if the device remains within spec, cumulative damage occurs.

  
  

• The varistor’s nominal clamping voltage (or breakdown voltage) slowly shifts downward (or in some cases upward) with repeated impulses, due to micro‐cracking, grain boundary changes, or heating effects see typical curves here: https://www.we-online.com/components/products/datasheet/820551406.pdf

  
  

• Leakage current may increase as the device ages, leading to higher self-heating under nominal voltage and accelerating degradation.

  
  

• Ambient temperature, duty cycle of surges, magnitude of surges, switching frequency, humidity or mechanical stress may all accelerate aging.

typical MOV derating curve :

Modelling the “life” of a varistor

When we speak of “lifetime” for a varistor, we typically mean the remaining useful service life under defined mission‐profile conditions before the varistor no longer meets its required clamp voltage, or before leakage/self-heating becomes unacceptable, or before outright failure.

A simple way to approach this is :

1. Define the mission profile

   • Mains line voltage (e.g., 230 V AC, ±10%)

   • The expected number of surge events per year above a defined magnitude (e.g., 1 kV/10 µs impulses - 100A peak current).

   • The expected amplitude and energy per impulse (or worst‐case scenario).

     
     

2. Define varistor degradation per impulse

   • Based on manufacturer data (if available) or published test results: e.g., after 1,000 impulses of X kA the clamping voltage drifts by Y%.

     not all the manufacturer provide this figure, most of them provide the maximum niumber of pulses per Pulse duration and maximum current :... e.g. https://www.we-online.com/components/products/datasheet/820443211E.pdf

     
     

3. Estimate remaining life

   • Use the number of impulses per year, the amplitude/energy distribution, together with the anufacturer datasheet to estimate the maximum life time of your product.

   • defined profile.

     
     

4. Go to the tetsing lab and try!

Final remarks

• lifetime estimate is not guaranteed – it depends heavily on actual surge environment, temperature, installation, etc.

Note for Medican equipments

• Consider periodic inspection or monitoring methods (e.g., measure leakage current). on mdeical equipment the maximum leakage current is usally below 100 uA and as MOX will deteriorate the leakage will deteriorate.