Answer
The failure of ZNR can be judged by measuring the varistor's voltage.
The voltage of the varistor can be measured from terminals at a constant current power supply of DC 1mA, within 1 sec. The varistor should be measured separately from the PCB to prevent from affecting other components. If the voltage change rate is over +/-10% after usage, the varistor can be judged as failure, however, it is difficult to obtain the initial value. In that case, the failure can be judged if the voltage of varistor is over 10% or more comparing with rated voltage. However, the judgment accuracy decreases.
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Circuit Protection (ESD, Surge, Fuse etc.)
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