The MOV is a ceramic based product and it will tend to degrade gradually after a large surge or multiple small surges. The degradation leads to increasing MOV leakage current, in turn raising its temperature under normal conditions like 120 VAC/240 VAC operating voltage. The built-in thermal element in the TMOV will sense the increase in MOV temperature as it continues to degrade to its end-of-life condition. Eventually, the thermal element will open and remove the degraded MOV from the circuit, preventing its catastrophic failure. Shown here is the standard MOV vs. TMOV test. MOD 1 and MOD 2 are standard MOVs and the LF MOD is the TMOV. The engineer applies the limited current abnormal overvoltage condition on these three components. The test result will tell the customer how different they are during and after the test.