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Although a rugged enclosure or backfilling will achieve the appropriate safety requirements, this solution can be costly. Using a higher rated suppression component to prevent break over can eliminate the safety risk during abnormal overvoltage testing, but will result in a higher VPR. This may not be a desirable trade off for the manufacturer. Fusing will address the short circuit test condition safety concerns, but typically is not sufficient to pass the abnormal overvoltage, limited current condition due to the reduced test current. A Thermal Cutoff device or TCO can be used in association with an MOV, but this will increase component count, reduce board space, and poses some difficulties in assembly due to the temperature sensitive nature of the TCO. Littelfuse TMOV Thermally protected varistors have an integrated thermally-active element to provide compliance to the abnormal overvoltage, limited current requirements of UL1449 in a single component. It is important to note, that the thermally-active element is not categorized as a TCO since it does not carry continuous current.
