Skip to main content

Thermally Protected Varistors Versus Standard MOVs

title

Standard MOVs (metal oxide varistors) have been providing transient voltage surge protection from surges for many years.

Surges can originate from lightning, electronic switching, electromagnetically induced surges and a myriad of other sources. MOVs are specifically designed to protect electronic equipment from overvoltage transient surges, however, they are not designed to protect equipment from a temporary abnormal overvoltage. This temporary overvoltage can be much longer in duration than the typical 8x20 μsec surge and can happen when a loss of neutral occurs in a three-phase system with imbalanced lines, or if an increase in the A-C line voltage occurs from an unstable electrical utility company. A temporary overvoltage can trigger the standard MOV into a continuous clamping condition and resulting fire, explosion or other hazardous condition. The figure above illustrates what can happen to an MOV that experiences a 20% increase in line voltage.

The illustrated MOV is rated at 90VAC maximum continuous line voltage and the temporary abnormal overvoltage applied away. This could cause a catastrophic event, especially if the MOV was housed in a plastic enclosure which could catch. Recently published Underwriter’s Labs 1449 3rd edition has increased the levels of limited current during the temporary abnormal overvoltage testing that Surge Protective Device manufacturers are required to pass. Thermally Protected Varistors address these concerns by integrating a thermo-fuse in series with a metal oxide varistor. When a temporary overvoltage limited current event occurs, the varistor will fail to a low resistance, and the thermo-link will open. This will effectively disconnect the damaged varistor from the circuit and prevent sustained follow current through the MOV portion. Proper design would be to incorporate an indicator circuit (LED) to alert the operator that the Thermally Protected MOV has failed and needs to be replaced. Also the AC line should be properly fused for the higher currents, and an enclosure must be used.

Leonard Drewes, Engineering Manager, World Products, LLC, Sonoma, CA International Appliance Manufacturing 2008 the thermo-fuse will open. This will effectively disconnect the damaged varistor from the circuit and prevent sustained follow current through the MOV portion. Proper design would be to incorporate an indicator circuit (LED) to alert the operator that the Thermally fused MOV has failed and needs to be replaced. Also the AC line should be properly fused for the higher currents, and a metallic enclosure should be used.

RS Components' new range of Thermally Protected MOV's

BUY these Products

Electromechanical Distributor
DesignSpark Electrical Logolinkedin