from--bo...@vnet.ibm.com Several years ago I tested MOV devices from several manufacturers. All had a nominal rating of 70 volts and 10 joules. My testing forced breakover with a 10-ohm limiting resistor. Source voltage was 120 mains and simple fill-wave rectifier. The results were quite interesting and (my opinion) point out that the circuit designer must consider all aspects of a part when used in a safety-sensitive application.
1. All parts failed rather violently, with a large plume of dense smoke and some flaming, as the overcoat materials decomposed and burned away. 2. Following the flaming stage was a 'cooking' stage. These parts collapsed to approximately 7 volts breakover zener characteristic and soaked up the high energy available. Judging by the white-hot color the surface temperature was approximately 1200 C. I let several of these tests run for 10 minutes--- these are very rugged parts, basically a ceramic sprayed with metallic coating for lead connection. Some of the parts lost connectivity and ended the tests, but this is not reliable. 3. In my opinion, use of these parts must include a disconnect means to cover the fault case.
