Some things to think about..... The rated operating voltage of the MOV needs to be greater than the PEAK AC voltage, not the RMS. During an over voltage event (spike or otherwise) the MOV conducts, generating heat. The higher the heat the lower the resistance causing them to go into thermal runaway quickly! This may result in a fire if the circuit is not properly protected by a thermal cutout. All recently UL listed surge protection devices (SPDs) are required to have a thermal cutout next to the MOV to disconnect power in the event of thermal runaway. I have seen many instances where an MOV has been installed without the thermal cutout and the result has been a fire. Many MOV devices will go into thermal runaway before they draw enough current to open a fuse of 10 or 15 Amps, typical of some of the larger power supplies. It only takes a small amount of current to cause thermal runaway where the MOV starts to flame and maybe cause damage to other items or cause a fire if not well contained in a non flammable compartment. As pointed out, the MOV degrades with every over voltage spike. Depending on the electrical environment, their effectiveness may be short lived. The best protection is achieved with a properly coordinated SAD (silicon avalanche diode) and MOV combination. UL1449, Edition 3 is an excellent reference. I would recommend that any surge protection device installed be UL1449, Edition 3, listed to ensure it does not cause a fire. I am referencing commercial devices, not individual components we may install in a project of our own as the individual components are typically not UL listed, only a complete product. When reviewing a product for purchase ensure that it has the UL1449 Ed 3 listing. Not just a UL seal as that could mean anything. Many protected outlet strips have a UL seal affixed but that only applies to the outlet strip portion being UL listed and not the surge protection portion. Generally if the manufacturer has a UL listing for the device as a surge protection device (SPD also previously known as TVSS devices) they will be proud of their UL listing and will have it printed on the label. Be sure you see Ed 3 appended to the UL1449 as the original 1449 of a few years ago did not require the thermal cutout device. Only after several fires was the standard changed to require the thermal cutout on the MOV (actually physically next to and electrically in series with the MOV)
With respect to the generator surges.... Just from reading the post, my thought is that the only load on the generator was the power supply or maybe with an additional small resistive load. I suspect this was a switched power supply without power factor correction. I also suspect that the generator governor was not designed with a tolerance suitable for powering sensitive electronic loads and that there was no voltage and little speed regulation built in. AC power generators like a power factor of nearly 1. Therefore, we must ensure that a goodly portion of the generator load is resistive or has a PF of nearly 1. Many switched supplies have a PF of as low as 0.7. Without an appropriate resistive load to offset this low PF and help get this closer to a PF of 1, the generator may do strange things. Also, generators may exhibit strange operating characteristics when used with power supplies that have PF correction circuits built in. The wrong combination of load and generator may cause damage to the load (power supply). Products designed for international distribution have power factor correction circuits installed, such as several Motorola products. Be cautious when powering these products from small generators that you may typically purchase at your local home center. Those generators are not designed to power sensitive electronic equipment and may cause damage to the equipment. I have seen that happen all too often. Don't try a Motorola Quantar on one of these generators. You may be disappointed. A generator suitable for powering your sensitive electronic loads must have suitable speed (frequency) and voltage regulation built into the product. A reminder to evaluate your field day generator with respect to your load. I have heard stories where folks have brought their valued home station to a field day site only to take the power supply home damaged. I hope this is helpful. 73 Bruce, W3YVV
