hsplab@tucc.UUCP (08/07/83)
David Anthony's remarks about GE MOVs do deserve some caution. We have been having some problems with high voltage transients produced during the testing of emergency power generators in the hospital (NC Memorial Hospital/Univ of NC-Chapel Hill). The GE MOVs which I put into several of these terminals failed to provide any protection. Upon inquiry with several engineers, it was concluded that the MOVs are relatively slow. In order for them to be effective in removing short duration spikes (or all of longer duration spikes), it is necessary to have a series resistor (low ohmage), parallel capacitor network to swamp out the spikes until the MOV has time to react. This is because of the varistor action of the device itself. A similar solid state device made by Panasonic call Transorbs are considerably faster. In our case, the Trans- orbs were effective without any additional swamping circuit. They are some- what more expensive, however. Finally, some of the noise reduction filters, such as the Corcom, can be extremely effective in reducing line spikes of short duration. To my chagrin, the protection of computer systems from line transients is not as trivial as I had initially imagined. Although many manufacturers are in- stalling Corcom type filters into computer devices (mostly for FCC certifi- cation and/or DIN approval for sales to Europe), I have seen almost no MOVs in computer equipment. Although this may be due to costs, I am also beginning to suspect that the overall benefits of the MOV are somewhat marginal. A direct lightning strike will destroy everything anyway, and the amount of protection provided by most devices may be not be useful except in cases such as ours where the equipment is subjected to repeated and regular high voltage spikes. David Chou Univ of North Carolina, Chapel Hill