Harvey White wrote: > > claude@bauv.unibw-muenchen.de (Claude Frantz) wrote: > > >jlundgre@delta1.deltanet.com (John Lundgren) writes: > > >>IMHO, the only place for MOVs is in the trash can. The only place to do > >>surge protection is where there is a good ground. And this is at the > >>breaker panel, not at the outlet strip. > > The best surge protection is at the breaker panel to the house. No > questions there. However, there are other places where MOVs can help. > John is absolutely right, there is very little that can help against a > direct strike, and a tiny little MOV will certainly not do it. The > only thing that might help in that case is to put enough impedance in > the line that the lightning just wants to go somewhere else. > > However, on those near misses and things, You will get some high > voltage spikes on the line. The MOVs are meant to clamp these, and > the gas tube surge supressors are meant to help as well. These spikes > are the ones that can damage most solid state equipment by punching > through the BE and BC layers of transistors, and anything else, > either. > > Harvey > Here's a couple of factoids to add to the above: 1. Most lightning strikes occur at some distance from your equipment. 2. These are fast rise-time impulses, so inductance in power lines can have a considerable effect on the amplitude and duration as seen at a distance. 3. Most wiring devices used in homes and offices have a limiting dielectric strength of about 6KV. This means that there are effectively a number of 'spark gaps' in most feeder systems. This, too, affects the amplitude seen by equipment. 4. Power lines also have resistance. Home wiring tends to have more than commercial wiring. This limits the current. 5. Feeders from service entrances connect radially to the breaker panel. These feeders each get a share of the power from transient surges. This limits the power seen by any particular piece of equipt. 6. Surges can have differential and common mode components. Surge suppression connected to earth ground typically only helps with common mode. Differential mode suppressors need not be grounded. 7. Dielectrically isolated components, provided that they have sufficient dielectric strength, will probably be undamaged by common mode transients. 8. MOVs are inexpensive and effective for transients up to a certain magnitude, but they suffer from gradual degradation. They are easiest to apply in differential mode. 9. Tranzorbs, which are groupings of high-powered avalanche breakdown diodes, are more rugged than MOVs. They can be effectively applied in either differential or common mode, or both. Best layout, as claimed above, is to have a massive tranzorb at the service entrance, set up in both modes, with a good ground. 10. Anything is better than nothing. There are lots of small transients that can damage equipt. if not suppressed. -- Paul Mathews, consulting engineer AEngineering Co. optoeng@whidbey.com non-contact sensing and optoelectronics specialists
Date: Mon, 19 Feb 1996 19:47:57 -0800
Original Subject: Re: Evaluate MOV's