At 09:21 AM 12/8/2006, Jim Brown wrote:
>On Fri, 08 Dec 2006 15:47:18 +0000, Phil Camera wrote:
>
>Each of Phil's posts have been right on about grounding. I'll add
>another important point that seems to be overlooked by many here on
>the topic of surge protection.
>
>MOV surge protection is a really bad idea on branch circuits. The
>only GOOD application of an MOV is a very beefy one AT THE SERVICE
>ENTRANCE. Why?
>
>Let's say you have an MOV "protecting" a computer at one outlet,
>interconnected to a printer at another outlet, and a cable modem at
>another outlet. When the MOV fires, it conducts the surge (lightning
>or otherwise) on the green wire, and the resulting IR drop in that
>green wire raises the voltage on the chassis of equipment connected
>to THAT OUTLET to a very high value. The two ends of the cable
>connecting equipment powered from a different outlet will see the
>very high voltage at one end and a very low voltage at the other
>(or, if MOV "protected," a different very high voltage). That
>potential difference can, and usually will, blow up equipment at
>either end. This is the likely mechanism by which Pete's computers
>were toasted.
There's yet another mechanism.. The MOV (or spark gap) fires, so
there's now a "short" across the line, so the stored energy in the
line up to that point discharges rapidly (just like in a tranmission
line pulse former). That rapid transient actually radiates, and
because it's in close proximity to the victim equipment, the magnetic
field couples quite well to the transmission line on the other side
of the MOV, actually increasing the spike amplitude at the victim equipment.
There's a great book on this by Ronald Standler: "Protection of
Electronic Circuits from Overvoltages" published by Dover press for
$27. All the theory, the practice, all about what's good and bad
about various strategies.
See also: http://www.rbs2.com/blitz.htm http://www.rbs2.com/fire.htm
>Another reason MOV's are a really bad idea is that they fail or
>"work" at reduced capability after having dissipated some finite
>number of joules, but you often don't know this until it's too late.
And can fail by catching fire. How nice...
>In the pro audio world, we have lots of expensive interconnected
>equipment at widely separated locations. Pro audio folks have
>learned the value of SERIES MODE surge protection. The SurgeX line
>of products use only series-mode surge protection, and have
>established an excellent track record. They are the only protection
>product I use on branch circuits. They are not cheap, and you won't
>find them at Sams Club or Costco. But neither is my ham gear, nor my
>computers, nor my home entertainment equipment. :)
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