On Oct 02, 1996 at 20:35 hrs, Russell Brooks of 3:640/305.55 wrote to Brett Vaughan:

Hello Russell,

 RB> A lot of good surge protectors do ....NOT
 RB> The common ones have a triggering voltage of 270Vac, so in my 
 RB> oppinion the extra 30Volts has already stuffed your equipment, before 
 RB> the Varistor is triggered.

Most equipment should be rated to handle that over-voltage ok.

 RB> If lightning strikes at your modem through the power or phone lines,
 RB> do you think that the super spark that just jumped 10 miles through
 RB> the sky is going to be stopped by a spark arrestor with a gap of a 
 RB> few microns or a wafer of composite material in a varistor.

Precicely why the gap is set to a few mm.  The arrestors are connected from
the active line (or phase) to earth.  The setting has to be big enough so
that it doesn't flash over during normal operation but does when say 300
volts is reached.

The main problem with them is that they take a few micro-Seconds to fire
during which time the over-voltage is presented to the computer/modem etc. 
The best systems have arrestors backed up by high speed zener diodes (which
react in nano-Seconds (1000th of a micro-Second).  The zener operates first
and holds the voltage down until the arrestor has time to fire.

Another problem is that it is not easy to determine if the arrestor/zener
combination is till operational or has failed for some reason.  If this has
happened, and you don't know about it, then you are no better off than
having nothing at all.

Perhaps the best arrangement is to also include a series fuse that blows
when the arrestor operates.  This way you know when the arrestor has fired
since the equipment won't work until the fuse is replaced.  At the same
time the arrestor/zener combination could be checked to see if healthy and
replaced.

Regards ... John
@EOT:

--- MsgedSQ/2 3.30