Thanks Ken, useful info.
Your remark about the isolation transformer (which I have installed)
as well as your url.

Regards, Len.


On Sun, 15 Jan 2006 09:24:30 -0700, Keith Hughes
wrote:

Len,

MOV's will help protect you from *large* spikes (like spikes from
lighting hitting the grid nearby, large transformers arcing, etc.) but
will not do anything for surges, sags, or common mode problems typically
seen from poor generation. For that, you need something like a UPS, or
a ferroresonant transformer backed by MOV's for major-spike protection.

If you use a varistor with a sufficiently low clamping voltage to
protect you from surges, you'll fry it pretty quick. Varistors protect
by providing a short circuit at high voltages, and if that short lasts
very long...poof.

Using a ferro as an isolation transformer will isolate you from the
generation plant, provide transient protection, carry you through
voltage sags (depending on duration of course), and dampen surges. The
biggest downside is that they are inefficient, especially under light
loads, and that translates into heat. They are also fairly intolerant
of frequency variation.

The link below has all the info about varistors you could want:

http://www.littelfuse.com/data/Appli...tes/an9767.pdf

Keith Hughes