mm wrote:

On Sun, 03 May 2009 16:48:03 -0400, Bryce
wrote:

Caesar Romano wrote:

If two surge protectors are connected in series, is the amount of
surge protection available at the down-stream protector approximately
equal to the sum of the two individual protections??

By 'surge protector', do you mean something like plug strips with
MOV peak voltage limiting? If so, the upstream device will limit
an overvoltage transient and the downstream device sees normal
waveform and provides added protection only if the upstream
device fails. Actually, the varistors don't provide an absolute
clamp at their trigger voltage, but the downstream guy will do
very little protecting.

If you're referring to connecting two MOV's in series across the
line, then the overvoltage clamping will begin when line voltage
reaches the sum of the individual MOV clamp voltages. The same
transient current will flow through each MOV, and each will
dissipate part of the transient (as heat). If they have the same
clamp voltage, then each MOV absorbs half of the energy.

You don't finish, but aren't you saying that the voltage that gets to
the appliance can reach twice the voltage with only one MOV across the
line, that it's much worse, much less, practically no protection with
two? I don't know enough to know, but that sounds conceivable and
sounds like the logical next sentence to what you wrote.

The peak let-thru voltage will be the sum of the series-connected MOV
clamp voltages (a bit higher actually). They are manufactured in several
clamp voltage ratings. Using a series-connected pair would probably
happen only if designing protection for an unusual line voltage or
to make do with what is in the junk box. Using two series-connected
MOV's, each intended for 120VAC to protect a 120VAC line would be a
bad idea. Using the same pair to protect a 240VAC line would be better.

By the way, parallel connection of two or more MOV's is a bad idea.
If they have different clamp voltages, the first to begin conducting
does all the work. Even two MOV's of the same rating will have
slightly different characteristics and won't play well together.

for protection of a 120VAC line