Uncle Peter presented the following explanation :
On Mon, 24 Feb 2014 14:15:06 -0000, Graham. wrote:

On Mon, 24 Feb 2014 13:48:25 -0000, "Uncle Peter" wrote:

I've got a few cheap micromark surge protectors, and noticed they have a
clamping voltage of 650V.
Anyone know where I can get a much lower one? I've heard of protectors as
low as 270V, but I can't find any to buy.

For 230V RMS?

Yes.

650 is the peal to peak equivalent, but I think it's just the peak
voltage that's relevant here, so anything less than 325v is no good,
and you are going to need some margin above that.

Don't know what micromark offer, but if it's just a few twopenny MOVs
I shouldn't bother.

This: http://petersphotos.com/temp/surge.jpg

It's to protect computers. Looking at other surge protectors up to 100 quid
(this one was 4 quid), nothing is lower than 650V. And some of the 100 quid
ones are 750V!

Am I right in thinking 650V is rubbish? I mean the surge could double the
peak supply voltage without the protector doing anything, and if the surge
was more, it would clamp it down to double, which is enough to **** up a
computer. Mind you if it's just the power supply caps that go bang, I
suppose it doesn't matter. Does a PC power supply stop surges getting past
it?

Absolutely not! I've just done a partial rebuild of a gaming machine
for someone that has cost United Utilities (their electricity supplier)
over £1,500 after it was hit by a voltage surge - same house also had
two TVs and a microwave oven blow up too. And about twelve other houses
were affected so not a happy time for UU.

Or.... does 650V mean peak to peak as opposed to zero to peak?