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wrote:
On 13 Jun 2006 14:58:56 -0700, "w_tom" wrote:

Granted it is a minor point. But the point is made because many
assume a plug-in protectors sitting between a transistor and a surge
will somehow stop or block what three miles of sky could not.
Effective protectors 'shunt'. Those protectors that don't have earth
ground hope one assumes it will instead 'stop' or 'absorb' a surge to
promote a myth. Even shunt mode plug-in protectors only shunt - divert
- a transient. To be effective, it must shunt to earth and not divert
into the appliance. A 'semantic' that can otherwise create confusion
- promote an ineffective product.

This is the only place I dissagree with W-Tom.
He is 100% right about the importance of a single point ground,
bonding of all point of entry protection plus the importance of solid
grounding electrodes and connections. You should try to stop the
surge at the service entrance but where I dissagree is I still think a
plug in protector has value to catch the things his point of entry
system didn't stop. These will usually be a very low order tranient if
the POE system was working but still enough to hurt you. It is better
to heat up a shunt wired MOV than to let this into a MOSFET.
Good point of use protectors also have a large ferro resonant
componant that will eat some heat too.
It still needs a good ground path back to your grounding electrode
system, just to stabilize the reference levels. He is right that it
does little good to clamp the transient if the resulting "zero" is 40v
above the ground reference of the interface lines.
You can't have too many layers of protection if you live in a
lightning area. I just like to add another layer to his system.
When I was doing physical planning for a large corporation that sold
business machines internationally we had a lot of lightning experience
is South Florrida. Layers of protection works for people who can't
turn off their system and unplug it every afternoon in the summer.

I agree. Tom starts off with good advice, but then goes down the path
of "If the whole house surge protection didn't work, then you did
something wrong and point of use protectors are useless." I and many
others have had experiences where it was pretty clear that plug-in
surge protectors did work. He's right, that a short ground path with
a service protector is clearly better, but that doesn't mean the
plug-in protectors are totally useless. A lot, for example, depends on
the rise time of the surge. The slower the rise time, the less
impedance there will be.

Also, for many folks, eg, those living in apartments, a whole house AC
protector is just not possible.