volts500 wrote:
Here in Florida we get a lot of serious lightning and I've been
called to repair a lot of that kind of damage. Almost always the
systems are not bonded before the fact. I've think that I may
have noticed a pattern. Many times the VCRs will get blown out
and the TV is OK. I've also noted that modems got spared while
the phones get fried. I'm beginning to wonder if it isn't a good
idea to leave those old, no longer used VCR's and telephones
connected first in line, to act as sacrificial lambs, so to speak.

Why two adjacent appliances but only one damaged? Question: which is
the better path to earth? A transient first constructs a complete
electrical path from cloud, incoming on utility, through household
appliance, to earth ground, and then to charges maybe located miles
away. Then electricity flows through everything in that path. Then
something in that path fails.

The VCR could have been a better path to earth. Or internal
protection inside the TV is better than protection inside the VCR.
Therefore VCR shunted a transient that would have otherwise overwhelmed
protection inside a TV. Multiple reasons why a VCR is damaged when
adjacent TV is not. But the bottom line: transient that should have
been earthed before entering a buillding, instead, found a destructive
path via that VCR. What is that better path to earth that next time
may take out the TV?

Appliances more often damaged include fax machines, modems, base
station for a portable phone, and alarm panels. Each may be a path
incoming from any utility and outgoing to earth ground via another
utility or other conductive material. For example, incoming on AC
electric to TV and VCR. But outgoing to earth ground via cable was
easier via VCR. Or maybe that conductive path was outgoing elsewhere.
Even linoleum tile floor or other materials normally not considered
conductive may become part of a destructive and outgoing path.

Obviously, finding every interior path to earth can be difficult. So
many items inside a building normally not considered conductive can,
instead, conduct destructive electrical transents. Therefore we earth
that transient before it can even enter the building. A protection
system must accomplish two things - equipotential and conductivity.

We make that earthing electrode as conductive as possible. But we
can never make it conductive enough. So we also create equipoential -
a single point earthing electrode, bonding everything together as short
as possible, or (even better) Ufer ground that surrounds a building.
But since we never do equipotential sufficiently, then we must improve
conductivity. Earthing (for transient protection) is an attempt to
accomplish both. Both so that neither TV nor VCR become a better path
to earth.

Of course, we are human. That means testing is necessary to find
mistakes. But testing a surge protection 'system' is not possible. If
damage results, we learn where an earthing system (and protectors) has
failed - and correct that mistake.

Other requirements such as not inside metallic conduit,
I believe that NEC 250.64(E) solves that problem by requiring
that the wire be bonded to the pipe at both ends.

Bonding the metallic pipe (or metallic raceways) at both ends also
addresses why wire inside a conduit is less conductive. However that
bonding now adds two connections. An earthing wire should not even
have splices. These actions are for lowering wire impedance. Why do
we bond a ground wire to both ends of the pipe? Because electricity
mostly flows outside - through the pipe - not through inside wire.
This bonding is a concern for human safety. Problem is greater when
wire must earth transients.