Pete C. wrote:
Example:

A common mode surge comes in along the 2 wire power cord to line lump
powering a laptop computer sitting on a wooden table. The laptop has no
connections to any other device i.e. WiFi network. Unless the surge is
of a large enough magnitutde to punch through the insulation of the
devices in question there should be no damage.

Punching through insulation (converting non-conductive material
temporarily into conductive material) is what surges do. Again,
appliances already contain any protection that will work on their power
cord. But a destructive surge creates conductive paths through items
(such as the wooden tabletop or church steeple) normally not considered
conductive.

Another classic example is a dialup modem. How are they damaged? A
most typical path goes into computer on AC mains, through modem via its
off-hook relay, then out to earth ground via telephone line. How does
it make a conductive path to a galvanically isolated phone line? Surge
creates a conductive path from relay's coil, across an isolation
barrier, to relay's wiper. IOW destructive surges are destructive
because they create conductive paths through non-conductive material.

So what is a building owner to do? The building is chock full of
potentially conductive paths to earth ... which is why protection must
earth before transients can enter that building.

A computer connected only to AC mains and using WiFi is less likely
to be damaged - just like the TV adjacent to a VCR might not be damaged
when VCR is destroyed.

Not only is earthing essential - so that protection inside that
laptop is not damaged. The protection is layered. A 'whole house'
protector earthed by a building electrode is secondary protection.
Primary protection must also be inspected:
http://www.tvtower.com/fpl.html

Surge protection is not installed for every transient. Protection
inside all appliances makes most all transients irrelevant - whether
laptop uses WiFi or phone line. But a destructive transient that would
otherwise punch through such protection is why effective protectors are
installed. Such surges occur typically once every seven years - a
number that can vary significantly even within a same town. We earth a
'whole house' protector so that the destructive surge does not punch
through insulation - protection that exists in all electronics.

Anything that a 'plug-in' protector would accomplish is already
inside electronics. Protection that can be overwhelmed by punching
through insulation. Just another reason why money spent on a plug-in
protector is better spent to enhance earthing for a 'whole house'
protector.