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Old 19-03-2019, 01:50 PM
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gary is offline
Join Date: Apr 2005
Location: Mt. Kuring-Gai
Posts: 5,170
Hi Peter,

You might like to look at a standard such as IEC 61024-1-2, "Protection of structures against lightning".

This is a vast topic but suffice to say unless a structure was designed
from the get-go to have best practice protection, any associated electrical
systems will always be vulnerable.

Broadly, there are what are termed "air termination networks" which are
connected to what are termed "down conductors". There are usually
multiple down conductors placed uniformly around the periphery of
a building.

Air termination networks are typically designed using either vertical or
horizontal conductors. It use to be thought that vertical pointed rods
were the best design for air termination conductors but in more
recent years studies have shown they are not as effective as they were
thought to be and so most modern air termination systems on larger buildings
use a horizontal mesh. For example a mesh size with 5m square

Owing to the very fast di/dt's, lightning doesn't always do what one
intuitively thinks electricity should do. For example, at such fast rise times
it will take the path of least impedance which often means the path of
least inductance rather than what you might measure with your
multimeter at DC as the path of least resistance.

For example, say one has diverted it through an air termination network
and the current path is starting to flow down one of the down conductors
toward the earth. If there is a nearby grounded structure, such as a pipe
or metal surface of a building, it will happily side flash across through air
via a path of lesser impedance. Side flashes such as these have been
known to start fires.
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