On Sun, 13 Jul 2003 22:44:21 +0100, "Bernard Burton"
wrote:

When there is a lightning flash close to any wire, buried or above ground,
there will be the risk of large induced currents, as a result of the huge
swings in the electro-magnetic field.

As an electrical engineer and a former meteorologist, I would say that
there is a danger of lightning induced high voltages (as distinct from
direct hits) in any wiring (indeed in anything conductive, such as a
wet washing line or wire fence), although it will obviously be
dependent on a number of factors.

I don't know for sure but any electronic equipment should, if normal
engineering practice has been followed, be protected at all inputs by
protective devices such as zener diodes, voltage dependent resistors
(VDRs or varistors) or spark gaps. These devices will handle voltage
surges but lightning induced spikes may overwhelm them.

The voltage induced does not have to be that high for damage to occur
and the charge can be very small indeed, microcoulombs or less.

Obviously overhead lines are the most prone, and even relatively
distant lightning may cause problems. Underground lines are not
immune, but the effect I would think must be less, as the lines are
"screened" for most of their length by moist soil at earth potential,
acting like a shield. The amount of line above ground at the property
is also a factor.

JPG