> I get the impression that in some readers minds buried cable is immune to
> the effects of lightning. However, the earth is not a magnetic shield. A
> cable a foot or two underground can have a surge induced in it of a
> thousand volts or so from a lightning strike a quarter mile away, or more.
> (Twice, one when the strikes magnetic filed expands outwards and again
> when the field collapses). Those surges can be on the shield of your
> buried coax cable and on the center conductor.
The main advantage of the earth is the cable is tightly coupled to
earth around the cable, and is very unlikely to have much potential
difference between it and earth from distant strikes. Put simply,
even if the shielding is less than perfect it all moves together in the
same direction at the same rate. Buried cables are mostly immune
unless very dense conducted charges along the earth's surface
from a close-by strike create problems.
The *induced* charges that are most problematic are induced when
the cable or metallic structure is vertical or suspended above
ground.
I think what mostly is missed is lightning is NOT dc, it is a varying
waveform. The energy peak is at 10kHz, with considerable energy
extending higher in frequency.
The key to lightning protection is proper grounding, and avoiding
ground loops through equipment. Virtually all of the problem deals
with connections to shields, there is very little differential mode
current or voltage induced inside a shielded cable. The exception
would be a case where you have a very large antenna element
connected to the center conductor.
If you depend on a lightning arrestor you are deluding yourself. The
vast majority of the problem is common mode, not differential
mode, voltage between conductors of incoming lines or cables! I'd
recommend reading the NAB (National Association of
Broadcasters) Handbook. They aren't trying to sell anything but
protection. In the section on antennas and towers, they cover site
lightning protection, and list other references.
73, Tom W8JI
W8JI@contesting.com
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