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Section 6.10
Little Known Lightning Information
- THE AVERAGE DISTANCE BETWEEN SUCCESSIVE FLASHES IS GREATER THAN
PREVIOUSLY KNOWN.
Old data said successive flashes were on the order of 3-4 km apart.
New data shows half the flashes are some 9 km apart. The National Severe
Storms Laboratory report concludes with a recommendation that: "It
appears the safety rules need to be modified to increase the distance
from a previous flash which can be considered to be relatively safe,
to at least 10 to 13 km (6 to 8 miles). In the past, 3 to 5 km (2-3
miles) was as used in lightning safety education."
Source: Separation Between Successive Lightning Flashes in Different
Storms Systems: 1998, Lopez & Holle, from Proceedings 1998 Intl
Lightning Detection Conference, Tucson AZ, November 1998.
- A HIGH PERCENTAGE OF LIGHTNING FLASHES ARE FORKED.
Many cloud-to-ground lightning flashes have forked or multiple attachment
points to earth. Tests carried out in both the USA and Japan verify
this in at least half of negative flashes and more than seventy percent
of positive flashes.
Present day lightning detectors may not be able to discriminate between
the several forks from the same flash.
Source: Termination of Multiple Stroke Flashes Observed by Electro-
Magnetic Field: 1998, Ishii, et al. Proceedings 1998 Int'l Lightning
Protection Conference, Birmingham UK, Sept. 1998.
- LIGHTNING CAN SPREAD OUT SOME 60 FT. UPON STRIKING EARTH'S
SURFACE.
Radial horizontal arcing has been measured at least 20 m. from the point
where lightning enters the earth. Depending upon soils characteristics,
safe conditions for people and equipment near lightning termination
points (ground rods) may need to be re-evaluated.
Source: 1993 Triggered Lightning Test Program: Environments Within
20 meters of the Lightning Channel and Small Are Temporary Protection
Concepts: 1993, SAND94-0311, Sandia Natl Lab, Albuquerque NM.
- LIGHTNING PROTECTION SYSTEMS PROVIDE LIMITED PROTECTION.
"What we found out was that the lightning protection system played
a limited role in directing current from a lightning strike…current
traveled through the rebar, through concrete, through pipes, through
cables, through vent stacks, and through the electrical system…"
- Results of rocket-triggered testing.
Source: Marvin Morris, Electromagnetic Test and Analysis Dept.,
as quoted in Sandia Lab News, April 25, 1997, Sandia Natl Lab, Albuquerque
NM.
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