ERICO Practical Guide to Electrical Grounding - Manual (Page 32)

New detection devices have been installed around the U.S.
which count the total number of lightning strokes reaching
the earth. This data results in precise occurrence of the total
strokes for a particular period of time for any particular area
rather than thunderstorm days per year.
Lightning is the nemesis of communication stations, signal
circuits, tall structures and other buildings housing
electronic equipment. In addition to direct strike problems,
modern electronics and circuitry are also highly susceptible
to damage from lightning surges and transients. These may
arrive via power, telecommunications and signal lines, even
though the lightning strike may be some distance from the
building or installation.
LIGHTNING PROTECTION
Lightning protection systems offer protection against both
direct and indirect effects of lightning. The direct effects are
burning, blasting, fires and electrocution. The indirect
effects are the mis-operation of control or other electronic
equipment due to electrical transients.
The major purpose of lightning protection systems is to
conduct the high current lightning discharges safely into the
earth. A well-designed system will minimize voltage
differences between areas of a building or facility and
afford maximum protection to people. Direct or electro-
magnetically induced voltages can affect power, signal and
data cables and cause significant voltage changes in the
grounding system. A well-designed grounding, bonding
and surge voltage protection system can control and
minimize these effects.
Since Ben Franklin and other early studiers of lightning,
there have been two camps of thought regarding the
performance of direct strike lightning protection systems.
Some believe that a pointed lightning rod or air terminal
will help prevent lightning from striking in the immediate
vicinity because it will help reduce the difference in
potential between earth and cloud by "bleeding off" charge
and therefore reducing the chance of a direct strike. Others
believe that air terminals can be attractors of lightning by
offering a more electrically attractive path for a developing
direct strike than those other points on the surface of the
earth that would be competing for it. These two thought
"camps" form the two ends of a continuum upon which you
can place just about any of the direct strike lightning
protection theories. The continuum could be represented as
shown below.
ACTIVE ATTRACTION SYSTEMS
On the left we have systems that are designed to attract the
lightning strike. The theory behind this practice is to attract
the lightning to a known and preferred point therefore
protecting nearby non-preferred points. The most common
way this is done is to have an air terminal that initiates a
streamer that will intercept the lightning down stroke leader
with a pre-ionized path that will be the most attractive for
the main lightning energy to follow.
PASSIVE NEUTRAL SYSTEMS
The middle of the continuum represents the conventional or
traditional theory of direct strike protection. Conductors
are positioned on a structure in the places where lightning
is most likely to strike should a strike occur. We have
labeled these systems as neutral since the air terminal or
strike termination devices themselves aren’t considered to
be any more attractive or unattractive to the lightning stroke
then the surrounding structure. They are positioned where
they should be the first conductor in any path that the
lightning strike takes to the structure.
ACTIVE PREVENTION SYSTEMS
The right third of the continuum is where we find the
systems that are designed to prevent the propagation of a
direct stroke of lightning in the area where they are
positioned. There are two theories as to how preventative
power occurs. The first is the “bleed off” theory mentioned
previously. The second is that the sharp points on the
prevention devices form a corona cloud above them that
makes the device an unattractive path to the lightning stroke.
There are some commonalities in these three approaches.
Each system’s design requires the following:
1. The air terminal or strike termination device must
be positioned so that it is the highest point on
the structure.
2. The lightning protection system must be solidly and
permanently grounded. Poor or high resistance
connections to ground is the leading cause of light-
ning system failure for each one of these systems.
To go further in our comparison, we must separate the
prevention systems from the other two. Obviously, if you
are counting on preventing a lightning stroke from arriving
near you, you don’t have to worry about how to deal with
the lightning current once you have it on your lightning
protection system. None of these systems claims to protect
against 100% of the possibility of a lightning stroke arriving
near you. A compromise must be made between protection
and economics.
24 Practical Guide to Electrical Grounding
Active Passive Active
Attraction
Early Streamer Emission Streamer Delay Franklin/Faraday Cage
Neutral Prevention
Dynasphere Spline Balls Blunt Ended Rods Sharp Pointed Rods
Grounding Book 4/14/99 10/5/99 6:01 PM Page 24 (Black plate)