ERICO Practical Guide to Electrical Grounding - Manual (Page 33)

25 Chapter 2: Building Lightning Protection
There is general agreement that the best theoretical
lightning system is a solid faraday cage around whatever it
is that is being protected. An airplane is an example of this.
But even in the case of the airplane, there are incidents
reported of damage from direct lightning strokes. On the
ground, a complete faraday cage solidly tied to ground is an
attractive protection scheme, but is expensive to
accomplish. If it is a general area, and not a structure that
you are trying to protect, the faraday cage approach is very
impractical.
This book will dwell basically on the passive “Franklin
Rod” theory for lightning protection. While lightning
cannot be prevented, it is possible to design a lightning
protection system that will prevent injury to people and
damage to installations in the majority of lightning strikes.
Standards and codes for passive lightning protection
materials and installations that ensure safety and minimize
damage and fire hazards in the great majority of lightning
strikes are published by Underwriters Laboratory (UL96 &
96A), the National Fire Protection Association (NFPA 780)
and the Lightning Protection Institute (LPI-175). Protection
for 100% of the lightning strikes is usually cost prohibitive.
Meeting the codes and standards does not necessarily
provide protection to sensitive electronic equipment and
data interconnections. These can be damaged or affected by
voltage levels below those that will harm people or start
fires. A well-designed lightning system exceeds the
minimum code requirements, providing not only safety to
people and protection against fire, but also providing
protection for equipment and the integrity of data and
operations. Manmade structures of steel, concrete or wood
are relatively good conductors compared to the path of
lightning through the ionized air. The impedance of a
structure is so low compared to that of the lightning path
that the structure has virtually no effect on the magnitude of
the stroke. As a result, lightning can be considered a
constant current source. The current may divide among
several paths to earth, along the outside walls, sides and
interior of a structure, reducing the voltage drop to ground.
Better protection is provided by multiple paths to ground,
including the many paths through the steel building
structure. All structural metal items must be bonded. Bolted
joints in steel columns are usually adequately bonded as are
properly lapped and tied or mechanical rebar splices.
Effective lightning protection involves the integration of
several concepts and components. In general, lightning
protection can be indexed as follows:
1. Capture the lightning strike on purpose designed
lightning terminals at preferred points.
2. Conduct the strike to ground safely through purpose
designed down conductors.
3. Dissipate the lightning energy into the ground with
minimum rise in ground potential.
4. Eliminate ground loops and differentials by creating
a low impedance, equipotential ground system.
5. Protect equipment from surges and transients on
incoming power lines to prevent equipment damage
and costly operational downtime (See Chapter 7).
6. Protect equipment from surges and transients on
incoming telecommunications and signal lines to
prevent equipment damage and costly operational
downtime (See Chapters 4 and 7).
My thanks to Dr. A. J. (Tony) Surtees, Manager - Facility
Electrical Protection, North / South America, ERICO, Inc.
who greatly assisted in the following section.
A NEW APPROACH TO LIGHTNING
PROTECTION
The overall purpose of a lightning protection system is to
protect a facility and it's inhabitants from the damage of a
direct or nearby lightning strike. Since ERICO believes that
trying to prevent a lightning strike is unreliable, the best
way to protect is to shunt the lightning energy “around” the
vital components/inhabitants of the facility and dissipate
that energy into the earth where it wants to go anyway. The
first step in that process is to make sure that lightning, when
approaching the facility, is attracted to the strike
termination devices that have been installed on the structure
for that purpose. The role of a lightning strike termination
system is to effectively launch an upward leader at the
appropriate time so that it, more so than any other
competing feature on the structure, becomes the preferred
attachment point for the approaching down leader
(lightning strike).
As the down leader approaches the ground, the ambient
electric field rapidly escalates to the point where any point
on the structures projecting into this field begin to cause air
breakdown and launch upward streamer currents. If the
ambient field into which such streamers are emitted is high
enough, the partially ionized streamer will convert to a fully
ionized up-leader. The ability of the air termination to
launch a sustainable up-leader that will be preferred over
any other point on the structure, determines it’s
effectiveness as an imminent lightning attachment point.
The Franklin Rod or conventional approach to lightning
protection has served the industry well, but since its
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