Lightning Protection Systems
Question: Our church has
old lightning rods. How can I tell if they are still in working condition? Do
I need them today?—Father Kevin O’Brien, St. James Roman Catholic Church, New
York, New York
Answer: Lightning has long
been a threat to buildings. Today, it is estimated that more than 30% of all
fires in religious properties are the direct result of lightning.
Lightning is a rapid exchange
of electrical charges between a cloud and the ground. Objects, especially tall
ones, which are considered a vertical extension of the earth’s surface, are
particularly vulnerable to this force. Lightning is classified in four categories:
negative descending, positive descending, negative ascending, and positive ascending.
Ninety per cent of all lightning is of the negative ascending type. The force
of a single bolt of lightning can contain as much as 100 million volts of energy
or the equivalent voltage of ten major power companies. Temperatures have been
found to be as high as 30,000°C.
Dense, gray column-shaped
clouds known as cumulonimbus are the source of lightning. As they grow, positively
charged electrical substances known as ions elevate towards the frigid upper
layers of the cloud while their negative counterparts tend to gather at the
lower levels. As the charged cloud moves closer to its eventual point of impact,
an electrical tension is created in the air by the attractive, opposite charges
in the ground and the clouds. Lightning seeks the path of least resistance to
the ground; elevated points such as church steeples are constantly at risk.
Lightning protection systems
have been around for centuries. The purpose of most systems is to conduct and
control the lightning current away from the building by providing a direct and
easy low-resistance electrical path to the ground. The path is insulated from
contact with the building to prevent fire or other damage. The most commonly
used system on spires is the Franklin lightning conductor developed by Benjamin
Franklin. A Franklin conductor protects a cone-shaped area in which the radius
of the base is equal to the height. For example, if the height from the ground
to the top of the conductor is equal to 50 feet, then the area protected by
the conductor extends 50 feet in all directions from the point on the ground
that the conductor is above. All building elements contained within the volume
of this cone are protected.
Starting at the highest point of the building, the basic components of a Franklin
lightning conductor system include a solid or tubular lightning rod or air terminal,
which is mounted on an insulated base and designed to intercept a direct stroke.
The rod or terminal is connected to the conductors, which are typically insulated
copper or aluminum cables, but can also be solid or tubular strips, bars or
rods. The conductors serve as downleads to at least two ground connections,
of which there are several types, each of which provides ample contact with
the earth (Figure 1).
Lightning protection systems
should be inspected periodically to insure that they will properly function
when needed. A system in poor condition, in fact, can actually attract lightning.
The Lightning Protection Institute (lpi) is a not-for-profit organization that
researches and disseminates information on lightning and lightning protection,
advises looking at critical check points at least every five years.
Since standards for the
industry have only existed since the 1950s, it is especially important for older
systems that materials be in sound physical condition. Outdated materials should
be replaced. System components should be properly situated and the route to
the ground be intact. Air terminals should not be bent, cracked, broken or otherwise
damaged.
All cable connections and
holders for the conductors should be tight and secure. The cable should be taut,
have no bends with an angle less than 90° and have no loose ends. The conductor
should be properly bonded to other metal bodies on the roof, including those
installed after the system to prevent lightning "side flash" or arcing
between metals. The condition of the grounding at the base is especially important
due to its need to receive over a million volts of energy and dissipate it into
the ground. Lpi recommends grounding at least one foot below grade. In addition,
a ground resistance check should be performed by a qualified engineer or lightning
system installer using an "ohms" resistance meter (Ohms are a unit
of electrical resistance). Every newly-installed system should meet contemporary
standards of lpi or the Underwriters Laboratory. Some insurance companies will
allow rebates on premiums for such certified systems.
Besides the Franklin protection
system, another type of system available is the ionizing lightning conductor
(ilc) or "Preventor." This is classified as a preventive system. It
sits on the highest point of the building. The ilc utilizes the principle that
lightning will readily travel along a stream of ions to reach the ground. By
using an ilc in a tension-filled atmosphere, the ions discharged by the ilc
unit bond with the charges and thus reduce the amount of electrical tension
present. However, it is only a temporary lowering of the tension and it is not
always sufficient to prevent a discharge of lightning. The ions in the ilc are
produced by such radioactive materials as radium and americum.