ERICO Practical Guide to Electrical Grounding - Manual (Page 37)

29 Chapter 2: Building Lightning Protection
The following are basic components for a lightning
protection system. Sketches at the end of this section depict
the many requirements discussed.
Air terminals, often called lightning rods, lightning points
or strike termination devices are blunt or pointed, solid or
tubular rods of copper, bronze, stainless steel or aluminum.
On large (over 500 sq. inch [0.323 sq. m] flue cross
section), tall (over 75 feet or 23 m) smoke stacks, the air
terminals must be stainless steel, monel metal or lead
jacketed solid copper. (Fig. 2-6) Air terminals are normally
between 10 and 24 inch (254 to 610 mm) long but may be
longer. Although they are normally pointed, a blunt rod has
been tested and found to be more effective. Since they are
usually thin pointed rods, protection should be provided to
minimize the danger of injury in areas where personnel
may be present. The protection can be in several forms but
the most common is the use of tall air terminals or blunt
rods. Terminals that are more than 24 inches (610 mm) high
require extra support other than the base mount.
Conductors connect the air terminals to each other, to the
metal structure of the building, to miscellaneous metal parts
of the building and down to the counterpoise and/or earth
electrodes. Building connections are made to the steel
columns or to the rebars (steel reinforcing bars) used in
concrete construction. In most large buildings, the heavy
steel structure provides a much lower impedance path to
earth than separate down conductors installed as part of the
lightning protection system. These steel columns can be
used as the down conductors. Since the lightning current is
not effected by the structure, multiple down conductor
paths in parallel will result in lower voltage differences
between the top of the building and the foundation. This
voltage differential can be important in buildings with
electronic equipment interconnected between floors, in
antenna towers and similar instances.
The size of the conductors is not too important although
they must meet the minimum requirements of the lightning
code. For example, a 4/0 conductor is only slightly better
(lower impedance) than a No. 6 AWG conductor for the
short duration (high frequency) of the lightning stroke.
Although the ampacity (DC resistance) of these two
conductors are different (by a factor of approximately 8),
short time impulses have voltage drops that are usually
within about 20% of each other.
The lightning down conductors must be bonded to the
building steel. Also included are any conductive items
which may cause side flashes resulting from instantaneous
voltages that exceed the voltage withstand capability of the
air or other insulating material between the conductor and
the conductive item. Side flashes can occur between
lightning conductors and building steel, permanently
mounted ladders, equipment, etc. even though all are
connected to a common ground or earthing point. The
instantaneous voltage difference can become dangerously
high because of the high impedance of the various paths to
the steep wave front lightning current, resulting in large
voltage drops. Even when no side flash occurs, the large
voltage differences can cause electronic noise and
component failure. Often, latent component failure, created
by repeated voltage stress, will cause equipment failure at a
time when no lightning or other outside influence is
present. This problem is likely to be made much worse
where there are separate equipment grounds, not bonded
together (which is a violation of the National Electrical
Code [NEC]).
A few general rules are that the conductors must be
horizontal or course downward from the air terminal to the
ground electrode; they cannot have a bend over 90° (Fig. 2-
7); they cannot have a bend radius tighter than 8 inch (200
mm) radius (Fig. 2-7); they cannot be coursed through the
Fig. 2-7
8" Minimum Bend Radius
90 Maximum Bend
Lead Coated
24"
Fig. 2-6
Grounding Book 4/14/99 10/5/99 6:01 PM Page 29 (Black plate)