Page 183 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 8 Circuit Protection 145

shows a typical faraday cage used to protect equipment
Point
exposed to high transient signals. Notice that the equipment is
100% enclosed within the housing. The pipe clamp and
ground wire are connected to an earth ground.
When testing highly sensitive electronics, any external sig-
nals may produce erroneous results. Therefore, lager faraday
cages can be built that shield the equipment from these stray Metal Rod
signals. The cage, as shown in Figure 8-37 is constructed with
a brass frame mounted to a grounded steel deck plate. The pan-
els are brass screens that are soldered to the brass frame work.
The door carries a special conductive seal. The inside of a cage Welded Lug
like this is an extremely clean electronic environment.
Cable Lug
Mount
Ground Wire
Cable Clamp
To Additional Ground
Conductive Door Seal Rods
Figure 8-38 Lightning Rod

Brass Screen

Brass Frame A lightning rod approximates a faraday cage. The volume
that is protected is in the form of a 90 cone, referred to as the
cone of protection. Figure 8-39 shows a lightning rod cone of
Steel Plate Deck
Ground
protection. For some applications the pole height may be
Figure 8-37 Faraday Cage for Servicing High Frequency restricted by the area that requires protection. In these cases,
Equipment multiple lightning rods are incorporated. Figure 8-40 shows a
house that is protected with three rods.
When protecting timber structures great care should be
taken in placing the ground wire and rod. Because of the
extreme energies involved, lightning will oftentimes travel
Lightning Protection

Lightning carries exceptionally high energies and the result-
ing strike damage can be severe. Buildings can burst into
Lightning Rod
flames, electrical and electronic devices can be completely
destroyed, metal fixtures can be melted, and a person can be
instantly killed. When trees are struck by lightning, so much
energy is absorbed that the water inside the trunks will flash
Height Cone of Protection
vaporize and the trees will explode. 90°
2× Height
Lightning Rods

Most of us are familiar with lightning rods. We have seen them
on top of buildings, barns, and radio towers. The idea behind Earth
the lightning rod is that the lightning strike is conducted to Figure 8-39 Lightning Rod Cone of Protection
ground over a predetermined and, therefore, harmless path.
Figure 8-38 shows a typical lightning rod that may be found on
top of any modern building. The top of tall parking garages is
an excellent place to inspect a lightning rod installation. Lightning Rods
Needless to say, inspections should only be carried out on a
clear day with no atmospheric electrical activity. The rods are
usually mounted along the perimeter of the highest part of the
building. The rods are connected to a heavy ground wire which
Ground
is connected to an earth ground. Do not connect a lightning rod Cone of Wire
to the internals of the building. The ground wire should be Protection Ground Rod
routed down the outside of the building. Figure 8-40 Multiple Lightning Rod Protection