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Lightning
Damage, Why? Where? How?
/ Back to Transient
Voltage
Portions of the following are taken
from a loss prevention manual, a major national insurance
company.
Direct
Damage is the obvious physical damage resulting
from lightning strike and is very rare. Grounding rods
are designed to conduct the discharge current and heat
away from the structure to ground. Every electrical system
should have a ground rod driven into the ground at the
AC power entrance. 
Secondary
Magnetic Effect is the difference in potential
caused by a lightning strike which can cause arcing and
induced currents. The current flow can arc across any
gaps in its path causing damage to machinery, equipment,
circuit boards, etc.
Electro-Magnetic
Effects are caused by recurring on/off action
of a lightning strike which causes the electro-magnetic
field surrounding the strike to expand and collapse with
each series of flashes. The changing motion of the electro-magnetic
field can induce electrical currents in nearby conductors
and wires causing damage to electric equipment. High tech,
solid-state equipment used today is highly vulnerable
because it is designed to operate at low internal currents.
Ground
Potential Surge is the difference in ground charge
potential caused by lightning strike which induces voltage
charges in the AC power lines. When the structure ground
takes a surge the difference in potential is great and
severe damage to electrical equipment can occur. The Environmental
Science Service Administration, U.S. Department of Commerce
records an average of between 50 and 60 thunderstorm-days
per year in Tennessee, North Georgia and North Alabama.
Most
of the time (99%), installing the Advanced Electronics
surge protection on all incoming AC power lines, sub-panels,
phone lines and data cables is sufficient. However, on
rare occasions, depending on the ground potential and
how data and or phone cables are installed, induced current
damage can occur even though the AC power, data and phone
lines are protected. For example, if the electro-magnetic
field is strong enough to induce currents on the metal
pipes, etc. in a facility and the data/phone cables are
connected to these pipes, damage can occur from induced/magnetic
currents. Damage from induced and/or electro-magnetic
currents inside a facility is very unusual, so our common
practice is not to protect equipment with no outside cable
connection. However, on rare occasions when a facility
is in an area where induced damage does occur, the only
way to eliminate the problem is to protect all solid state
equipment.
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