Lightning
One of Nature’s Most Fascinating Phenomena
• Lightning is one of the most beautiful and
spectacular displays in nature.
• It is also one of the most deadly natural
phenomena known to man.
• With bolt temperatures hotter than the
surface of the sun and shockwaves radiating
out in all directions, lightning is a lesson in
physical science and humility.
Lightning
What is lightning?
• Lightning is a massive electrostatic discharge
caused by unbalanced electric charge in the
atmosphere, either inside clouds, cloud to
cloud or cloud to ground, accompanied by the
loud sound of thunder.
– Electrostatic discharge?
• Think of those annoying finger zaps you get when you
drag your feet across the carpet…only many more
magnitudes more powerful and awesome.
Lightning
Where does it occur?
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How often does it occur?
• Less than 0.1 flash/km2 per year (<0.26
flash/mi2 per year) in Western Washington.
• Eastern Washington might see as high as 0.5
flash/km2 per year (1.3 flash/mi2 per year).
• Whereas Florida might see 14 flash/km2 per
year (37 flash/mi2 per year).
– That’s 10-100 times more often than what we’d
see in the Pacific Northwest.
Lightning
Did you know?
• Lightning strikes 40–50 times a second worldwide,
possibly more.
– That’s at least 1.5 billion flashes per year.
• The strike (arc) can be over 5 miles (8 km) long.
• It can raise the immediate air temperature to over
50,000 oF.
• It can be over 100 million Volts.
– Typical residential and commercial electrical systems are
240/120 Volts and 480/277 Volts or some combination of
the two.
• It is also believed to have played an vital role in life on
Earth.
Lightning
On a personal note…Did you know?
• The odds of becoming a lightning victim in the
U.S. in any one year is 1 in 700,000.
• The odds of being struck by lightning in your
lifetime is 1 in 3,000.
• If you can hear thunder, you are within 10 miles
(16 kilometers) of a storm and can be struck by
lightning.
– Sound travels about 1 mile every 4.6-4.7 seconds,
hence the “count the seconds and divide by 5 rule” for
estimating how far away the lightning strike was.
Lightning
How does it all work?
• Lightning is not confined to thunderstorms.
– It's been seen in volcanic eruptions, extremely intense
forest fires, surface nuclear detonations, heavy
snowstorms, and in large hurricanes.
• Typically, as a thundercloud moves over the
surface of the Earth, an electric charge equal to
but opposite the charge of the base of the
thundercloud is induced in the Earth below the
cloud.
– The induced ground charge follows the movement of
the cloud, remaining underneath it.
Lightning
How does it all work?
• The more common “negative discharge”
occurs when negative charge accumulates at
the base of thundercloud, inducing a positive
charge on the ground.
• Leaders form at the base of the thundercloud
and travel towards the ground.
– The negatively charged leader proceeds
downward in a number of quick jumps (steps).
Lightning
How does it all work?
• This initial phase involves a relatively small electric
current (tens/hundreds of amperes).
• The leader is almost invisible when compared with the
subsequent lightning channel.
• The progression of stepped leaders takes a
comparatively long time (hundreds of milliseconds) to
approach the ground.
• When a stepped leader approaches the ground, the
presence of opposite charges on the ground enhances
the strength of the electric field.
– If the electric field is strong enough, a positive streamer
can develop .
Lightning
How does it all work?
• As the field increases, the positive streamer
may evolve into a hotter, higher current leader
which eventually connects to the descending
stepped leader from the cloud.
• It is also possible for many streamers to
develop from many different objects
simultaneously, with only one connecting with
the leader and forming the main discharge
path.
Lightning
How does it all work?
• Once a channel of ionized air is established
between the cloud and ground this becomes a
path of least resistance and allows for a much
greater current to propagate from the Earth
back up the leader into the cloud.
• This is the return stroke and it is the most
luminous and noticeable part of the lightning
discharge.
Lightning
How does it all work?
• When the electric field becomes strong enough, an electrical
discharge (the bolt of lightning) occurs within clouds or between
clouds and the ground.
• During the strike, successive portions of air become a conductive
discharge channel as the electrons and positive ions of air
molecules are pulled away from each other and forced to flow in
opposite directions.
• The electrical discharge (averaging 30 kA for negative or 300 kA for
positive lightning, and travelling at around 1×108 m/s) rapidly
superheats the discharge channel, causing the air to expand rapidly
and produce a shock wave heard as thunder.
– Typically, a single lightning strike is made up of multiple individual
strokes.
– The rolling and gradually dissipating rumble of thunder is caused by
the time delay of sound coming from different portions of a long
stroke.
Lightning
What does lightning damage look like?
Lightning
What does lightning damage look like?
Lightning
What does lightning damage look like?
Lightning
What does lightning damage look like?
Lightning
Really?
• Not all lightning forms in the negatively
charged area low in the thunderstorm cloud.
• Some lightning originates in the top of the
thunderstorm, the area carrying a large
positive charge.
– Lightning from this area is called positive lightning
and is even more powerful than “normal” or
negative lightning.
Lightning
Really?
• Positive lightning is particularly dangerous,
because it frequently strikes away from the
rain core, either ahead or behind the
thunderstorm.
• It can strike as far as 5 or 10 miles (8 or 16
kilometers) from the storm, in areas that most
people do not consider to be a lightning-risk
area.
– This is known as the “…the bolt came from out of
the blue…” phenomena.
Lightning
The Danger to Life
• Lightning strikes injure humans (and animals) in several
different ways:
– Direct strike, which is usually fatal.
– Contact injury, when the person was touching an object
that was struck.
– Side splash, when current jumps from a nearby object to
the victim.
– Ground strike, current passing from a strike through the
ground into a nearby victim.
• A strike can cause a difference of potential in the ground (due to
resistance to current in the Earth), amounting to several thousand
volts per foot.
– Blast injuries, including hearing damage or blunt trauma by
being thrown to the ground.
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Example of Contact Injury
Lightning
Example of Ground Potential Rise from Ground Strike
Lightning
The Danger to Life
• According to the National Oceanic and Atmospheric
Administration (NOAA), over the last 20 years, the
United States averaged 51 annual lightning strike
fatalities.
• Lightning strikes can produce severe injuries, and have
a mortality rate of between 10% and 30%, with up to
80% of survivors sustaining long-term injuries.
– These severe injuries are not usually caused by thermal
burns, since the current is too brief to substantially heat up
tissues, instead nerves and muscles may be directly
damaged by the high voltage producing holes directly in
their cell membranes.
Lightning
The Danger to Life
• A person injured by lightning does not carry
an electrical charge, and can be safely
handled.
• Administer first aid before emergency services
arrive.
– Lightning can affect the brainstem, which controls
breathing.
• If a victim appears lifeless, it is important to begin
artificial resuscitation immediately to prevent death.
Lightning
Personal Safety
• If you can hear thunder, you are within 10
miles (16 kilometers) of a storm and can be
struck by lightning.
• If outside, follow the 30-30 rule:
– Take appropriate shelter when you can count 30
seconds or less between lightning and thunder.
– Remain protected, out of the weather or sheltered
for 30 minutes after the last sound of thunder.
Lightning
Electrical and Structural Damage
• Telephones, modems, computers and other
electronic devices (TVs, VCRs, DVDs etc.) can
be damaged by lightning.
– Harmful overcurrent can reach them through the
phone jack, Ethernet cable, or electrical outlet
(electrical system).
• In addition to electrical wiring damage, the
other types of possible damage to consider
include structural, fire, and property damage.
Lightning
Electrical and Structural Damage
• Surge protectors do not provide protection
against a lightning transient.
– The energy from a lightning strike is too great to
be absorbed by typical transient voltage surge
suppressors (TVSS) that are part of the protection
scheme within surge protectors.
Lightning
Lightning Protection
• The NEC requires certain grounding, bonding
and protection features which are intended to
protect against lightning.
– These safeguards greatly reduce the risk of shock
or electrocution to a person in the house, and the
risk of fires caused by lightning.
– However, they are totally inadequate to prevent
damage to electrical or electronic equipment.
Lightning
Lightning Protection
• Lightning effects can be direct and/or indirect.
– Direct effects are from resistive heating, arcing
and burning.
– Indirect effects are more probable and include
capacitive, inductive and magnetic behavior.
• Lightning protection, in the absolute sense, is
impossible.
Lightning
Lightning Protection
• Lightning rods
– In Benjamin Franklin’s day, lightning rods
conducted current away from buildings to Earth.
– Now known as air terminals, they are now
believed to send streamers upwards at varying
distances and times according to their shape,
height and other factors.
• Air terminal design may alter streamer behavior
– Blunt pointed rod vs. sharp pointed rod.
– Air terminal design and performance is a
controversial and unresolved issue.
Lightning
Lightning Protection
• Down conductors, Bonding and Shielding
– Down conductors should be installed in a safe manner
through a known route on the outside of a building.
• Building steel may be used in place of down conductors
where practical as a beneficial part of the earth electrode
subsystem.
– Bonding assures all that all metal is at the same
electrical potential.
• All metallic conductors entering structures should be
integrated electrically to the earth electrode subsystem.
– Shielding is an additional line of defense against
induced effects.
Lightning
Lightning Protection
• Grounding
– The grounding system must address low Earth
impedance as well as low resistance.
• A single point grounding system is achieved when all
equipment within the structure is connected to a
master bus bar which in turn is bonded to the external
grounding system at one point only.
– Earth loops must be avoided.
– The grounding system should be designed to reduce AC
impedance and DC resistance.
Lightning
Lightning Protection
• Transients and Surges
– Ordinary fuses and circuit breakers are not
capable of dealing with lightning induced
transients.
• Lightning protection equipment may shunt current,
block energy from traveling down the wire, filter
certain frequencies, clamp voltage levels, or perform a
combination of these tasks.
Lightning
Lightning Protection
• Transients and Surges (continued)
– Voltage clamping devices capable of handling
extremely high surge current as well as reducing
the extremely fast rising edge of the transient are
recommended.
• Adopting a multi-faceted defense against surges is
prudent.
– Protect the main electrical service entry, protect all relevant
secondary distribution panels, and protect all valuable plug-in
devices.
– Further, protect all incoming and outgoing data and signal
lines.
Lightning
Testing for Damage
• Typically, lightning damage to electrical
systems and attached equipment is readily
detectable because you can see it.
• However, in-wall wiring is a different issue
altogether.
– Romex type wiring is only rated for 600 Volts.
– Hidden wiring must be tested to verify the
integrity of the insulating system.
• Testing is conducted using a Megger.
Lightning
In Summary
• Lightning safety should be practiced by everyone
during thunderstorms.
• Organizations should adopt a Lightning Safety
Policy and integrate it into their overall Safety
Plan.
• A systematic hazard mitigation approach to
lightning safety is a prudent course of action.
– For starters, check out IEEE’s, “How to Protect Your
House and its Contents from Lightning.”
• It’s IEEE’s guide for surge protection of equipment
connected to AC power and communication circuits.
Lightning
Q&A
• Questions?
• Contact Information:
– Doug Barovsky, P.E.
– MDE Inc.
– 700 S. Industrial Way
– Seattle, WA 98108
– 206.622.2007
• Thank you.