WHAT IS LIGHTNING PROTECTION(LP)?
...AND HOW CAN WE HELP?
WHAT IS LIGHTNING?
Lightning is a sudden electrostatic discharge that occurs during a weather event
such as a thunderstorm. This discharge occurs between electrically charged regions
of a cloud (called intra-cloud lightning or IC), between two clouds (CC lightning), or
between a cloud and the ground (CG lightning).
The charged regions in the atmosphere temporarily equalize themselves through
this discharge referred to as a strike (if it hits an object on the ground), and a flash,
(if it occurs within a cloud). Lightning creates light in the form of black body radiation
from the very hot plasma created by the electron flow, and sound in the form of
thunder. Lightning may be seen and not heard when it occurs at a distance too great
for the sound to carry.
In order for an electrostatic discharge to occur, two preconditions are necessary: firstly, a
sufficiently high electric potential between two regions of space must exist, and secondly a
high-resistance medium must obstruct the free, unimpeded equalization of the opposite charges.
It is well understood that during a thunderstorm there is charge separation and aggregation in
certain regions of the cloud; however the exact processes by which this occurs are not fully
understood.
The atmosphere provides the electrical insulation, or barrier, that prevents free equalization
between charged regions of opposite polarity. This is overcome by "lightning", a complex process
referred to as the lightning "flash".
The electric current within a typical negative CG lightning discharge rises very quickly to its peak
value in 1–10 microseconds, then decays more slowly over 50–200 microseconds. The transient
nature of the current within a lightning flash results in several phenomena that need to be
addressed in the effective protection of ground-based structures.
The rapidly changing currents also create electromagnetic pulses (EMPs) that radiate outward
from the ionic channel. This is a characteristic of all electrical discharges. The radiated pulses
rapidly weaken as their distance from the origin increases.
However, if they pass over conductive elements such as power lines, communication lines, or metallic
pipes, they may induce a current which travels outward to its termination. This is the "surge" that,
more often than not, results in the destruction of delicate electronics, electrical appliances, or
electric motors. Devices known as surge protectors (SPD) or transient voltage surge suppressors
(TVSS) attached in parallel with these lines can detect the lightning flash's transient irregular current,
and, through an alteration of its physical properties, route the spike to an attached earthing ground,
thereby protecting the equipment from damage.
WHAT IS LIGHTNING PROTECTION?
A lightning protection system is designed to protect a structure from damage due to lightning strikes by intercepting such strikes and safely passing their extremely high currents to earth. A lightning protection system includes a network of air terminals, bonding conductors, and ground electrodes designed to provide a low impedance path to earth for potential strikes.
Lightning protection systems are used to prevent or lessen lightning strike damage to structures. Lightning protection systems mitigate the fire hazard which lightning strikes pose to structures. A lightning protection system provides a low-impedance path for the lightning current to lessen the heating effect of current flowing through flammable structural materials. If lightning travels through porous and water-saturated materials, these materials may literally explode if their water content is flashed to steam by heat produced from the high current. This is why trees are often shattered by lightning strikes.
Because of the high energy and current levels associated with lightning (currents can be in excess of 150,000 amps), and the very rapid rise time of a lightning strike, no protection system can guarantee absolute safety from lightning. Lightning current will divide to follow every conductive path to ground, and even the divided current can cause damage. Secondary "side-flashes" can be enough to ignite a fire, blow apart brick, stone, or concrete, or injure occupants within a structure or building. However, the benefits of basic lightning protection systems have been evident for well over a century.
Originally, scientists believed that such a lightning protection system of air terminals and "downleads" directed the current of the lightning down into the earth to be "dissipated". However, high speed photography has clearly demonstrated that lightning is actually composed of both a cloud component and an oppositely charged ground component. During "cloud-to-ground" lightning, these oppositely charged components usually "meet" somewhere in the atmosphere well above the earth to equalize previously unbalanced charges. The heat generated as this electric current flows through flammable materials is the hazard which lightning protection systems attempt to mitigate by providing a low-resistance path for the lightning circuit.
RISK MITIGATION. No lightning protection system can be relied upon to "contain" or "control" lightning completely. Nor thus far, to prevent lightning strikes entirely.
Steel framed structures can bond the structural members to earth to provide lightning protection. A metal flagpole with its foundation in the earth is its own extremely simple lightning protection system. However, the flag(s) flying from the pole during a lightning strike may be completely incinerated.
The majority of lightning protection systems in use today are of the traditional Franklin design. The fundamental principle used in Franklin-type lightning protections systems is to provide a sufficiently low impedance path for the lightning to travel through to reach ground without damaging the building.This is accomplished by surrounding the building in a kind of Faraday cage. A system of lightning protection conductors and lightning rods are installed on the roof of the building to intercept any lightning before it strikes the building.
The electrical earth installed by your electrician is there to protect the internal workings of the electrical system in your building to accommodate everyday electricity usage. The electrical earth is not designed to handle the mega electricity (100 million + volts of power or 200 kA of electrical energy) that a typical lightning strike can pack.
IT IS CRITICAL THAT YOU UNDERSTAND YOUR RESPONSIBILITY AS THE SITE OWNER AND THE COMPLIANCE REQUIREMENTS AS SET OUT IN AS/NZS 1768: 2007.
TALK TO AN EXPERT. LP SYSTEMS ARE COMPLEX AND REQUIRE EXTENSIVE ENGINEERING AND DESIGN.
HOW CAN WE HELP?
Our approach is simple:
AUDIT + TEST + SPECIFICATION = DESIGN
Once we have gathered together all of the information required and validated
the findings of our audit under test, we work with our engineering team to
provide detailed design and drawings, including specification of equipment.
If you have been specified lightning protection equipment without conducting
the necessary audits and tests, then you may be operating with equipment that is unsuitable for the application.
By U.S.A. governement. noaa.gov - http://www.lightningsafety.noaa.gov/science/science_electrification.
By Anynobody - Own work by uploader CC BY-SA 3.0,
