The Science of Lightning

 LIGHTNING

Fig: Lightning Strikes

Lightning is a transient, high- current electric discharge phenomenon which develops when the upper atmosphere becomes unstable due to the convergence of a warm, solar heated, vertical air column on the cooler upper air mass. Lightning is a naturally occurring spark discharge or short-lived electric arc. A lightning strike, even if it is indirect, creates a surge voltage on the cable lines and transmits a momentary high voltage impulse to the sensors/transmitters in the field or the control room inputs.

Across the U.S., lightning strikes on average 25 million times per year and occurs at a rate of 100 flashes per second worldwide. And according to the National Weather Service, the average strike has an average magnitude of nearly 30,000 Amps. The temperature of such lightning can reach over 28,000°C.

FORMATION OF LIGHTNING

Contrary to popular belief, lightning does not always strike the tallest object. Instead, the energy in a strike attempt to find the path of least resistance to ground.
The first step in lightning formation is Cloud Electrification, in which opposite-charged charge particles split and generate a quasi-static electrical field between the cloud and the ground. The electrical field then grows to the point of start of upward streamers, followed by a down-leader. To complete an ionized channel between the cloud and the earth, the upward leader propagates toward a down-leader. Until one of the competing upward leaders connects with the lower leader, the downward leader propagates. Between the cloud and the ground, a highly ionized path forms, allowing for a large charge transfer to the ground. This event takes few milliseconds to complete, and a "flash" lasts about 0.5 seconds.

Schematic diagram of Lightning stroke between clouds and ground

90% of all lightning discharges between a cloud and the ground are negative, i.e., "negative cloud-to-ground strikes”. Here, the lightning begins in an area of negative charge in the cloud and spreads to the positively charged ground. However, the vast majority of discharges take place within clouds, or from one cloud to another. The less common types of discharge are:

• Negative ground-to-cloud lightning
• Positive cloud-to-ground lightning
• Positive ground-to-cloud lightning

Various types of Lightning Strokes

Data from International Council of Large Electrical Systems (CIGRE) indicates that 5% of first, negative lightning strokes exceed 90 kA (average is 33 kA), 5% of positive lightning strokes exceed 250 kA (average is 34 kA) and 5% of negative subsequent strokes exceed a rate of current rise of 161 kA/μs.

Only around 10% of lightning discharges occur from cloud to ground; the large majority of lightning discharges occur between clouds during thunderstorms. Discharges within clouds often provide general illumination known as ‘sheath lightning’. Measurements of stroke currents at ground have shown that the high current is characterized by a fast rise to crest (1 to 10 μsec) followed by a longer decay time of 50–1000 μsec to half-time.

ENERGY IN LIGHTNING

To estimate the amount of energy in a typical lightning discharge let us assume a value of potential difference of 10^7 V for a breakdown between a cloud  and ground and a total charge of 20 coulombs. Then the energy released is 20* 10^7 Ws or about 55 kWh in one or more strokes that make the discharge. Ionization of molecules, excitations, radiation, and other processes consume small amounts of this energy. The sudden expansion of the air channel consumes the majority of the energy. The struck earthed objects are heated by some fraction of the total energy. In general, lightning activities restore the energy that was utilized to build the charged cloud to the global system.

SOURCE OF DAMAGE

The actual sources of damage are lightning strikes that are subdivided into four groups depending on the point of strike. 

1. Direct lightning strike to communication line.
2. Direct lightning strikes nearby outside equipment.
3. Direct lightning strike near a structure.
4. Lightning strikes to a structure.

Fig: Source of Damage

Source: electrical-engineering-portal.com

LIGHTNING PROTECTION SYSTEM (LPS)

Lightning strikes are unpredictable and random events. Lightning protection is a serious, yet often underestimated issue, which requires professional equipment, installed professionally. LPS is meant to protect your structure or location from any harm caused by a lightning strike. The purpose of a LPS is to protect buildings from direct lightning strikes and possible fire or from the consequences of lightning currents (non-igniting flash). LPS is based on statistical probabilities and the risk tolerance of the property owner.

The IEC 62305 series of standards are primarily design standards, giving the user a tool kit of rules and options to provide lightning protection for a structure. The standards cover structure protection and equipment protection with regard to the effects of direct and indirect lightning flashes. “IEC 62305 Protection Against Lightning” is comprised of 4 parts (documents):

I.   I. IEC 62305-1 Part 1: General Principles

This part presents general information on lightning and its characteristics and general data, and introduces the other documents.

II. IEC 62305-2 Part 2: Risk Management

This part presents the analysis making it possible to calculate the risk for a structure and to determine the various protection scenarios in order to permit technical and economic optimization.

III.  IEC 62305-3 Part 3: Physical Damage to Structure and Life Hazard

This part describes protection from direct lightning strokes, including the lightning protection system, down-conductor, earth lead, equipotentiality and hence SPD with equipotential bonding.

I.        IV.  IEC 62305-4 Part 4: Electrical and Electronic Systems within Structures

This part describes protection from the induced effects of lightning, including the protection system by SPD (Types 2 and 3), cable shielding, rules for installation of SPD, etc. This series of standards is supplemented by:

• the IEC 61643 series of standards for the definition of surge protection products

• the IEC 60364-4 and -5 series of standards for application of the products in LV electrical installations.

COMPONENT OF LPS:

There are five elements that need to be in place to provide an effective lightning protection system.

• Strike termination devices must be suitable to accept direct lightning attachment and patterned to accept strikes before they reach insulated building materials.

• Cable conductors route lightning current over and through the construction, without damage, between strike terminations at the top and the grounding electrode system at the bottom. 

• The below grade Grounding Electrode System must efficiently move the lightning to its final destination away from the structure and its contents. 

• Bonding or the interconnection of the lightning protection system to other internal grounded metallic systems must be accommodated to eliminate the opportunity for lightning to sideflash internally. 

• Surge protection devices must be installed at every service entrance to stop the intrusion of lightning from utility lines, and further equalize potential between grounded systems during lightning events.

TYPES OF LPS

i.    External LPS:

The function of an External LPS is to intercept, conduct and disperse a lightning strike safely to earth. The External LPS consists of an air-termination system, down conductors and the earthing system. With these components it is able to perform the functions required of it, namely intercepting direct lightning strikes, discharging the lightning current to earth and distributing it in the ground.

        i.            ii.   Internal LPS:

The function of internal LPS is to ensure the avoidance of dangerous sparking occurring within the structure to be protected. This could be due, following a lightning discharge, to lightning current flowing in the external LPS or indeed other conductive parts of the structure and attempting to flash or spark over internal metallic installations. Carrying out appropriate equipotential bonding measures for ensuring there is a sufficient electrical insulation distance between the metallic parts can avoid dangerous sparking between different metallic parts. It consists of equipotential bonding and surge protection system.