Lightning, the visible discharge of electricity that occurs when a region of a cloud acquires an excess electrical charge, either positive or negative, that is sufficient to break down the resistance of air. Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions in the atmosphere or ground temporarily equalize themselves, causing the instantaneous release of as much as one gigajoule of energy. This discharge may produce a wide range of electromagnetic radiation, from very hot plasma created by the rapid movement of electrons to brilliant flashes of visible light in the form of black-body radiation. Lightning causes thunder, a sound from the shock wave that develops as gases in the vicinity of the discharge experience a sudden increase in pressure. Lightning occurs commonly during thunderstorms and other types of energetic weather systems, but volcanic lightning can also occur during volcanic eruptions.
Lightning is a very common event in nature. At any one time, there are some 1700 electrical storms active throughout the world, producing in excess of 100 flashes per second. This equates to an aggregate of some 7 to 8 million flashes per day. Of these, approximately 90% are cloud-to-cloud flashes and the remaining are predominately cloud-to-ground flashes. It is a natural phenomenon where the charge generated due to cloud/ air movement and other turbulent atmospheric conditions. When these is separated charged clouds get very large, the air between the positive and negative regions breaks down in a giant spark (an intra-cloud stroke), or a charged region breaks down to ground (a cloud ground stroke). The resulting current being extremely high (it can vary from 30 kA to 100 kA and may sometimes even scale up to 450 kA) ionizes and heats the air along the path to ~30,000 DegC. The ionized air glows brightly (the lightning), and the sudden increase in temperature expands the channel and nearby air, creating a pressure wave that makes the thunder. Globally approximately 90% of all lightning flashes are cloud-to-cloud & only 10% being cloud-to-ground flashes (in which negative charge is transferred from the cloud to ground).
Ground-to-cloud flashes are extremely rare and generally only occur from high mountain tops or tall man-made structures – they are typically positive strokes (positive lightning). Strokes between clouds are relatively rare.
To be capable of generating lightning, the cloud needs to be 3 to 4 km deep (cumulonimbus), the taller the cloud, the more frequent the lightning is. The center column of the cumulonimbus can have updrafts exceeding 120 km/hr, creating intense turbulence with violent wind shears and consequential danger to aircraft. This same updraft gives rise to an electric charge separation that ultimately leads to the lightning flash.
Need of Lightning Protection System (LPS)
But the big question is, if lightning is a natural phenomenon and is inevitable, why to invest in a safety system to handle this? Why not just let the lightning strike as it may and let the building dissipate the energy to ground?
To understand it, we must understand the Lightning first. Lightning is nothing but a discharge of atmospheric electricity, which is generated by an accumulation of differing charges within a cloud. It is one of the most common causes of weather-related deaths in India. Apart from causing deaths each year lightning strikes cause crore of monetary losses in property damage and devastation. Because of sheet intensity & short duration for which lightning current flows, the related energy has high level of damaging power severity, despite the fact that it flows for a few µ seconds. It is well recorded that an average lightning strike may contain energy up to 1 Billion Joules and the core temperature of a lightning strike is approximately 30,000 DegC, which in normal energy consumption terms is only about 300 kWh – but since the lightning current has very short average time (~350 µs or so), the energy flow per unit of time (i.e. power burst or rate of energy flow) becomes too large (approx. 800 kW). Such large power flow has a massively destructive impact on the structure or any live body, be it human or animal, through which it flows. The precise details of such impacts can never be predicted however, in humans or animals, it can damage the central nervous system, heart, lungs, and other vital organs (like any other electrical shock leading to even deaths in most of the cases. Buildings, structures and power lines may get affected by the excessively high voltages, intense electromagnetic forces and heat, getting generated by electromagnetic forces when lightning current passes through the structure. The rise system id voltage of electrical power supply may also damage house wiring or appliances & may also start the fire indirectly by causing power faults (short circuit or wire burning). The non-availability or poorly designed LPS will also not limit the rise in surface potential (see pic below) creating an unsafe condition leading to unsafe surroundings and may cause loss of lives.
Life Loss Due to Lightning in India
As per Mid Monsoon 2019 Lightning Report By Indian Meteorological Department, between 01.04.2018 to 31.07.2019, there were a total of 64,55,540 lightning strikes, out of which 23,52,614 (36%) were cloud to ground lightning while rest of 41,02,926 (64%) were In-Cloud (IC) lightning strike (See graphs below).
According to the data analyzed by the Indian Institute of Tropical Meteorology (IITM), Pune, on 16.04.2018 there were almost 41,000 clouds to ground (CG) lightning strikes all over India, in which about 89 people in 11 states were killed. The data from 1999 from the reports of National Crime Records Bureau, MoHA (Accidental Deaths & Suicide in India) shows an alarming trend of deaths by lightning in absolute numbers and more steeper in percentage of deaths due to natural calamities. The percentage rise probably proves that despite the good work of National Disaster Management Program, though India could reduce the number of deaths due to other natural calamities, the deaths by lightning has remained at the same level. Hence there is a greater need to understand the Lightning phenomenon, and spread awareness to put a proper LPS, in all places in order to have a much safer social environment in which we are able to save more lives when lightning strikes. It is important to note here that as of now there is no formal system in India to account & record property losses due to lightning.
Lightning Protection System (LPS)
The purpose of a LPS is to safely direct the lightning current with intense electrical energy, travelling from the clouds to the ground after breaking the natural barrier of atmosphere. The system must be designed to make an isolated & safe conducting path with the lowest possible electrical resistance in order to guide the lightning current to ground, so as to reduce its impact on nature, infrastructure and/ or human/ animal lives.
Although any LPS cannot guarantee a 100% protection and at best can only avoid loss of lives and damage to infrastructures to a certain extent if lightning strikes yet a well-planned LPS would certainly help in reducing its impact (and in eliminating in some cases), along with meeting the basic purpose of saving lives. However, it may be noted that when a lightning strikes a structure, its electrical potential with respect to earth is also raised (see the picture) and thus the lightning current, while travelling down through the LPS, may seek alternative paths during this travel through anything where it finds lower resistance to ground leading side-flashing. To avoid this, IS: 2309 1989 strongly recommends reduction of the resistance to earth to a value below 10 Ohm to help in reducing the potential gradient around the earth electrodes to reduce the risk of side-flashing to metal in or on a structure. For this the IS: 2309 1989 recommends two different ways of preventing side flashing, namely: a) isolation, and b) bonding as described below:
Isolation: Isolation requires either providing a large gap or providing an insulation between the LPS Conductor and any nearby metal structures (e.g. water or other services) to ensure it does not get connected to any other metal objects connected
Bonding: In bonding, adjacent metal works need to be connected with LPS (so that the lightning does get diverted to them) with careful consideration of possible effects of such bonding that it might have upon metalwork which has been protected.
Lightning Flash/ Current
Almost everybody is aware of the two most common waveforms of electricity as shown – while DC waveform remains at fixed level depending upon the level of current flowing in circuit, the AC current has sinusoidal wave form whose amplitude shows the amount of current flowing however when many harmonics get mixed in AC Current the wave form also gets distorted.
However, it has been a challenge for researchers to arrive at the wave form of Lightning current, because of its highly short duration of existence & transient nature due to which the shape of the lightning current may vary from event to event, place to place. Still for study and application purposes, based on statistical probability, the researchers have defined a wave shape of these lightning currents.
Understanding Wave Form of Lightning Current
One of the most fundamental requirements of any concept of protection is to define the nature of the hazard. Lightning is one of its kind, an uncontrolled natural phenomenon, which is a variability so high that giving its definition is not easy. In order to be able to test protection networks and provide comparison data, various standard waveforms have been proposed over the years. These are all defined in terms of magnitude and wave shape, and can refer to current (normally associated with short-circuit conditions) or voltage (associated with open-circuit faults). Waveforms are usually double exponential rise and decay shapes, specified by two time periods; rise-time to peak value and decay-time to 50% peak value. The wave form is defined as T1/T2 µs waveform where T1 refers to Wave Front Time (µs) & is the time taken by current to achieve 90% of its peak value while T2 refers to Wave Tail Time (µs) & is time taken by the current to diminish down to 50% of that peak value.
The lightning is a very short burst of voltage and as such no accurate waveform for these currents can be defined (even mathematically), however based on statistical data of numerous measurements of actual lightning strikes, researchers have arrived at conclusion that the 10/350 µs waveform is a reasonable approximation for the current waveform for the primary lightning stroke. IEC 60060-2 describes a particular method of determining slopes by choosing the 10/350 µs waveform to represent the electrical and mechanical stress associated with direct conducted lightning.
Both the IEC and the IEEE have standardized on two types of current wave forms:
- 10/350 µs waveform to represent direct lightning stroke
- 8/20 µs waveform to represent induced lightning stroke
Risk Assessment & Threat Exposure from Lightning
Detailed programs are available for risk assessment for designing the LPS, however they all are based on following 10 sets of losses/ risks categories:
Lightning protection level (LPL) is a number assigned to represent maximum and minimum lightning parameters that should not be exceeded by natural lightning. While LPL I offers the highest protection level (greatest level of protection), with LPL IV offering the lowest level of protection as shown below:
For example, LPL I positions terminals have the probability that 99% of all lightning flashes are intercepted (all those of 3 kA or greater). There is only a 1% probability that lightning may be smaller than the 3 kA minimum, and may not be close enough to an air-terminal to be intercepted. It should be noted that flashes of less than 3 kA are rare, and typically would not be expected to cause damage to the structure.
The table also gives details the rolling sphere radius used in the rolling sphere design method (which is the most preferred method for determining positioning of air-terminals) as well mesh sizes for the mesh methods for designing LA protection system. There is one method known angle method. …to be continued
Prabhat Khare possesses a BE (Electrical) degree from IIT Roorkee (Gold Medalist). Now, he is the Director of KK Consultants. He is also a BEE Certified Energy Manager and a Lead Assessor for ISO 9K, 14K, 45K & 50K.