Lightning is a high-voltage discharge (usually negative) within clouds (intra-), to each other (inter-), or to the earth. The cloud-to-ground (CG) flash is the one we are usually concerned with lightning protection. The charged cells in clouds normally attract charges of opposite polarity on the earth's surface or on high objects located directly below them. When the charge reaches a critical level causing the insulation between cloud and earth breaks down, it develops a stepped ionized path, frequently to the earth, resulting in a high current discharge (stroke) that neutralizes, for the moment, these cloud and earth charges. The discharge current increases from zero to a maximum of 1 us to 10 us, and then declines to half the peak value of 20 us to 1000 us. Lightning flashes usually consist of a sequence of individual return strokes that transfer significant electrical charge usually from the cloud to earth.
Lightning protection is essential for buildings, transmission lines, and electrical equipment from lightning discharges and surges. The protection to be given for a structure or facility against lightning strikes is based on the probability of lightning strike and the extent of risk of damage or disruption that a lightning strike can cause. Based on the latter criterion, structures can be divided into various classes in ascending order of protection requirement.
- Class 1
- Structures, which need very little or no additional protection except connecting them to an effective ground electrode, come under this category. These are all-metal structures, buildings with metallic roofing, side cladding and metallic frame work, stand-alone metallic masts, etc.
- Class 2
- Structures that have a metallic roof, side cladding and non-conductive framework are in this category. Protection to these structures is provided by down conductors bonded to the roof and side members and connected to ground electrodes.
- Class 3
- These include metallic frame buildings with non-metallic roof and side cladding. In this case, air terminations on the top of the building and on other non-conducting surfaces connected to the metal frame of the building are required to protect the insulating surfaces from being punctured by lightning.
- Class 4
- This class includes completely non-metallic structures such as buildings and tall chimneys/stacks constructed of reinforced concrete or masonry. These structures need extensive protection using air terminations, down conductors and grounding electrodes.
- Class 5
- Buildings of historic or public importance or those containing valuable materials, places where a large number of people can gather at a time and public utilities such as power plants, water works, etc. come in this category and need utmost attention while planning protection.
Lightning Protection Standards
Lightning is one of the most widely studied and documented natural phenomena. It is also one of the main causes of transient over-voltages in electrical systems. A proper understanding of lightning is essential for planning protection against lightning strikes so that no untoward damage is caused to buildings and electrical installations.
A lot of research has been done over a number of years worldwide and several publications as well as national and international standards have evolved which give us a good insight into this phenomenon.
Some of these are:
- AS 1768 Australian standard on lightning protection
- ANSI/NFPA 780 National lightning protection code
- IEEE 142 IEEE green book
- IEC 1024 Protection of structures against lightning.
Lightning is the sudden draining of charge built up in low-cloud systems. The flow of charge creates a steep fronted current waveform lasting for several tens of microseconds. The flow is more usually that of negative charges though at times it may involve positive charge flow too. The latter are generally of lower magnitude.
The occurrence of lightning flash starts with a buildup of charge in a cloud system close to the ground. This charge is usually of the order of several million volts and usually of negative polarity at the bottom. Though the exact mechanism of charge separation is not clear, observations indicate that the ice particles in the top portion of the cloud are positively charged whereas the heavier water particles in the bottom portion of the cloud carry a negative charge.
The high electrical field causes ionization of air and creates a conducting path. This usually happens near the cloud and the ionized path is called the downward leader. The leader precedes in steps of 20–30 m toward the ground, each step forming further ionization of the subsequent step. Simultaneously, from the high points or structures on the ground upward leaders of positive charges start forming. The interception of downward leaders and upward leaders completes the conducting path between the cloud and the ground and results in a lightning strike.
The lightning flash along the ionized path causes a very high peak of current amounting to several kilo amperes and dissipates its energy in the form of heat (temperatures up to 20 000°C for a few microseconds), sound and electromagnetic waves (light, magnetic fields and radio waves).
Incidence of lightning
Lightning depends on both atmospheric and geographical factors. It is usually associated with areas having convection rainfall. It requires presence of high moisture levels in air and high surface temperatures on ground. Since the protection to be given to buildings is a function of the probability of lightning strikes, the frequency of lightning occurrence has been extensively studied and the results are published in the form of annual isokeraunic maps for different world regions. These are contour maps, which show the mean annual thunderstorm days of the region involved.
A thunderstorm day for this purpose is defined as one when thunder is heard at the point where it is measured. This obviously cannot indicate whether it is a result of inter-cloud or cloud to ground discharge. It does also show the frequency/number of instances or severity of cloud to ground strikes. Further studies are under way to gather such data and may result in modifications to the present methods of lightning risk assessment.
Probability of lightning strike
The probability of lightning strike depends on two factors:
- the incidence of lightning strikes in the geographical area where the structure is situated and;
- the attractive area offered by the structure for lightning. The attractive area can be defined as the horizontal area within which a downward leader will be intercepted by an upward leader originating from the structure.
Method of lightning protection
Lightning protection to any structure consists of providing a low impedance conducting path for flow of lightning discharge currents to ground without allowing them to flow through the structures. Lighting protection consists of an air termination, down conductors and ground electrodes.
A lightning mast independent of the structure but near enough to divert any lightning occurring in the vicinity is one example of protection. The protection offered may not be complete if the attractive radius of the building to lightning extends well beyond that of the mast. In such a case, it is possible that some of the strikes may hit the building rather than the mast.
A sphere is rolled over the protecting structure and the shaded areas which the sphere cannot touch are within the protection zone. The radius of the sphere can vary between 20 and 60 m depending on the degree of protection required. The standard protection will consider a radius of 45 m and increased degree of protection can be obtained by reduction of the radius.
Not all buildings can be protected by a free standing mast, the approach in this case is to have the conductors placed on the building itself called as air terminals then connect them to the ground through down conductors. The air terminals can be vertical spikes placed on the top periphery of the building. These air terminations can be connected by a flat conductor run on the roof thereby offering multiple paths for the lightning current to flow to ground. The down conductors are connected to dedicated ground electrodes to offer a short conducting path to ground.
Lightning behaves like a current source. In other words, the flow of current is independent of the circuit impedance. If the current path to ground has high-impedance elements, the lightning current, which is of the order of several kiloamperes as we saw earlier, produces a very high voltage drop. This voltage appearing on the conducting elements can cause secondary flash to nearby earthed objects. It can also cause damage to building structures by forcing a path through non-conducting building elements. This explains why the lightning conductors should be of as low impedance or in other words as
low a length as possible.
To Protect or not to protect
Standard AS 1768 provides clear guidelines to take a decision to provide or not to provide lightning protection to a building or structure based on an assessment of risk involved.
The assessment is done in terms of the likelihood of the structure being struck and the consequences of any such strike. The use of the structure, the nature of its construction, the value of the contents and the prevalence of thunderstorms in the area can all be considered in making the assessment.
A decision to provide lightning protection may, however, be taken without any risk assessment, for example, where there is a desire that there should be no risk to a structure at all.
Examples of such structures are:
- Those in or near which large numbers of persons congregate
- Those concerned with the maintenance of essential public services
- Those in areas where lightning is prevalent
- Very tall or isolated structures and
- Structures of historic or cultural importance.
Where it is thought that the consequential effects will be small and that the effect of a lightning flash will most probably be merely slight damage to the structure, it may be economic not to incur the cost of protection but to accept the risk. Even then, it is better to make an assessment so as to give some idea of the magnitude of the risk that is being taken.
The need to protect electronic equipment and to protect persons against potential differences associated with metallic services increases with the building area. In such cases even though the construction of the structure does not warrant protection, appropriate measures must be taken to avoid risk to persons and equipment.
The standard also stipulates that any structure which is entirely within a zone protected by an adjacent object or objects (whether protected or not) should be deemed to be protected, that is no separate protection is necessary for such structures.
- Practical Grounding, Bonding, Shielding and Surge Protection
IEEE Std 80-2000
IEEE Guide for Safety in AC Substation Grounding
IEEE Std 142-2007
IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems