Equipment and building protection is provided by low impedance grounding and bonding between electrical services, protective devices, equipment and other conductive objects – so that faults or lightning currents do not result in hazardous voltages within the building. Also, the proper operation of over current protective devices is frequently dependent upon low impedance fault current paths.
Earthing is primarily used to avoid electric shocks. Grounding is primarily used for unbalancing when the electric system overloads. Earthing is located under the earth pit, between the equipment body and underground. It is located between the neutral of the equipment being used and the ground.
Protective earthing includes measures for protecting the metal parts that neither belong to the circuits, nor are they in direct electrical contact with them, but in the event of a defect, a voltage can arise. Earthing reduces that voltage and prevents the emergence of conditions that are dangerous to the equipment, as well as the lives of people handling that equipment or which can be affected by the defect or by moving in the vicinity.
The grounding includes measures for protecting the part of the circuit, which provides the desired function or the working feature of that circuit. Grounding can be performed directly or indirectly. Direct grounding is carried out by direct connection of the grounding system. Indirect grounding is performed by binding to the grounding system through impedance.
There are three types of earthing: Pipe earthing, Plate earthing, Strip earthing. The size of the pipe used for earthing is of diameter 40 mm and 2.5 meters in length for ordinary soil or greater length in case of dry and rocky soil. The depth at which the pipe must be buried depends on the moistures of the ground. The minimum depth at which the pipe must be buried in pipe earthing is 3.75 meter.
A grounding system provides a low-impedance path to the ground. This low-impedance path is installed to encourage the lightning to travel through it instead of through your expensive electronic equipment. Grounding rods and wires are also used to create what is referred to as a common ground. IS 3043 (1987) is Code of practice for earthing.
Lightning protection and earthing equipment are usually made up of earth rods either copper bonded, solid copper or stainless steel and also forming the earthing system are copper earth plates, copper lattice earth mats or 25 x 3mm copper earth tapes.
Lightning is a visible electrical discharge that occurs within a cloud, between two clouds, or between a cloud and the surface of the earth. As lightning passes through the air it heats the air quickly. Lightning is essentially a huge electric spark that occurs between the cloud and the ground. The charge and lightning develops because of tiny collisions between ice particles within the cloud. This happens millions of times per second and when this charge gets large enough a lightning strike occurs.
Lightning begins as static charges in a rain cloud. Wind trapped inside the cloud is very turbulent. Water droplets in the lower region of a cloud get attached to the updrafts and lifted to higher regions where the extreme cold atmosphere freezes them. In the meantime, downdrafts in the cloud push ice particles below from the top. Where the ice going down meets the water coming up, and then electrons are stripped off. A single lightning flash can be made up of a number of strokes or pulses of current. A lightning strike is essentially a cloud to ground lightning stroke where it strikes the ground. Lightning is a direct current and a typical lightning flash is about 300 million Volts and about 30,000 Amp. The lightning conductor works on the principle of induction. Whenever a charged cloud passes by the building, the conductor gets charge opposite to that of the cloud through the process of induction. Now, this acquired charge moves to the earth through the earthing system.
Lightning Protection System (LPS) usually consists of both external and internal lightning protection systems. An external LPS is intended to: (a) intercept a lightning flash to the structure, with an air-termination system; (b) conduct the lightning current safely towards earth, using a down-conductor system; (c) disperse the lightning current into the earth, using an earth-termination system. An internal LPS prevents dangerous sparking within the structure using either equipotential bonding or a separation distance between the external LPS components and other electrically conducting elements internal to the structure.
Types of Lightning
Intra-cloud lightning is the most common type. It occurs when lightning moves quickly inside a cloud having opposite charges in different parts of the cloud.
When a positive charge associated with one cloud meets with the opposite polarity charge from another cloud they create a bolt of lightning known as Cloud to cloud lightning.
Cloud to air lightning occurs at the top of clouds when the positive charged particles from the higher region thunder cloud reaches out to negatively charged particles around the cloud.
Since lightning has no specific path to reach the ground, it may choose to utilize any conductor available inside a building, house or a tree. This also include any electrical equipment like phone, cable, electricity lines, the water or gas pipes, or in case of a steel-framed building the whole structure itself. These are the most commonly used paths utilized by the lightning to reach the ground. As a result, lightning presents several hazards to our houses, commercial buildings and human beings.
When a lightning strike occurs on a tree or other object, much of the energy travels outward from the strike in and along the ground surface. This is known as the ground current. Anyone outside near a lightning strike is potentially a victim of ground current. Lightning protection system for a building, consists of five parts: air terminals or lightning rods, conductors, ground connections or electrodes, bonding, and lightning arrestors.
The first mention of lightning rods was a note published in Gentleman’s Magazine, May 1750 and in the London edition of this book on electricity, published in 1751, where Franklin recommended the use of lightning rods to avoid Lightning. In 1876, James Clerk Maxwell suggested that Franklin’s lightning rods attracted more lightning strikes than the surrounding area. Many developments took place during the last two decades. Conceptually there are four methods LPS for buildings -(1) Air termination method (2) Protection angle method (3) Mesh method and (4) Rolling sphere method.
Copper and aluminium main cable conductors for lightning protection are designed to a smooth weave or rope-lay standard using smaller gauge individual wires. This construction allows a maximum surface area per unit weight of conductor to accommodate lightning which travels quickly on the surface. A conventional arrester works as a conductor which is mounted on the top of a tower or building and is connected to the ground via earthing strips and or through a wire, to protect the building or a tower at the time of sky lightning. The function of an external lightning protection system is to intercept, conduct and disperse a lightning strike safely to earth. Without such a system a building’s structure, electronic systems and the people working around or within it are all at risk. LPS is to be connected to the main grounding system. Adequate separation, or separate bonding, should be provided at other levels to minimise the risk of flashover. Many electricians are unaware that there is a requirement to provide adequate separation between conductors of a lightning protection system and other conducting material, such as electrical wiring. Lightning Protection Systems are highly effective at preventing lightning damage to buildings.
IEC62305 (2007) series provides comprehensive guidelines on the design and installation of LPS for buildings. The design standard comprises general principles, risk management, physical damage to structures and life hazard and electrical and electronic systems within structures. It is the apex level document that informs the standards for lightning protection around the world. A lightning protection system is designed to protect a structure from damage by intercepting such strikes and safely passing their extremely high voltage currents to “ground.” This system includes a network of air terminals, bonding conductors, and ground electrodes designed to provide a low impedance path to ground.
It is now well established that properly installed and maintained lightning rod-based protection systems significantly decrease lightning damage. According to this mesh method of lightning protection, a conducting mesh with a cell size determined by the minimum return stroke current that is allowed to strike the protected structure.
In order to avoid a direct strike the mesh has to be located at a critical distance above the flat surface to be protected. A lightning protection system is composed of four main components: Lightning Rods or Air Terminals, Conductor Cables, Ground Rods, Surge Protection Devices. Radius protection, recommended installing the lightning conductor in a height of 5 to 6 meters because then achieving a high level of protection. LPL refers to the case of maximum surge current, based on the 10/350µs pulse current waveform, which is applied to the case of Type I protectors used in power entry level into a building.
Lightning arrester and surge arrester are the same, but they are different. The former one is installed outdoors whereas the latter one is installed indoors. However, both of them keep your electrical appliances safe from accidents. The earthing continuously performs its function during the operation of the power system, while the lightning protection functions only for the duration of the overvoltage, and the grounding only for the duration of insulation failure.
Dr. Gopalkrishna Dhruvaraj Kamalapur is an Adjunct Faculty of Department of Electrical and Electronics Engineering at Shri Dharmasthala Manjunatheshwar College of Engineering and Technology, Dharwad-580002 (Karnataka state) INDIA.