Typically LPS follows two installation approaches
Non Isolated System: Here, LPS is directly installed on the structure that needs to be protected.
Isolated System: Here, LPS is isolated from the structure under protection by a specified distance, sufficient enough that energy flowing through the LPS does not spark with the protected structure. They are mainly used for structures that store combustible materials or are parts of telecommunication sites.
Air Terminal Placement
For air-terminal placement, the main consideration is the minimum value of expected current and the ability of the LPS to intercept these smaller flashes. As described earlier, as the lightning downward leader approaches the ground or structure, the electric field increases to the point that the ground or structure launches an upward leader that may eventually intercept the downward leader. This is termed the “striking distance”.
The larger the amount of charge carried by the lightning leader, the greater will be the distance at which this happens (as the larger the charge of the leader, the larger the resulting lightning current).
This formula shows that it is more difficult for an air-terminal to intercept a smaller lightning flash than a larger flash, as the smaller flash must approach closer to the air-terminal before the upward leader is launched. To protect the structure against smaller lightning flashes, air-terminals must be spaced closer together.
For smaller lightning flashes there is a risk that an air terminal may not be close enough to intercept the down leader, thus a closer structural point releases an upward leader which intercepts the flash (i.e. the building is struck). For each of the lightning protection levels, a minimum current level to be protected against has been statistically determined along with their probability percentages that lightning may be greater – than these levels.
However, no LPS can claim to be 100% effective or safe as lightning protection is an issue of statistical probabilities and risk management. A system which is protected by a well-designed system will statistically reduce the chances of risk, which may occur when we consider lightning hit. Some of them are:
Step Potential: When lightning strikes and the lightning current flows into the earth, a large voltage gradient builds up around the earth electrode & the voltage rise is greatest near the electrode where current density is the highest. The normal “step distance” of a person is about 1 metre. At the time of discharge being close to the earth electrode means the voltage differential across this distance can be large enough to be lethal – depending upon circumstances such as condition of footwear, etc., substantial current can flow through one lower leg to the other. In the case of animals, a larger risk exists. The distance between the front and rear legs of larger animals can be in the order of 2 metres, and the current flows through the more sensitive region of the heart.
Touch Potential: Touch potential is due to a similar reason as step potential, but the voltage differential being considered is that which exists between the hand and (generally) feet. The risk of electrocution due to touch potential is greater than for step potential, as the passage of current flows close to the heart region.
Side Flashing: all down-conductors have a resistance and when lightning current flow through them, their surface voltage may rise to a level to cause a Flashover to a nearby conductive object. This can be controlled by using a number of parallel down-conductors or ensuring the separation distance between the two objects is sufficient.
Basics of Designing Method for LPS
Rolling Sphere Method (RSM): In this method the radius of the sphere is considered as equal to the striking distance associated with the minimum current level for the chosen lightning protection level. This imaginary sphere is rolled over the structure. The surface contact points traced out by the sphere define possible points that may launch an upward leader to intercept with the downward leader. All these points are deemed to require protection, whilst the untouched points do not. Generally, an LPS is designed such that the rolling sphere only touches the LPS and not the structure. This is the most effective way of designing LPS.
Protection Angle Method (PAM): Air-terminations (rods/masts and catenary wires) are located so the volume defined by the protection angle covers the structure to be protected. The height of the air-termination is measured from the top of the air-termination to the surface to be protected. The protection angle method is limited in application to heights that are equal to or less than the corresponding rolling sphere radius. Where the protection angle method alone is employed, multiple rods are generally required for most structures. However the protection angle method is most commonly used to supplement the mesh method, providing protection to items protruding from the plane surface. The protection angle method can be used on inclined surfaces, where the height of the rod is the vertical height, but the protection angle is referenced from a perpendicular line from the surface to the tip of the rod.
Additionally, the Protective Angle Method is only valid up to heights equal to the radius of the rolling sphere as defined by the class of LPS defined for the structure.
Mesh Method: The mesh method is recommended for flat roof surfaces in which meshed conductors are placed on edges as well as on surface to form a mesh & then from each edge joint, minimum two separate paths are made to direct to ground. Natural components may be used for part of the mesh grid, or even the entire grid. It is also recommended for the protection of the sides of tall buildings against flashes to the side. The mesh method should not be used on curved surfaces, but can be used on non-horizontal plane surfaces and compound surfaces e.g., on the vertical sides of tall buildings for protection against flashes to the side, or on compound surfaces such as industrial roofs. For compound surfaces, conductors should be placed on the roof ridge lines if the slope exceeds 1/10. Natural components are typically metallic structural items that will not be modified during the life of the structure, such as reinforcing steel, metal framework and roofing/cladding. Natural components must meet minimum material requirements and be electrically continuous with secure interconnections between sections such as brazing, welding, clamping, seaming, screwing or bolts.)
Due to highly unpredictable nature of lightning, there is no single method that can be said to be fool proof and it is always a combination of system that is preferred for lightning protection. It is shown below with volume protected by combination of all three methods:
Air Terminals & Their Use
Air-terminations are the main front end devices that specifically placed at specified locations to capture the lightning flashes. Some of air-terminations types are:
Also, all items on the roof that contain electronic or electrical equipment require protection via air-terminations. Surge protective devices must be installed on the circuits to limit current entering into the internal environment.
Earthing & Its Importance in LPS
The most critical part of LPS is earthing, which if not made correctly or maintained correctly in which case lightning will cause the havoc despite, investing money on all other items. The reliable performance of the entire LPS is dependent upon an effective earthing system. Following points must be kept in mind:
- Providing a low impedance network to dissipate the fast-rising lightning impulse
- Minimization of touch and step potential hazards
- Long term performance of the system – i.e. quality of materials and connections
While the earthing system of LPS is normally installed and tested as a dedicated system, it is required by most codes that the earthing system of any LPS must be kept separately yet interconnected with other earthing systems. Isolation from the telecommunication, power and other earthing systems is highly recommended.
The earthing system (and down-conductors) should be located away from entrances and exits of the structure and places where people may congregate. If the earthing system is in locations accessible to the public, then measures should be taken to minimize step potential risks.
- The earthing system should be located away from other metallic buried items (e.g. bore well, Gas & water pipelines and services).
- Test joints should be installed between the local electrode sections and down-conductor to enable isolation and measurements of sections of the system during future inspection and testing.
- It is recommended that IS: 3043 1987 must be followed while making an earth pit.
- And preferably multiple paths to multiple earth pits should be provided for diverting the Lightning current.
Earthing Resistance Requirements: The general requirement is that the earthing of LPS must have a resistance of less than 10 ohms measured at a frequency different from the power system frequency or multiple thereof.
Epilogue
When we look at a Lightning Protection System in its most elementary form, it is quite simple in which simple metal rods (air terminal) are installed to divert the striking lightning to ground by providing a low resistance conducting path. The system provides protection from lightning to the structures. The overall concept seems very simple yet considering the unpredictable nature of lightning, which till no one is able to define till date, it is very complicated. It is this unpredictability, which brings one of those very difficult challenges of designing a protection system which requires use of historical data & statistical model to forecast its behaviour & nature. Yet this statistical model is based upon a great amount of science and theoretical investigation developed over the last two plus centuries. These researches & studies have created modern theories that guide engineers to work out engineering solutions and develop LPS to provide a statistically derived protection system by optimizing the overall cost and designed to work silently 24X7 without any recurring cost. ...Concluded
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.
