Presentation on theme: "Lightning and Lightning Protection"— Presentation transcript:
1Lightning and Lightning Protection Prof. Mohamed Adel AbdallahUmm AL-Qura University
2The Lightning ProcessLightning is a complex and not completely understood phenomenon. The process that creates a storm cell has not been completely defined, although there is general agreement as to the overall process.Activity within the cloud cell causes charge separation within the cell. The cell develops an electrical charge between the top and its base. As a result, the cloud becomes an electrostatic battery where the upper levels are predominantly positively charged and the base of the cloud takes on a predominately negative charge (see Figure). That charge tends to concentrate at the base of the cloud cell, producing a very strong electrostatic field between the cloud and the earth's surface.
3Under a mature storm, that electrostatic field can achieve levels of between 10,000 and 30,000 Volts per meter of elevation above the earth's surface.The electrostatic field created by the storm cell induces a positive charge on the earth's surface between the cell that is of equal charge but of opposite polarity, normally positive.It should be pointed out that the ionosphere also creates an electrostatic field of about 150 Volts per meter, normally leaving a negative charge (with respect to the ionosphere) on the earth's surface
5The Lightning Discharge Air dischargeA form of lightning discharge probably similar to a cloud discharge in which the lightning channel propagates away from a cloud charge center into apparently clear air where it terminates.Thus, cloud charge is moved away from its original location and space charge of opposite sign outside the cloud may be neutralized
7Atmospheric transients The atmospheric transient is a secondary effect resulting from varying electrostatic fields generated by thunderstorms and caused by discharges from direct lightning stokes.Any conductors suspended above the earth and immersed within an electrostatic field will be charged to that same voltage potential related to the object’s height (ie. height times the field strength) above local grade (see electrostatic field). As discharges (lightning stroke) occur nearby, the electrostatic field changes, sending voltage transients searching for a path to earth, along these conductors. Unless the path is properly protected, it will damage sensitive electric and electronic equipment along the way
8BreakdownThe process by which electrically-stressed air is transformed from an insulator to a conductor.Breakdown involves the acceleration of electrons to ionization potential in the electric field imposed by the thundercloud, and the subsequent creation of new electrons which avalanche and expand the scale of enhanced conductivity. Breakdown precedes the development of lightning or higher current-carrying processes during lightning flashes.
10r High Density Charge 10 < 40 Coulombs r =Striking Distance Step Meters= Striking Distance (r)+++r =Striking DistanceHigh Density ChargerPoint of Discrimination10 < 40 CoulombsStriking DistanceUpward Streamers
11Cloud electrification The process by which clouds become electrified. This process separates positive and negative electric charge and develops potential differences occasionally sufficient to produce lightning.Cloud flashA lightning discharge occurring between a positively charged region and a negatively charged region, both of which may lie in the same cloud.Cloud-to-ground flashA lightning flash occurring between a charge center in the cloud and the ground. On an annual basis, negative charge is lowered to ground in about 95% of the flashes, in the remaining 5% flashes a positive charge is lowered to ground.
12CoronaA faint glow enveloping the high-field electrode in a corona discharge, often accompanied by streamers directed toward the low-field electrode.corona dischargeA luminous, and often audible, electric discharge that is intermediate in nature between a spark discharge and a point discharge. It occurs from objects, especially pointed ones, when the electric field strength near their surfaces attains a value near 100,000 volts per meter.Aircraft flying through active electrical storms often develop corona discharge streamers from antennas and propellers, and even from the entire fuselage and wing structure.
13Electromagnetic pulse (EMP) The electromagnetic pulse is a secondary effect from a direct lightning stoke and is the result of the transient magnetic fields that form from the flow of intense transient current through the lightning channel.Ground-to-cloud dischargeA lightning discharge in which the original leader process starts upward from some object on the ground; the opposite of the much more common cloud-to-ground discharge.Induced ground chargeWhen a thunderstorm with electrified clouds (see charge separation) moves over an area, it induces a similar charge intensity of opposite polarity upon the ground surface below it.
14LightningLightning is a transient, high-current electric discharge whose path length is measured in kilometers.The most common source of lightning is the electric charges separated in ordinary thunderstorm clouds (cumulonimbus).lightning channelThe irregular path through the air along which a lightning discharge occurs.
15Peak currentThe maximum current measured in kilo Amperes (kA) of the lightning discharge is typically in the 20 to 30 kA range with a maximum of 310kA.secondary effectsLightning activity (direct lightning strikes resulting in lightning discharges) within close proximity to wiring, plumbing, electrical and electronic equipment generate destructive and disruptive secondary effects.Secondary effects include intense electromagnetic pulses (EMP), earth current transients, atmospheric transients and bound charges.
16ThunderSound is generated along the length of the lightning channel as the atmosphere is heated by the electrical discharge to the order of 20,000 degrees C (3 times the temperature of the surface of the sun). This compresses the surrounding clear air producing a shock wave, which then decays to an acoustic wave as it propagates away from the lightning channel
20Lightning Related Secondary Effects In addition to the potential damage created by the direct strike return stroke, there are five potential secondary effects that are the result of strike termination.All or any of these results can create significant damage to electrical and electronic circuits.
221- Earth Current Transients The earth current transient is the direct result of the neutralization process that follows stroketermination as illustrated by the Figure, The process of neutralization is accomplished by the movement of the charge from the location where the charge is induced to the point where the stroke terminates.
23Earth Current Transients +Buried Data & Phone LinesEarth Current TransientsNearby Strike
24Any conductors buried in the earth within the charge will provide a more conductive path from where it was induced to the point nearest the stroke terminus.This induced voltage is called an earth current induced transient. It will be found on wires, pipes or other forms of conductors. If the wires are shielded, the internal wires will experience the first derivative of the shield current flow. Since the discharge process is fast (20 to 100 microseconds) and the rate of rise to peak can be as little as 50 nanoseconds in that case, the induced voltage will be very high.
252- Atmospheric Transients Atmospheric transients or electrostatic pulses are the direct result of the varying electrostatic field that accompanies an electrical storm. As illustrated in the Figure, any wire suspended above the earth is immersed within an electrostatic field and will be charged to that potential related to its height (i.e. height times the field strength) above local grade.For example, a distribution or telephone line suspended at an average of 10 meters above earth in an average electrostatic field during a storm will take on a potential of between100kV and 300 kV with respect to earth.
26Atmospheric Transients VaryingElectrostaticFieldNearbyStrike*Suspended Power Lines*Inter-PlantData Lines*Will Experience Induced Transients
273- Electromagnetic Pulse (EMP) The electromagnetic pulse is the result of the transient magnetic field that forms from the flow of current through the lightning stroke channel, as illustrated by the Figure. After the lightning stroke channel is established between the cloud and earth, it then becomes a conductive path like a wire.The neutralization current flows very rapidly, with the rate dependent on the channel path impedance and the charge within the cloud. The rate of rise of these current pulses varies by orders of magnitude. They have been measured at levels of up to 510 kA per microsecond. A practical average would be 100 kA per microsecond.
28Stroke Channel EMP 100 kA di us dt = EMP Field Computer System IT PBXSatellite SystemEMP FieldITIT: Induced Transientsdidt=100 kAusStroke Channel EMP
29The EMP also has a related secondary effect as a result of the current flowing into the grounding system, as illustrated in the Figure.In this situation, the fast-changing current in time (di/dt) creates the magnetic field which is now mutually couples to any underground (within ground) wiring that passes nearby, over or parallel to any part of the grounding system.
30Ground Current EMP 100 kA di us dt = Ground Cables Underground UtilitiesGround Current EMPdidt=100 kAus
31Lightning Strike Protection Traditional lightning protection has sought to collect and divert the energy of a lightning strike into the ground.The industry standards groups agreed that the “cone of protection” theory was optimistic at best and various groups seemed to agree independently that the logic should switch to the “rolling sphere” concept. This theory was based on the premise that by reducing the assumed volume of protection from a cone to one where the height of the collector was used as the radius of a ball rolled against the collector, thereby reducing the assumed protected volume to that illustrated by the Figure.
35Air Terminal Height. The tip of an air terminal shall be not less than 254 mm (10 in.) above the object or area it is to protect.
36(A) The graph shows the protected distance (“horizontal distance”) as measured radialy from the protected structure.(B) The horizontal distance thus determined shall apply only at the horizontal plane of the “height protected.”Under the rolling sphere model, the horizontal protected distance found geometrically by (“horizontal distance, ft”) also shall be permitted to be calculated using the formula:
37Roof Types and Pitch. For the purpose of this standard, protection for the various roof types shall be as shown
45The zone of protection shall include the space not intruded by a rolling sphere having a radius of 46 m (150 ft).(A) Where the sphere is tangent to earth and resting against a strike termination device, all space in the vertical plane between the two points of contact and under the sphere shall be considered to be in the zone of protection.(B) A zone of protection shall also be formed where such a sphere is resting on two or more strike termination devices and shall include the space in the vertical plane under the sphere and between those devices, as shown(C) All possible placements of the sphere shall be considered when determining the zone of protection using the rolling sphere model.For structure heights exceeding 46 m (150 ft) above earth, the zone of protection shall be the space in the vertical plane between the points of contact and also under the sphere where the sphere is resting against a vertical surface of the structure.
46Geometric model for structures of selected heights up to 46 m (150 ft ). Based on the height of the strike termination device for a protected structure being 7.6 m (25 ft), 15 m (50 ft), 23 m (75 ft), 30 m (100 ft), or 46 m (150 ft) above ground, reference to the appropriate curve shows the anticipated zone of protection for objects and roofs at lower elevations.
54Ground Terminals.Each down conductor shall terminate at a ground terminal dedicated to the lightning protection systemGround rods shall be not less than 12.7 mm ( in.) in diameter and not less than 3 m (10 ft) into the earth.
56Dissipation Array System (DAS) The Dissipation Array System (DAS), generically known as a Charge Transfer System (CTS) the only true lightning strike prevention system. That is, the system actually prevents the termination of lightning strikes within any area defined as “protected”.
57It consists of the three basic subsystems The Ground Charge Collector (GCC) is deployed such that it will collect the charge induced on the area or facility to be protected.The Charge Conductor (CC) is analogous to the conventional down conductor; but should be thought of as an “Up Conductor” because its function is to conduct the collected charge to the Ionizer, providing a low surge impedance path in the process.The Charge Transfer Mechanism (The Ionizer) is the Charge Transfer component, and the most design sensitive. Its function is to transfer the collected charge to the adjacent air molecules via a principle known as “Point Discharge”. The resulting ions make up what is known as “Space Charge”,
58This Space Charge forms a shield between the protected site and the storm cell.
60DAS Ionizer can react and prevent stroke termination by generating a combination of the pre-strike space charge and a high density charge.Any existing space charge will have been neutralized by current flow through the ionized air moleculesDAS Ionizer can and does produce a very strong Space Charge and prevent leader termination