EXTERNAL LIGHTNING PROTECTION SYSTEM AS PER INDIAN STANDARDS IS/IEC 62305-3 & NBC-2016 General There are no devices or methods capable of modifying the natural weather phenomena to the extent that they can prevent lightning discharges. Lightning flashes to, or nearby, structures (or lines connected to the structures) are hazardous to the structures, their contents and installations as well as to lines. This is why lightning protection measures are essential Lightning Protection System shall be in accordance with IS IEC 62305-3 & NBC-2016. Lightning Protection consists of external Protection for the building with Air termination, Down Conductors and Earthing and Internal protection for power lines with Surge Protective devices. Generally lightning between cloud and ground creates failures. How ever inter-cloud and intra-could lightning also can create potential differences and failures in electronic installation. More than 95 % of Lightning strikes are of Negative impulse and less than 5 % are of positive impulse. Positive impulses are mainly due to dry lightning in cold areas. Current parameters as per IS/IEC 62305 and the effects of lightning are as below Current Parameters Symbol Unit Lightning Protection Level Effect I II III IV First positive Impulse Peak Current I kA 200 150 100 Mechanical Impulse charge QSHORT C 100 75 50 Thermal (arc) Specific Energy W / R MJ/Ω 10 5.6 2.5 Mechanical & Thermal Average Steepness di / dt kA / µS 20 15 10 Surges and flashover Time Parameters T1 / T2 µS / µS 10/350 First Negative Impulse Peak Current I kA 100 75 50 Mechanical Average Steepness di / dt kA / µS 100 75 50 Surges and flashover Time Parameters T1 / T2 µS / µS 1 / 200 Damages from lightning strike are due to Peak Current (I), Charge (C), Specific Energy (W/R) & Rate of change of current (di/dt). Lightning protection is designed to take care of these effects of lightning and hence the following parameters shall be strictly followed. Effect of Lightning on External LPS (Air termination, Down Conductor and Earthing) Effects on air-termination systems arise from both mechanical and thermal effects. Effects on down-conductors are thermal effects due to resistive heating & mechanical effects in parallel conductors and in Bends. The real problems with earth-termination electrodes are linked with chemical corrosion and mechanical damage caused by forces other than electro dynamic forces. Sizing and fixing of Materials are selected to handle the mechanical and thermal effects. Bends in down conductor shall strictly NOT be at 90 degree (right angles) & should have a curved path of 45 degree bend. Earth electrodes are selected based on the current handling capacity up to 1 second. To avoid corrosion problems as explained in IS/IEC 62305 (clause E.4.3.4 and E.5.4.3.2), GI is strictly not recommended inside concrete and in soil. Effect of Lightning on Internal LPS (SPD’s for POWER, DATA lines etc) Effect on internal LPS is mainly due to coupling and the rate of change of current. Due to Very high di/dt of the first negative stroke. The expected problem is the response time of SPD and the voltage drop in the connecting wires. SPD’s at the incoming panels shall have a response time less than 1 nano sec & shall be of BUSBAR Mounted type to avoid connecting wire length. LPL (Lightning Protection Level) LPL is a number associated with a set of lightning current parameters relevant to the probability that the associated minimum & maximum values do not exceed the normally occurring lightning. LPL can be determined by Risk analysis as explained in IS IEC 62305-2 or can be selected based on the guideline in NBC-2016. Application LPL* Computer Data Centers, Military Applications, Nuclear Power Stations, High raise Hotels/Hospitals, airports, essential services such as telecom towers 1 EX-Zones in the industry and chemical sector, Low raise Hospitals & Hotels, fuel retail outlets, gas station, compressor station etc 2 Schools, Banks, Residential Buildings, Temple, Churches, Mosques 3/4 LPL levels, probability and basic design consideration: Class of LPS Lightning current MINIMUM Lightning current MAXIMUM Interception probability Rolling sphere radius (m) Mesh size (m) Down Conductor Spacing 1 3 kA 200 kA 98% 20 5*5 10 2 5 kA 150 kA 95% 30 10*10 10 3 10 kA 100 kA 88% 45 15*15 15 4 16 kA 100 kA 81% 60 20*20 20 Protection angle w.r.t Height Air termination system: Material, Configuration and Minimum cross sectional area of air terminal & down conductors Air Termination mesh conductor and down conductors: 8 mm Aluminium alloy round conductor (50 mm2) Air Termination Rod: 10 mm, 16 mm & 40 mm solid Aluminium rods (combination of sizes) (tubes are not allowed) Joints / Connectors / Fixing materials: Connection materials Connector type GI fixing materials shall not be used Aluminium to Aluminium Aluminium or SS Aluminium to Steel SS Aluminium accessories if connection is between Aluminium materials are necessary. SS accessories if connections are between aluminium and copper / copper coated materials. Earth Termination Conductor: 10 mm solid copper coated steel conductor (100 microns min coating) Earth Termination Joints in soil: Exothermic welding GI material for earthing shall not be used as per the recommendation in IS/IEC62305 as well as Kerala electrical inspectorate guideline. If the structure height is more than 60 meters, top 20% of the height of the structure shall be protected with a lateral air termination system. This is needed because the probability of flashes to the side is generally more for structures more than 60 meters in height. More importance need to be provided to Corners, Edges and significant protrusions such as balconies. Metallic handrails/ Aluminium frame of wall cladding if used in balconies shall be conned to air termination / down conductors. In PEB / Steel buildings where GI sheet roofing, air termination mesh / Rod shall be directly mounted on the sheet. Fixing materials used shall be in good electrical contact with the sheet, shall not create water leakage. No drilling is allowed in the terrace for fixing the air terminal, if the roof is made of concrete. Parapet wall is exception to this. Air terminal holder: Concrete Roof structure: Conductors shall be securely fixed on the terrace by means of concrete air terminal holders with suitable fixing materials which is fixed on the roof by adhesive or cement mortar taking care of varying weather conditions. Plastic air termination conductor holder is not allowed. The minimum height of this air terminal holder shall be 50 mm to avoid the contact of conductor with water Metal Roof structure: Conductors shall be securely fixed on the terrace by means of air terminal holder which is fixed on the roof by metal conductor holder made of Stainless steel. As metal roof structures are normally tapered at an angle, there are no min height criteria for metal conductor holder. Recommended fixing distance of air terminal and down conductors Arrangement Recommended distance TAPE / Strip ROUND Horizontal conductor on horizontal surface 500 mm 1000 mm Horizontal conductor on vertical surface 500 mm 1000 mm Vertical conductor from Ground to 20m height 1000 mm 1000 mm Vertical conductor above 20m height 500 mm 1000 mm If antenna, Chillers or any other roof top electrical equipment is present in terrace, the same have to be protected by using vertical air terminal after calculating the safety or separation distance. The vertical air terminal has to have suitable supports to hold it. Wind speed need to be taken into account. Vertical air terminal must be connected to horizontal air terminal by using suitable connectors. At the crossings of the horizontal air terminals, suitable Cross connector has to be used. Safety or Separation distance: (not required for LPS using structural natural components) To avoid flash over to electrical/electronic apparatus, this equipment shall be kept at a distance away from LPS components more than the safety distance as per the following calculation. Safety/Separation distance (S) in m = (ki * kc*L) / km Coefficient ki depends on class of LPL/LPS (ki = 0.08 for LPL1, 0.06 for LPL 2, 0.04 for LPL3 and 4) Coefficient kc depends on no of down conductors: kc = 0.66 for 2 down conductors, kc = 0.44 for 3 or more down conductors Value of coefficient km = 1 Value of L is the total distance between the equipment to be protected (for e.g. Antenna) to the equi-potential bonding bar situated just above the ground. Expansion piece In order to take care the expansion of the metal in summer and contraction of the metal in winter, expansion piece with suitable connectors have to be used at every 20m distance of horizontal air termination mesh. Joints and Bends The lightning protective system shall have few joints as far as possible & air terminal & down conductor have to be straight. Where it is not possible, it should NOT be bent at 90 degree (right angles) & should have a curved path of 45 degree bend. Down conductor system In order to reduce the probability of damage to electronic/electrical equipment, the down conductors shall be arranged in equi distance in such a way that from the point of strike to earth, several parallel current paths should exist & length of the current path should be minimum. Down conductors should be installed at each exposed corner of the structure as a minimum. Maximum distance between down conductors shall be as per the table above. Test joints: At the connection to the earth conductor, a test joint should be fitted on each down conductor at a height of 1 m from the ground, except in the case of natural down conductors combined with foundation earth electrode. The purpose of test joint is to measure the earth resistance value. The remaining portion of down conductor (i.e., after the test joint should be mounted inside a plastic pipe of minimum 3 mm thickness.) Earth Terminations For earth termination system, 2 basic types of earth electrode arrangements are applicable. Type A & Type B arrangement. Type A arrangement: Comprises of horizontal or vertical earth electrode installed outside the structure to be protected connected to each down conductor. Minimum Length of vertical earth electrode shall be as below Class of LPS Typical Length of each vertical earth electrode based on Soil resistivity Up to 500 Ω M 1000 Ω M 2000 Ω M 3000 Ω M 1 2.5 meter 10 meters 25 meters 40 meter 2 2.5 meter 5 meter 15 meters 22 meter 3 2.5 meter 2.5 meter 2.5 meter 2.5 meter 4 2.5 meter 2.5 meter 2.5 meter 2.5 meter If horizontal electrodes are used, the length shall be double. In type A arrangement, the total number of earth electrodes shall not be less than two. Type A arrangement is suitable in places where electronic equipment are not located. Type B arrangement: This type of arrangement comprises either a ring conductor external to the structure to be protected, in contact with the soil for at least 80% of its total length or a foundation earth electrode. Ring earthing must be 1 meter away from the building and 0.5m below the ground as a closed loop. Such earth electrodes can also be meshed. For structures with extensive electronic systems or with high risk of fire, type B earthing is most preferable method. There is no limit in the resistance of Ring Earthing if the ring radius of the ring is larger than 50 meters or 80 meters for LPL 1 and 2. For LPL 3 and 4 this radius is about 5 meters. The overall resistance of earthing system shall not exceed 10 ohms. Galvanised steel (GI) as earthing material shall not be used. Lightning Counters: At least 2 down conductors in an installation shall have a lightning counter tested as per IEC 62561. The counter shall be digital type with replaceable battery. Battery life shall be minimum 3 years. The minimum measuring current is 1KA (8/20) and the maximum is 100 KA (10/350). The counter shall be outdoor type, IP65 and shall be able to record date, time and no of strikes. Quality and Confirmations All materials and accessories shall be tested as per IEC 62561 for its mechanical / corrosion resistant / electrical conductivity. Vendor shall provide test reports along with completion certificate. GI (Hot dip galvanised or zinc electroplated) fixing materials and fasteners are not allowed. References: IS/IEC62305 – PROTECTION AGAINST LIGHTNING: Part 1: General Principles Part 2: Risk Management Part 3: Protection of structures Part 4: Protection of Electrical & Electronic equipment within structure NBC-2016: National Building Code of India – 2016 IS3043: 1987: Code of practice for earthing.