A Guide to BS EN 62305:2006 Protection Against Lightning Part 6 pps

BS EN 62305-3 Physical damage to structures and life hazard If the metallic and electrical services enter the structure at different locations and thus several bonding bars are required, these bonding bars should be connected directly to the earth termination system, which preferably should be a ring (Type B) earth electrode arrangement If a Type A earth electrode arrangement is used then the bonding bars should be connected to an individual earth electrode (rod) and additionally interconnected by an internal ring conductor If the services enter the structure above ground level, the bonding bars should be connected to a horizontal ring conductor either inside or outside the outer wall and in turn be bonded to the external down conductors and reinforcing bars of the structure Where structures are typically computer centres or communication buildings where a low induced electromagnetic field is essential, then the ring conductors should be bonded to the reinforcing bars approximately every metres b) If the roof mounted equipment cannot withstand a direct lightning strike then a separation (ie isolation) distance needs to be calculated (explained in more detail, later in this section) If this separation distance can be achieved, (ie there is sufficient space on the roof) then an air rod or suspended conductor should be installed (see Figure 4.19) This should offer sufficient protection via the protective angle or rolling sphere method and is so spaced from the equipment, such that it complies with the separation distance This air rod/suspended conductor should form part of the air termination system If the equipment has metallic services entering the structure (gas, water etc) that can be bonded directly, then these should be bonded to the nearest equipotential bonding bar If the other electrical services not have an effective outer core screen, then consideration should be given to bonding to the nearest equipotential bonding bar, via Type II overvoltage SPDs If the electrical services are effectively screened but are supplying electronic equipment, then again due consideration should be given to bonding, via Type II overvoltage SPDs Protection measures for roof mounted equipment containing electrical equipment This is an issue that has already caused some debate Applying the guidance from BS 6651 the designer/installer would bond the metallic, roof mounted casing into the mesh air termination system and accept that if the metallic casing suffered a direct lightning strike, then the casing, if not sufficiently thick, could be punctured What it did not address to any great degree was the solution to the possibility of partial lightning currents or induced overvoltages entering into the structure, via any metallic services that were connected to the roof mounted equipment BS EN 62305-3 significantly elaborates this topic Our interpretation of the lightning protection requirements can be summarised by the flow chart shown in Figure 4.41 There are several scenarios that can occur: a) If the roof mounted equipment is not protected by the air termination system but can withstand a direct lightning strike without being punctured, then the casing of the equipment should be bonded directly to the LPS If the equipment has metallic services entering the structure (gas, water etc) that can be bonded directly, then these should be bonded to the nearest equipotential bonding bar If the service cannot be bonded directly (power, telecom, cables) then the ‘live’ cores should be bonded to the nearest equipotential bonding bar, via suitable Type I lightning current SPDs If the electrical services are effectively screened but are not supplying electronic equipment, then no additional measures are required c) If the roof mounted equipment cannot withstand a direct lightning strike, then again a separation distance needs to be calculated If this separation distance cannot practically be achieved, (ie there is insufficient space on the roof) then an air rod or suspended conductor should be installed This still needs to meet the protective angle or rolling sphere criteria but this time, there should be a direct bond to the casing of the equipment Again, the air rod/suspended conductor should be connected into the air termination system If the equipment has metallic services entering the structure (gas, water etc) that can be bonded directly, then these should be bonded to the nearest equipotential bonding bar If the service cannot be bonded directly, (power, telecom, cables) then the ‘live’ cores should be bonded to the nearest equipotential bonding bar, via suitable Type I lightning current SPDs The above explanation/scenarios are somewhat generic in nature and clearly the ultimate protection measures will be biased to each individual case We believe the general principle of offering air termination protection, wherever and whenever practical, alongside effective equipotential bonding and the correct choice of SPDs where applicable, are the important aspects to be considered when deciding on the appropriate lightning protection measures 64 BS EN 62305-3 | Lightning equipotential bonding www.furse.com Separation (isolation) distance of the external LPS A separation distance (ie the electrical insulation) between the external LPS and the structural metal parts is essentially required This will minimise any chance of partial lightning current being introduced internally in the structure This can be achieved by placing lightning conductors, sufficiently far away from any conductive parts that has routes leading into the structure So, if the lightning discharge strikes the lightning conductor, it cannot ‘bridge the gap’ and flash over to the adjacent metalwork This separation distance can be calculated from s = ki × kc km ×l (4.5) Where: ki Relates to the appropriate Class of LPS (see Table 4.13) kc Is a partitioning coefficient of the lightning current flowing in the down conductors (see Table 4.14) km l Material km Air Concrete, bricks 0.5 When there are several insulating materials in series, it is good practice to use the lower value for km The use of other insulating materials is under consideration Table 4.15: Values of coefficient km (BS EN 62305-3 Table 12) If the structure has a metallic framework, such as steel reinforced concrete, or structural steel stanchions and is electrically continuous, then the requirement for a separation distance is no longer valid This is because all the steelwork is effectively bonded and as such an electrical insulation or separation distance cannot practicably be achieved Type of air termination system Earthing arrangement Type A Earthing arrangement Type B Single rod Is the length in metres along the air termination or down conductor, from the point where the separation distance is to be considered, to the nearest equipotential bonding point 1 Wire 0.66 Mesh Mesh ki I 0.08 II 0.06 III and IV 0.04 Table 4.13: Values of coefficient ki (BS EN 62305-3 Table 10) Number of down-conductors n Detailed values (see Table C.1) kc and more and more, connected by horizontal ring conductors 0.5 (see Figure C.1) a) 0.44 d) 0.25 0.5 (see Figure C.2) b) 0.44 d) 1/n 0.5 (see Figure C.3) c) d) a) Values range from kc = 0.5 where c