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A Guide to BS EN 62305:2006 Protection Against Lightning Part 7

Khi hai đám mây tích điện trái dấu lại gần nhau, hiệu điện thế giữa chúng có thể lên tới hàng triệu von. Giữa hai đám mây có hiện tượng phóng tia lửa điện và ta trông thấy một tia chớp. | íùrse Structural LPS not required If the risk evaluation shows that a structural LPS is not required ie RD is less than RT but there is an indirect risk Rj ie Rj is greater than RT any electrical services feeding the structure via an overhead line will require lightning current Type I SPDs tested with a 10 350ps waveform of level l2.5kA 10 350ps . For underground electrical services connected to the structure protection is achieved with overvoltage or Type II SPDs tested with an 8 20ps waveform in accordance with the Class II test within the BS EN 61643 standard on SPDs . Such underground electrical services are not subject to direct lightning currents and therefore do not transmit partial lightning currents into the structure. Underground electrical services therefore do not have a requirement for lightning current Type I SPDs where no structural LPS is present. The relationship between differing types of SPDs their testing regimes and typical application is illustrated in Table 5.3. Type of SPD Description Test classd Test waveform ps Typical application I Equipotential bonding or lightning current SPD I 10 350 current Mains distribution board II Overvoltage SPD II 8 20 current Subdistribution board III Overvoltage SPD III Combination 1.2 50 voltage and 8 20 current Terminal equipment 1 Test class to BS EN 61643 series Table 5.3 Test class and application of SPDs Enhanced performance SPDs - SPD Table NB.3 of Annex NB BS EN 62305-2 details the use of improved performance SPDs to further lower the risk of damage. It should be clear that the lower the sparkover voltage the lower the chance of flashover causing insulation breakdown electric shock and possibly fire. It therefore follows that SPDs that offer lower and therefore better voltage protection levels UP further reduce the risks of injury to living beings physical damage and failure of internal systems. This subject is discussed in detail on page 80 Coordinated SPDs. Other considerations Once an LPZ is defined