基础防雷外文资料翻译(编辑修改稿)

基础防雷外文资料翻译(编辑修改稿)内容摘要：

y than a sharp pointed rod. Faraday Cage and overhead shield designs produce yet other effects. Air terminal design and performance is a controversial and unresolved issue. Commercial claims of the elimination of lightning deserve a skeptical reception. Further research and testing is ongoing in order to understand more fully the behavior of various air terminals. Downconductors, Bonding and Shielding Downconductors should be installed in a safe manner through a known route, outside of the structure. They should not be painted, since this will increase impedance. Gradual bends (min. eight inch radius) should be adopted to avoid flashover problems. Building steel may be used in place of downconductors where practical as a beneficial part of the earth electrode subsystem. Bonding assures that all metal masses are at the same electrical potential. All metallic conductors entering structures (AC power, gas and water pipes, signal lines, HVAC ducting, conduits, railroad tracks, overhead bridge cranes, etc.) should be integrated electrically to the earth electrode subsystem. Connector bonding should be thermal, not mechanical. Mechanical bonds are subject to corrosion and physical damage. Frequent inspection and ohmic resistance measuring of pression and mechanical connectors is remended. Shielding is an additional line of defense against induced effects. It prevents the higher frequency electromagic noise from interfering with the desired signal. It is acplished by isolation of the signal wires from the source of noise. Grounding The grounding system must address low earth impedance as well as low resistance. A spectral study of lightning39。 s typical impulse reveals both a high and a low frequency content. The high frequency is associated with an extremely fast rising front on the order of 10 microseconds to peak current. The lower frequency ponent resides in the long, high energy tail or followon current in the impulse. The grounding system appears to the lightning impulse as a transmission line where wave propagation theory applies. A single point grounding system is achieved when all equipment within the structure(s) are connected to a master bus bar which in turn is bonded to the external grounding system at one point only. Earth loops and differential rise times must be avoided. The grounding system should be designed to reduce ac impedance and dc resistance. The shape and dimension of the earth termination system is more important a specific value of the earth electrode. The use of counterpoise or crow39。 s foot radial techniques can lower impedance as they allow lightning energy to diverge as each buried conductor shares voltage gradients. Ground rings around structures are useful. They should be connected to the facility ground. Exothermic (welded) connectors are remended in all circumstances. Cathodic reactance should be considered during the site analysis phase.。