兰花集团同宝煤业矿区防雷设计外文翻译资料

Fundamentals of Lightning Protection

Introduction

Lightning is a capricious, random and unpredictable event. Its physical characteristics include current levels sometimes in excess of 400 kA, temperatures to 50,000 degrees F., and speeds approaching one third the speed of light. Globally, some 2000 on-going thunderstorms cause about 100 lightning strikes to earth each second. USA insurance company information shows one homeowners damage claim for every 57 lightning strikes. Data about commercial, government, and industrial lightning-caused losses is not available. Annually in the USA lightning causes more than 26,000 fires with damage to property (NLSI estimates) in excess of $5-6 billion.

The phenomenology of lightning strikes to earth, as presently understood, follows an approximate behavior:

1. The downward Leaders from a thundercloud pulse towards earth seeking out active electrical ground targets.

2. Ground-based objects (fences, trees, blades of grass, corners of buildings, people, lightning rods, etc., etc.) emit varying degrees of electric activity during this event. Upward Streamers are launched from some of these objects. A few tens of meters off the ground, a 'collection zone' is established according to the intensified local electrical field.

3. Some Leader(s) likely will connect with some Streamer(s). Then, the 'switch' is closed and the current flows. We see lightning.

Lightning effects can be direct and/or indirect. Direct effects are from resistive (ohmic) heating, arcing and burning. Indirect effects are more probable. They include capacitive, inductive and magnetic behavior. Lightning 'prevention' or 'protection' (in an absolute sense) is impossible. A diminution of its consequences, together with incremental safety improvements, can be obtained by the use of a holistic or systematic hazard mitigation approach, described below in generic terms.

Lightning Rods

In Franklins day, lightning rods conducted current away from buildings to earth. Lightning rods, now known as air terminals, are believed to send Streamers upward at varying distances and times according to shape, height and other factors. Different designs of air terminals may be employed according to different protection requirements. For example, the utility industry prefers overhead shielding wires for electrical substations. In some cases, no use whatsoever of air terminals is appropriate (example: munitions bunkers). Air terminals do not provide for safety to modern electronics within structures.

Air terminal design may alter Streamer behavior. In equivalent e-fields, a blunt pointed rod is seen to behave differently 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 on-going 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 compression and mechanical connectors is recommended.

Shielding is an additional line of defense against induced effects. It prevents the higher frequency electromagnetic noise from interfering with the desired signal. It is accomplished 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 lightnings 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 component resides in the long, high energy 'tail' or follow-on 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 'crows 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 recommended in all circumstances.

Cathodic reactance should be considered during the site analysis phase. Man-made earth additives and backfills are useful in difficult soils circumstances: they should be considered on a case-by-case basis where lowering grounding impedances are difficult an/or expensive by traditional me

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防雷基础

介绍

雷电是一种反复无常、随机且不可预测的现象。它的物理特性为：电流有时会超过400kA，温度可以高达50000华氏度，速度接近光速的三分之一。从全球范围来讲，2000多次持续雷暴每秒造成大约100次雷击。美国保险公司的信息显示，基本上每57次对房主的赔偿中有一次是因为雷电。这些数据还不包括商业、政府和工业雷电造成的损失。在美国，每年有超过26000起火灾，造成的财产损失超过50-60亿美元。（NLSI估计）

地球雷击现象，据目前的了解，遵循着如下的一个规律：

1.顶层雷云脉冲下行至地面，寻找地面的电气目标。

2.地面物体（围栏、树木、草叶、建筑物角落、人员、避雷针等）在受到雷击时会发出不同强度的电力活动。从这些地基对象向上发送电力波动，在离地面几十米处会出现一个“收集区”加剧的电场。

3.当带有异种电荷的雷云相遇时，相当于电路“开关”被关闭，于是有电流流过。这就是我们看到的闪电。

雷电的影响可以是直接或间接的。直接的影响是使电阻发热、电弧燃烧。间接的影响在于对电容、电感和电磁的影响。闪电的“预防”或“保护”（在绝对意义上的）是不可能的。可以通过使用整体或系统的方案来减小雷电的影响，进行安全防护，下面就对常用的方案进行描述。

避雷针

从富兰克林研究雷电开始，就开始使用避雷针将雷电引入大地，保护建筑物。避雷针是现在最常用的防雷装置，根据建筑物形状、高度等，可以设计不同的避雷针来达到避雷效果。例如，一些公共事业倾向于变电站架空屏蔽电缆。在有些情况下，没有任何合适的避雷装置。

高空架设避雷装置可以改变雷电的动作。在等效电子领域，钝尖杆被视为是一种有效的避雷针类型。空气终端的设计和性能，是一个有争议且未解决的问题。“消除”闪电是一个存在质疑的方法。进一步的研究和测试仍在进行中，为的是更加充分地了解各种高空避雷装置的作用。

引下线、接地和屏蔽

引下线应以一种安全的方式来安装，敷设在建筑物外部。引下线不可以涂漆，因为这样会使阻抗增加。应采用圆钢（半径至少8英寸）等，以避免闪络问题。有些情况下可利用建筑物中的钢筋等作为引下线，但要保证所有的金属物共同接地。

交流电以及燃气和水管、信号线、空调管道、铁路轨道、桥式起重机等金属导体都应处于相同的电位，即共用接地系统。金属导体之间的连接应该是焊接，而不是机械连接。机械连接时容易受到腐蚀和物理损害。建议对压缩和机械连接器进行定期检查和欧姆电阻测量。

屏蔽是防止雷电感应的另一道防线。它可以防止较高频率的电磁噪声干扰信号。它通过隔离噪声源的信号线来完成。

接地

接地系统必须解决低阻抗和低电阻的问题。雷电典型脉冲频谱的研究揭示了雷电中的高频和低频部分。高频的频谱变化是非常迅速的，可达到10mu;s的峰值电流，处于前端。低频频谱则存在于脉冲中高能量的“尾部”或后续电流中。接地系统就是将雷电导入大地来减少雷电的危害。

单点接地系统是将结构内的所有设备都连接到一个主汇流排，再将主汇流排直接连接到外部接地系统。必须避免接地回路和差分上升时间。接地系统的设计应能够减小交流阻抗和直流电阻。接地系统所在位置的形状和尺寸决定特定接地极的选择。使用平衡或“乌鸦足”辐射技术可以降低阻抗，因为它们会使雷电能量发散，每个接地体电位相等。建筑物周围的接地环也很有用，应将其连接到设施地面。在所有情况下都可以使用放热（焊接）连接器。

在现场分析阶段应考虑接地电阻。应根据具体情况考虑使用人工降阻剂，比如用于土壤高电阻地区。降低接地电阻的传统方式难以实现或费用很高。防雷方案中应包括对接地电阻的定期检查和测试。

瞬变和浪涌

普通熔断器和断路器不能应对雷电引起的瞬变。避雷装置具有分流电流、阻止能量沿着导线行进、过滤某些频率和钳制电压的作用。防雷设备是可以使得电流分散，过滤某些频率，钳制电压水平，或执行这些任务的组合。电压钳位器件能够限制瞬时高电流，还能减小瞬时高电压。采用浪涌保护器是一种可取的方法：它可以保护主面板，保护所有相关的二次配电板，保护贵重的电子设备，如过程控制仪表、计算机、打印机、火灾报警器、数据记录和SCADA设备等；此外，还能保护进出的数据线和信号线。保护主要电子设备，如井口、安全监控报警器、监控摄像头、高杆照明灯等。在建筑物之间的HVAC通风口也不应被忽视。

浪涌保护器的最小的引线长度取决于电气面板。在短时间内，长导线上会感应出瞬态高电压。

在任何情况下都要考虑保护器件的高质量、高速度和自我诊断能力。瞬态限制器件优先使用电弧间隙分流器、金属氧化物变阻器、硅雪崩二极管的组合。还要了解器件的箝位电压要求。确认供应商的产品已经过严格的ANSI / IEEE / ISO9000测试标准测试。避免低价，低价的产品扩大市场（警告空洞）。

发现

闪电探测器可用于不同的成本和技术，有时可用于早期预警。一个有意义的作用是在闪电到来之前，它可以断开交流线路电源并启用备用电源。用户应该谨防对这些不完善的设备的过度信任，它并不是总能达到完全的防雷效果。

教育

所有人都应当接受在雷雨天气时的防雷安全教育。在室内或汽车内时，要避免接触水和一些的金属物件；避免制高点；避免在孤木下避雨；避免在雨天室外打电话。如果在附近有闪电发生，应当躲在安全的位置，丢掉金属物件，蜷缩起来，低着头，手放在耳朵上，以减少声震。

测量闪电的距离很容易。使用“闪光/爆炸”（F / B）技术。每五次从看到雷击到听到雷声，雷电的距离为一英里以上。10英里的F / B = 2英里; 20英里的F / B等于4英里。从雷击点A到雷击点B到雷击点C的距离可以高达5-8英里。 如果可能的话，在第一次听到雷声时不要到户外活动，直到20分钟后最后一次可观察到的雷电已经过去，才能恢复户外活动。

相关机构应采取防雷安全政策，并用于整体安全计划。

测试

现代诊断测试可用来模拟雷电传导装置的性能，并显示闪电通过建筑物的一般路线。这种测试通常是低功耗的，最多50瓦。它是可追踪的，但不会跳闸MOV、气体管避雷器或其他瞬态保护装置。发生之前，了解事件发生前的行为是每个商人的殷切期盼。采用这种技术，可以可靠地预测闪电路径。

规范与标准

市场充斥着对产品完美性的夸大宣称。经常引用的代码和安装标准是不完整的、过时的和由于商业利益颁布的。另一方面，IEC、IEEE、MIL-STD、FAA、NASA和类似文件由背景工程、同行评审过程支持，并且是技术性质的。

总结

在雷电安全分析中考虑所有上述主题是很重要的。 在防雷方面没有乌托邦。 雷电带来的危害可能会超过每个研究人员的想象。采取系统性的防雷减灾措施是一个十分有效的方法。

参考文献

1. API 2003, Protection Against Ignitions Arising out of Static, Lightning, and Stray Currents, American Petroleum Institute, Washington DC, December 1991.

2. Golde, G.H., Lightning, Academic Press, NY, 1977.

3. Hasse, P., Overvoltage Protection of Low Voltage Systems, Peter Peregrinus Press, London, 1992.

4. Hovath, Tibor, Computation of Lightning Protection, John Wiley, NY, 1991.

5. IEEE Std 1100, Powering and Grounding of Sensitive Electronic Equipment, IEEE, NY, NY. 1992.

6. KSC-STD-E-0012B, Standard for Bonding and Grounding, Engineering Development Directorate, John F. Kennedy Space Center, NASA, 1991.

7. Morris, M.E., et.al., Rocket-Triggered Lightning Studies for the Protection of Critical Assets, IEEE Transactions on Industry Applications, Vol. 30, No. 3, May/June 1994.

8. Sunde, E.D. Earth Conduction Effects in Transmission Systems, D. Van Nostrand Co., NY, 1949.

9. Towne, D., Wave Phenomena, Dover Publications, NY.

10. Uman, Martin, Lightning, Dover Publications, NY, 1984.

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