外文翻译-雷电防护的基本原则.doc

1、外文文献：Basic Principles of Lightning ProtectionA lightning conductor is a device invented by Benjamin Franklin to protect buildings from lightning strikes. It provides a low-resistance path for the lightning current to flow to the ground, preventing any damage that would have resulted if the lightning

2、 current had passed through the building to the ground. Let us now consider how a lightning conductor acts during a lightning strike. Recent research has shown that when allowed to compete with each other, lightning rods with moderately blunt tips perform better than rods with sharp tips.The electro

3、-geometrical method (EGM) is a simple procedure used by lightning protection engineers to evaluate the attractive range of a lightning conductor 1. Let us first examine the basic assumptions of this method.Two basic assumptions are made in EGM. The first assumption is that inevaluating a lightning a

4、ttachment to structures one can neglect the effect of connecting leaders. Since connecting leaders are an integral part of lightning attachment, this assumption is not justified. However, the length of the connecting leader depends on the height of the structure. It increases with increasing structu

5、re height. This assumption is therefore justified only in the case of short structures where the influence of the connecting leader on the lightning attachment is slight. With increasing structure height the effect of connecting leaders on lightning attachment increases, and one would expect the res

6、ults obtained from EGM to deviate from reality. The second assumption is that a stepped leader is attracted to a grounded structure when it reaches a critical distance from the grounded structure. This critical distance is called the striking distance. It can be described as the distance of the fiel

7、d of vision of the stepped leader. If the distance from a grounded structure to the stepped leader is larger than the striking distance, then the stepped leader is not aware of the grounded structure. The reason for this assumption in EGM is as follows. As the distance between a grounded structure a

8、nd the tip of the stepped leader decreases, the average electric field in the space between the grounded structure and the tip of the stepped leader increases. When this average electric field reaches a critical value, the positive streamers (in the case of a negative stepped leader, of course) gene

9、rated from the extremity of the structure will be able to propagate in the space between the structure and the stepped leader connecting the stepped leader to the grounded structure. When the electric field that exists between the stepped leader and the grounded structure reaches this critical value

10、, the attachment process is said to have reached the final jump condition. Once the final jump condition is established beween a point on the structure and the stepped leader the attachment of the lightning flash to that point is imminent.As is apparent from Eq. 17.1, the striking distance is a func

11、tion of the prospective return stroke peak current, as is the radius of the sphere that should be rolled around the structure to determine the location of lightning conductors. So what is the radius of the sphere to be selected in lightning protection? To solve this problem, lightning protection eng

12、ineers have categorized lightning protection of structures into four levels. These levels are presented in Table 17.1. If a structure is protected at Level I, then any return stroke peak current larger than 3 kA will be stopped by the lightning protection system. That is, the radius of the sphere us

13、ed in designing the lightning protection system will be 20 m. If a structure is protected at level II, then any peak current larger than 5 kA will be intercepted by the lightning protection system. At level II the safety level is 10 kA and at level IV 16 kA.Consider a structure that is protected at

14、level IV. This does not mean that allleaders that come down over the structure having prospective return stroke currents smaller than 16 kA will strike it. Many of these stepped leaders will also be captured by the lightning protection system. For example, any stepped leader having a prospective ret

15、urn stroke current less than 16 kA approaching the structure within its striking distance from a lightning conductor will be intercepted by the lightning conductor。Observe that having a lightning protection system does not mean that the structure will not be struck by lightning. It will be struck by

16、 lightning having current peaks smaller than one associated with the level of protection. Thus, before installing a lightning protection system, one must have some idea of the magnitude of the peak return stroke current that the structure can safely withstand. Once this decision is made, the level of protection necessary can be selected. Observe that the higher the sensitivity of the structure (or its contents) to lightning flashes, the higher the level of protection that shoul