Patent Publication Number: US-6657120-B2

Title: Lightning shelters

Description:
BACKGROUND TO THE INVENTION 
     1. Field of the Invention 
     The present invention relates to shelters, particularly shelters for individuals or groups of people caught in the open in inclement weather especially when there is a danger of lightning. 
     2. Summary of the Prior Art 
     Persons caught in the open during inclement weather may be at risk of being struck by lightning. The present applicant proposed in GB 2 332 458A a lightning shelter having a cover supported by an electrically-conductive frame and a floor of electrically-conductive mesh. The frame comprised a plurality of hollow metal poles which are anchored to the ground by earthing or grounding spikes. As shown in FIG. 1, the mesh floor  10  was welded to a collar  12  surrounding the lower end of one frame pole  14  which was anchored to a grounding spike  16 . The collar  12  was fixed to the metal pole by bolts  18 . 
     SUMMARY OF THE INVENTION 
     The present applicant has appreciated that extremely high voltage gradients may be generated when grounding a lightning bolt through a spike in the ground. In fact, the spike dissipates to ground a current pulse associated with the lightning strike, generating a high voltage potential centred on the spike. Points on the ground spaced from the spike will be at a much lower potential, giving rise to a voltage gradient, with voltage decreasing rapidly in a radial direction from the spike. The voltage gradient may be lethal to humans and animals since an electric shock. across vital organs e.g. heart may be produced when one leg is on the ground at a much higher potential than the other (so-called step voltage). 
     In accordance with the present invention, there is provided a shelter comprising: an elongate electrically-conductive member for supporting a canopy, the elongate electrically-conductive member being configured as a lightning conductor; an electrically-conducting floor; and a coupling member for electrically connecting the elongate electrically-conductive member to the electrically-conducting floor, wherein the coupling member includes a flange extending laterally away from the elongate electrically-conducting member, over the electrically-conductive flooring, to provide an enlarged footprint of engagement with the electrically-conducting floor. 
     The footprint should be of such a size as to ensure complete electrical conduction between the elongate member and the floor. The flange may surround the elongate electrically-conductive member, and may be substantially annular. By avoiding sharp points or corners, voltage concentrations capable of starting electrical breakdown (sparks) are prevented. In this way, the flange may ensure a uniform potential (equi-potential) is produced across the electrically-conducting floor, thereby preventing the occurrence of a lethal “step voltage” during a lightning strike. 
     The electrically-conductive flooring may comprise a metal mesh. The coupling member may comprise a further flange which opposes and is movable relative to the aforementioned flange. The flanges may thus define a pair of jaws which are configured to engage opposite sides of the electrically-conductive flooring. The flanges may be urged together using bolts to provide a clamping action. Electrical resistance between the flange(s) and the electrically-conductive flooring may be below 100 mΩ; in this way explosive resistive heating caused by a high current lightning pulse may be avoided. 
     For larger shelters, the electrically-conductive flooring may comprise a first and a second metal mesh, with one overlapping the other at the coupling member. In this way, the flange may be used with an opposing part to clamp the first and second metal meshes together, urging one into intimate contact with the other. 
     Each metal mesh may be supplied from a roll, and may therefore have a length which is greater than its width. In the case of first and second metal meshes from a roll, one metal mesh may be aligned with its roll direction at an angle to that of the other. In other words, the meshes may be positioned so that their longitudinal axes are at an angle to each other. The angle may be 90°. In this way, a plurality of metal meshes may be used and coupled together to cover an area which could be greater than 500 m 2  e.g. large enough to cover a tennis court. 
     A portion of the elongate electrically-conductive member which extends away from the coupling member may have an electrically-insulating cover, e.g. polyurethane sheath. The cover may be configured to provide protection for an individual from a touch voltage of up to 9 kV e.g. by employing a cover of thickness of about 3 mm. The touch voltage is the voltage difference between a position 2 m from ground level up the elongate electrically-conducting member to ground potential. When lightning strikes a conductor a touch voltage of up to 9 kV may be generated, and thus an individual may need to be insulated from it. 
     The electrically-conducting floor may further comprise a layer of insulation on top of the metal mesh. The layer of insulation may be at least 25 mm thick to prevent spikes on sports shoes (e.g. golf shoes) from penetrating the layer of insulation and making contact with the metal mesh. The layer of insulation may comprise rubber, e.g. granulated rubber bonded with resin. 
     The layer of insulation may be configured to provide for rigidity, for example by comprising a hard material such as concrete. 
     The shelter may comprise a foundation member under the coupling member which is configured to provide ballast for the elongate electrically conductive member. For example, the foundation member might comprise a dense material such as concrete. The foundation member may help maintain the integrity of the shelter by reducing movement of the portion of the electrically conductive member which is below ground, thereby preventing the generation and enlargement of holes or voids at the bottom end of the electrically conductive member. The foundation member may surround and extend laterally away from the elongate electrically conductive member at least as far as the bolts that clamp the flanges together. 
     The elongate electrically-conductive member may comprise a pole and a spike member driveable into the ground, the spike member being configured to anchor one end of the pole to the ground when in use. One end of the pole may be a push fit onto an end of the spike member which remains exposed when driven into the ground. One part of the clamping member may be coupled (e.g. welded) to the spike member. 
     The elongate electrically-conductive member may further comprise a elongate connecting member configured at one end to be a push fit onto the lower end of the pole and at the opposite end to be a push fit onto the upper end of the spike member. The connecting member may help the pole and spike member to register with each other during assembly. In addition, the connecting member may provide a robust connection between the spike member and the pole. 
     The shelter may comprise at least one further electrically-conducting member configured to be driveable into and anchor in the ground around the shelter&#39;s perimeter. When anchored in the ground, the at least one further electrically-conducting member may be used to measure the electrical potential of the ground to thereby test the performance of the shelter. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a view showing detail of a prior art shelter; 
     FIG. 2 is a schematic perspective view of a shelter embodying the present invention; 
     FIG. 3 is a cross-sectional view showing detail of the shelter of FIG. 2 according to a first embodiment; 
     FIG. 4 is a plan view showing detail of a clamping plate of FIG. 3; 
     FIG. 5 is a cross-sectional view showing detail of the shelter of FIG. 2 according to a second embodiment; 
     FIG. 6 is a plan view showing detail of a clamping plate of FIG. 5; 
     FIG. 7 is a plan view of the shelter of FIG.  2  and three test points; and 
     FIG. 8 is a view in profile of the arrangement of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Specific embodiments of the present invention will now described, by way of example, and with reference to the accompanying drawings. 
     A shelter  20  comprises a plurality of elongate metal embers or poles  22 , the upper ends  24  of which are coupled to a metal spike  26  at the apex of the canopy  28 . The lower ends  30  of the poles  22  are electrically connected to metal earthing or grounding spikes  32  so that each respective pole/spike pair is configured as a lightning conductor  34 . The shelter  20  is provided with a metal mesh floor  36  which is electrically connected via coupling members  38  to each lightning conductor  34 . The metal mesh floor  36  extends outwardly of the lower ends  30  of poles  22  by about 0.75 m. 
     FIG. 3 shows in cross section according to a first embodiment the way in which the coupling member  38  electrically connects the metal mesh floor  36  to the lightning conductor  34  (comprising pole  22  and grounding spike  32 ). A metal hub  40  is bolted over the top of the grounding spike  32  and is a snug fit inside the lower end  30  of pole  22 . (Bolts  42  are provided to secure the pole  22  to the hub  40 ). The coupling member  38  comprises an annular metal flange  44  extending laterally of and around hub  40 , and over metal mesh floor  36 . An annular plate  46  of an equivalent size to flange  44  is positioned beneath the metal mesh floor  36  (in registration with the annular flange  44 ) and is bolted at  48  to the annular flange  44 . The metal mesh floor  36  is thereby clamped between the annular flange  44  and annular plate  46 , ensuring intimate electrical contact between metal parts thereof. In fact, coupling member  38  provides an annular enlarged footprint  50  of engagement with the metal mesh floor  36 . 
     The clamping action of coupling member  38  is particularly useful when the metal mesh floor  36  comprises two or more layers  36 A,  36 B. For example, if metal mesh is provided from a roll of such material, the two layers  36 A,  36 B may be orientated with the roll direction (long axis) of one perpendicular to that of the other, with the two layers overlapping at least where clamped by the coupling member  38 . In this way, large areas may be covered by the shelter without jeopardising the protection afforded to anyone sheltering therein during a lightning strike. 
     The pole  22  is sheathed in a pipe  54  of insulating material (e.g. polyurethane pipe). A 30 mm insulating layer  56  of rubber crumb/resin may be provided on top of mesh  36 . Not only does the insulating layer  56  prevent objects coming into contact with the mesh  36 , even sharp objects such as shoe spikes which may stick into the insulation, but also it may raise the floor level when the mesh  36  is located beneath the ground on which the shelter stands. Furthermore, a skirt or gaiter  58  of insulating material is provided around the lower end  30  of pole  22  to shield individuals from direct contact with the coupling member  38 . 
     A second embodiment of the present invention is shown in FIGS. 5 and 6. Where the components of the second embodiment are the same as those of the first embodiment, the same reference numbers have been used. The following differences in the embodiments are to be noted: 
     Instead of the 30 mm insulating layer  56  of rubber crumb/resin of the first embodiment, the second embodiment has a layer of concrete  60  over the metal mesh floor  36 . The layer of concrete may improve upon the rigidity of the shelter. A thin layer, e.g. 6.5 mm (0.25 inch), of rubber crumb/resin  56  is provided over the concrete layer  60  to protect spiked footwear from being damaged by the concrete. 
     A foundation member  62  is provided underneath the coupling member  38 . The foundation member  38  surrounds the lightning conductor  34  and extends laterally beyond the footprint  50  of engagement created by the coupling member to provide ballast for the lightning conductor. The foundation member  62  typically comprises concrete. 
     Instead of the hub  40  and bolt  42  arrangement of the first embodiment, the pole  22  of the second embodiment is connected to the grounding spike  32  by means of an elongate connecting member  64  of circular cross-section. As shown in FIG. 5, the opposite ends of the connecting member  64  are inserted into the lower end of the pole  22  and the upper end of the grounding spike  32  respectively. The ends of the connecting member should be a snug fit inside the pole and grounding spike to ensure secure connection of pole and grounding spike. 
     The grounding spike  32  of the embodiment of FIG. 5 comprises a elongate hollow cylinder, which is terminated by a solid spike  66  which is a snug push fit into the open end of the elongate hollow cylinder. 
     FIGS. 7 and 8 show an arrangement which allows the performance of the shelter  20  to be tested. Three earth electrodes  68  are installed in a triangular pattern in the ground around the perimeter of the shelter  20 . The electrodes may be installed permanently and be provided with protective coverings. To test the shelter, test equipment is connected to the base of the metal spike  26  at the top of the shelter&#39;s canopy and to a first electrode  68  and the performance test carried out. The test equipment is disconnected from the first electrode and connected to each of the other electrodes in turn whereupon the performance test is repeated. The performance test should be in accordance with IEE 613-4 Appendix 15. The electrodes  68  should be positioned in relation to the shelter on the basis of the so-called ‘100 foot arc’, which determines the ‘safe zone’ around the outside of the shelter. As shown in FIG. 8, the ‘100 foot arc’  70  contacts the shelter at the top of the metal spike  26  and at the poles  22 . The point at which arc  70  contacts the ground determines the extent of the safe zone. 
     It is to be appreciated that the shelter and other features described with reference to the embodiments discussed above can be combined in other embodiments of the present invention.