GB2206005A - Lightning conductor or device for discharging static electricity - Google Patents

Abstract

A lightning conductor or device for discharging static electricity comprises laser (10) arranged to project a laser beam (17) upwards, preferably vertically, through an earth conducting metal ring (19) to provide an ionized path for the passage of an electrical charge. The laser (10) may be substantially horizontal and its output beam be turned by a mirror (16) before passing through the ring (19). <IMAGE>

Description

LIGHTNING CONDUCTOR OR DEVICE FOR DISCHARGING STATIC ELECTRICITY Lightning takes the form of a violent electrical discharge which occurs either between an electrically charged cloud and the earth, between clouds charged to opposite polarities, or internally in charged clouds. The means by which the clouds become electrically charged to such a high voltage level is not fully understood and therefore at the present time cannot be prevented with any certainty, even in the most local of situations. Conventional equipment provides for a safe, or reasonably safe, discharge path directly to earth through a copper or similar high conductivity metallic element to a very low resistance ground terminal or mat. The element forming the discharge path, or lightning conductor as it is usually known, is normally secured to and, in all installations, projects above the highest point to be protected.For permanent installations the standard conventional type of solid stripped earth lightning conductor is usually more than adequate. However, in many military and civilian installations which are necessarily housed not in permanent buildings, but open to the elements, such conventional means of protection are, in the majority of cases, less effective than required.

According to the present invention, a laser beam is projected upwardly, preferably vertically, to provide an ionized path for the passage of an electrical charge, and passes through a massive, earth conducting metal ring. An electrical discharge down the ionized path is automatically diverted to the conducting metal ring, in preference to the longer, higher resistance path presented by the ionized beam so that the laser and its associated power source and control network are protected from damage. This enables the build-up of static electricity in vulnerable areas to be either discharged before their levels reach lightning strike proportions or provides an ionized path for a lightning flash to earth or other suitable point.The very low resistance ionized path to earth or similar ground point provided by the laser beam therefore reduces the voltage level and gradients to one lower than "leader-stroke" proportions.

The leader-stroke is the initial, relatively slow, probing electrical discharge from, say, a statically charged cloud to ground which creates a low resistance ionized path along which the main discharge can flow.

The path in a natural leader-stroke is not necessarily straight, but is dependent upon natural phenomena in the atmosphere such as temperature gradients and humidity levels. By replacing the unpredictable leader-stroke path by a straight ionized path provided by the laser beam, any lightning strike is controlled and kept away from any vulnerable points.

Preferably the laser beam is projected substantially horizontally and is then diverted upwardly by an angled mirror or mirrors before passing through the massive earthed conducting metal ring. It is not essential that a mirror should be used in this way.

Instead, the laser may be aimed directly upwardly through the earth ring. Although this simplifies the reflecting system it creates the problem of laser-to-ring alignment and creates difficulties in keeping the lens of the laser clean if it is mounted close to the ring. On the other hand, a mirror or mirrors can be cleaned much more easily by sluicing with water and wiping with an efficient wiper mechanism of the windscreen wiper type.

The invention will now be described in more detail, with reference to the accompanying drawings, in which: Figure 1 illustrates the protection afforded by a vertical lightnIng conductor mast; Figure 2 (a) shows a system in accordance with the invention in which a laser beam is projected substantially horizontally and then deflected upwardly through an earthed ring; and, Figure 2 (b) shows an alternative form of system in accordance with the invention in which the laser beam is projected directly upwardly.

Figure 1 shows a vertical lightning conductor mast 1 of height h. The maximum ground plane area protected by this mast is circular in shape and with the mast as its centre, is of a base radius r equal to the height h of the mast. Under average lightning conditions the protected volume is a vertical cone indicated by the dotted line 2, having the highest point of the mast 1 as its apex and the projected ground plane as its base. For permanent installations appropriate to buildings of normal geometry which house no complex electronic, explosive, flammable or sensitive mechanical equipment, this form of lightning conductor is usually more than adequate since there is no difficulty in fitting the building in within the protected volume.As already explained, however, the same does not apply to installations which are not housed in permanent buildings but are open to the elements, and Figures 2 (a) and 2 (b) show alternative forms of system in accordance with the invention for protecting this latter type of installation.

As shown in Figure 2 (a) a laser 10 is arranged to project an ionized beam 11 in a generally horizontal direction. The laser 10 has a power supply 12, a laser pump 13 and normal control equipment 14. The beam 11 is directed on to a deflecting mirror 16 mounted at approximately 450 so as to deflect the beam 11 upwardly in a substantially vertical direction as shown at 17.

The beam passes upwardly through an opening 18 in a conductive metal bracket 19, which thus defines a ring around the beam 17. The bracket 19 has a stud 20 for an earth connection capable of carrying the highest discharge likely to be expected.

The mirror 16 may be made, for example, of polished copper or other metal treated to give a highly reflective surface. The use of polished metal is preferable to glass since with the latter the reflecting surface is formed by the "silvering" on the back face and the beam has to pass twice through the thickness of the glass. Consequently the attenuation of the signal by the glass leads to an inefficient reflector system. The mirror 16 may be cleaned without difficulty by sluicing with water and then wiping with an efficient wiper of the windscreen wiper type. The bracket 19 may be made of copper, aluminium or other high conductivity metal and any electrical discharge down the beam 17 is automatically diverted to the bracket 19, thus passing directly to earth in preference to the longer, higher resistance path presented by the beam 11. This keeps the discharge away from the laser 10 and thus protects the laser and the associated equipment 12, 13 and 14 from damage.

The alternative system shown in Figure 2 (b) makes use of a similar bracket which is again shown as 19 and has an opening 18 and an earth connection stud 20 as described with reference to Figure 2 (a). The mirror 16 is not included, however, and the laser 10 is mounted in its place so as to project the beam 17 directly upwardly without the need for any reflection. Although this simplifies the optical system, it raises the problem of correct alignment between the laser and the ring 18 owing to the proximity of the laser 10 to the bracket 19 and the risk of moving the laser when cleaning it. From the point of view of operation, however, the tszo alternative systems are identical and the bracket 19 provides the same degree of protection for the laser and associated equipment in both types of system.

Although the ionized beam 17 is shown as being substantially vertical, this is not necessarily the case and in a static situation where there is only one object to be protected, the beam may be projected at an angle and in a position such that the cone of protection envelopes the object without subjecting the structure of the object to unnecessary ground effects. Where large areas are to be protected, a number of beams may be projected upwardly so that the individual cones of protection overlap to blanket the area. Beams projected at an angle to the vertical may be used in conjunction with rotating or oscillating mirror reflectors to form a complete shield above the area.

The projected height of the beam and its ionizing effect are for all practical purposes, presuming the beam is parallel, proportional to the laser power.

Conservation and maximum utilisation of power is achieved by pulsing the laser only when the forecast or detected atmospheric conditions are likely to cause static build-up and concentrations of lightning flash proportions. Megawatts of power for periods of 1 microsecond (1 joule) at one second intervals are considered adequate to establish and maintain a relatively staticfree zone, particularly when considering the sequential switching of multiple units incorporating angled scanning devices.

Although the primary function of a system in accordance with the invention is for discharging a static electrical charge which would otherwise produce lightning, other forms of static build-up may also be discharged such as dangerous static charges in the vicinity of milling operations, e.g. cocoa, flour and explosive powders or other operations involving similar particulate material which result in high fire-risk areas. The neutralisation of static electrical charges greatly reduces the explosive and fire potential of such areas.

Similarly electrically static free areas are desirable in the efficient manufacture of high gain, high input impedance semiconductor devices, high density semiconductor chips and integrated networks. Rejection rates can be appreciably high even after conventional elaborate, and often cumbersome methods and precautions are taken to neutralise the static charges. Laser beam ionized paths may be utilised to create numerous discharge or equalisation zones in such manufacturing areas. Protection by laser beam ionization may also be afforded against electromagnetic pulses caused by nuclear explosions. Ionizing beams, caging or surrounding the device or area to be protected will reduce the voltage gradient in and around the immediate vicinity of the beam to effectively screen the device or area and act similarly to a Faraday Shield.

Protection and restoration of signals from radio communication aerials (antennas) can be achieved by using the aerial as a mount for the ionizing static discharge system. The laser will itself not give rise to static interference as the site of intermittent charge coupling will be a considerable height (e.g. 100 feet) above the highest point of the aerial. In order to ensure that the signal attenuation is minimised when the laser is pulsed to create an ionized channel, the beam may be changed in conductive length by altering the power to a level which results in resonance at 1/4 wavelength.

At this point the impedance is theoretically infinite at the receiving frequency.

Claims (8)

1. A lightning conductor or device for discharging static electricity comprises a laser arranged to generate a beam projecting upwards to provide an ionized path for the passage of an electrical charge, and passing through a massive, earth conducting metal ring.

2. A lightning conductor or device for discharging static electricity according to claim 1, in which the laser is mounted and aimed directly upwards through the earth ring.

3. A lightning conductor or device for discharging static electricity according to claim 1, in which the laser is mounted substantially horizontally so that the laser beam is initially projected substantially horizontally and the conductor includes an angled mirror or mirrors to divert the beam upwards before passing through the massive earthed conducting metal ring.

4. A lightning conductor or device for discharging static electricity according to claim 3, in which the mirror includes a wiper mechanism of the windscreen wiper type.

5. A lightning conductor or device for discharging static electricity according to any of the preceding claims, in which the laser is a pulsed laser.

6. A lightning conductor or device for discharging static electricity according to claim 5, in which the laser pulses have a power of the order of a mega watt and a duration of the order of a microsecond and are repeated at intervals of substantially one second.

7. A lightning conductor or device for discharging static electricity in accordance with any of the preceding claims, including a number of laser beams projecting upwards to provide blanket protection over an area.

8. A lightning conductor or device for discharging static electricity substantially as described with reference to the accompanying drawings.