WO1998018186A1

WO1998018186A1 - An improved lightning downconductor

Info

Publication number: WO1998018186A1
Application number: PCT/AU1997/000696
Authority: WO
Inventors: Stephen John Gumley; John Richard Gumley; Brian J. C. Burrows
Original assignee: Erico Lightning Technologies Pty. Ltd.
Priority date: 1996-10-18
Filing date: 1997-10-16
Publication date: 1998-04-30
Also published as: AUPO307296A0; TW359840B

Abstract

A lightning downconductor (9) forming part of a lightning protection system, the downconductor (9) has an upper end connected to a lightning collector (10) through an upper termination device (11) and a lower end connected to an earthing system (15) through a lower termination (14). The downconductor (9) comprises an inner electrical conductor (3), an insulating layer (5) surrounding the inner electrical conductor (3), a resistive semi-conductive layer (6) surrounding the insulating layer (5) and a conductive layer (7) surrounding the resistive semi-conductive layer (6). A length of the conductive layer (7) is omitted or removed from the upper end of the downconductor (9) adjacent the upper termination device (11) to expose the resistive semi-conductive layer (6) as an outer layer of the downconductor (9).

Description

AN IMPROVED LIGHTNING DOWNCONDUCTOR

FIELD OF THE INVENTION:

This invention relates to downconductor apparatus for lightning protection, in particular for buildings and structures.

BACKGROUND OF THE INVENTION:

Direct strike lightning protection apparatus is generally well known and consists of a collector device or devices for attracting or receiving lightning (some types are called lightning rods), one or more down-conducting devices or systems for transporting the electrical charge to ground which may be part of the structure being protected or added to it, and an earthing system to ensure charge dissipation into the ground. This invention discloses a new concept for the downconducting part of the overall direct strike lightning protection system.

Lightning downconductors have traditionally been made of copper, aluminium, stainless steel, or other metallic tapes and strips, run down the side of buildings and structures. The downconductors connect lightning collectors or air termination systems to earthing systems. Alternatively, the structural metalwork inside buildings can be used as a downconductor system. Simple electrical cables with outer insulation have also been used. Hybrids of various combinations of tapes, strips, simple cables and structural steelwork are also used by some practitioners.

Each of these systems may allow electrification of the building or structure and the possibility of sideflashing or electrical discharge to other metallic parts of the building or structure or to power or data cables therein. Modern buildings often contain large amounts of sophisticated electronics which are adversely affected by stray lightning currents running through the fabric of such buildings and magnetically or capacitively inducing into electrical cables. An improvement to tape downconductors was alleged in US Patent 3919956 (Nov 18, 1975) wherein a biaxial format lightning downconductor was proposed which had a grounded outer conducting sheath separated from an inner conducting core by an insulating layer. The biaxial arrangement does not function in the classical manner of a radio type coaxial cable in that no return current can flow in the outer conductor as it is an open circuit where the downconductor connects to the collector device. Moreover it can be shown that the core conductor rises to very high voltage where lightning currents are injected. This makes very problematical and expensive the design of an upper high voltage termination apparatus connecting the lightning rod to the biaxial cable.

A further improvement was a triaxial format downconductor cable first manufactured in 1987 by Olex Cables, Melbourne Australia under agreement with designers, Lightning Protection International, Hobart Australia. This cable had a current carrying central core, a layer of high voltage insulation, a metallic sheath, another layer of insulation, a further metallic sheath, and then a semi-conductive layer of outermost material, designed to be attached to the side of the building or structure with metallic clamps. This cable better controlled the capacitive grading between conductive layers and reduced risks of sideflashing to other metallic parts of the building or to electric power or data cables. The semi-conductive outermost sheath was taken over the full length of the cable, to within about one metre of the collector.

Electrical stresses were highest at the top of the triaxial format cable where it connected to a lightning rod or equivalent lightning termination system. Specially designed upper cable termination assemblies were required to aid the entry of the lightning current into the top of the triaxial cable, in a way that minimised risks of electrical arcing or flashover at or near the top of the cable.

Even with such well-engineered terminations, high levels of electrical stress were evident during the lightning discharge at the upper termination with the lightning rods. This made the upper termination methods expensive and problematical, especially on very tall structures. Highest peak voltages occurred during the sharp risetime of the lightning strike. A further problem was that installation practice needed to be almost perfect because slight imperfections or incorrect installation caused local high electrical stress concentrations that could cause the failure of insulation in the triaxial format cable. Furthermore, the actual voltages reached were difficult to calculate or estimate, being highly dependent on detail of installation practice.

SUMMARY OF THE INVENTION: It is therefore desirable to provide a lightning downconductor in which the problem of electrical stress is alleviated.

It is also desirable to provide a lightning downconductor which does not require an expensive upper termination.

In one broad aspect, the present invention provides a lightning downconductor which is deliberately designed and manufactured as a very lossy transmission line and which uses a voltage grading system at its upper end which is to be connected via an upper termination to a lightning collector.

The downconductor preferably includes one or more electrical conductors, at least one insulating layer and one or more partially or semi-conductive layers between the electrical conductor or at least one of the electrical conductors and the insulating layer or layers of the downconductor to form the lossy transmission line and the voltage grading system preferably utilises a resistive, semi-conductive layer which is exposed as an outer layer at the upper end of the downconductor.

In accordance with another aspect of the invention there is provided a lightning downconductor having an upper end adapted for connection to a lightning collector and a lower end adapted for connection to ground comprising: an inner electrical conductor; an insulating layer surrounding the inner electrical conductor; a resistive, semi-conductive layer surrounding the insulating layer; and a conductive layer surrounding the resistive semi-conductive layer; wherein the conductive layer is omitted or removed from a length at said upper end of the downconductor to expose the resistive, semi-conductive layer as an outer layer of the downconductor so as to provide voltage grading along the length of the downconductor at said upper end.

In accordance with a further aspect of the invention, there is provided a lightning protection system comprising a lightning collector, an upper termination device connected to the lightning collector and a downconductor connected between the upper termination device and an earthing system, wherein the downconductor comprises an inner electrical conductor, an insulating layer surrounding the inner electrical conductor, a resistive semi-conductive layer surrounding the insulating layer, and a conductive layer surrounding the resistive semi-conductive layer, wherein a length of conductive layer is omitted or removed to expose the resistive semi-conductive layer as an outer layer of the downconductor at the upper end of the downconductor adjacent the upper termination device so as to provide voltage grading along the length of the downconductor at said upper end.

Preferably, the resistive, semi-conductive layer is exposed at the upper end of the downconductor for a length of at least about 0.1 metres, more preferably for a length of at least one metre, and up to a practical limit of around 5 metres. The length of exposed semi-conductive layer of the downconductor which is exposed may, however, vary from different applications and different sizes and heights of structures which require lightning protection.

In a preferred embodiment, the upper end of the downconductor which has the exposed semi-conductive layer and the upper termination device a nonconductive support for the lightning collector. The main inner electrical conductor may comprise a core of copper, aluminium or other electrically conductive, metallic material, but preferably the inner electrical conductor is of annular form surrounding a filler. The filler may include a strain component of high strength material, such as steel or kevlar, to provide mechanical strength to the downconductor for handling, transportation and installation. Preferably, the construction and cross-sectional area of the main inner electrical conductor are such that it can carry large lightning currents associated with lightning strikes whilst avoiding excessive ohmic heating.

Preferably, an at least partially electrically conductive layer is provided between the inner electrical conductor and the insulating layer in order to reduce or smooth local electrical stress concentrations in the main inner conductor which may result from imperfections or from strands of a braided conductor.

The insulating layer surrounding the inner electrical conductor (and any intermediate partially conductive layer) preferably has a high dielectric constant falling substantially in the range from about 2 to about 6, and a high dielectric strength to provide good insulation properties and performance.

The resistive, semi-conductive layer surrounding the insulating layer which is exposed at the upper end of the downconductor is an important feature of the present invention in that it provides voltage grading along the length of the downconductor at its upper end. For a given input current which is required to enter the downconductor from a lightning collector, a high voltage upper termination (connected between the main conductor and the resistive inner sheath) is exposed to a lower voltage with such a voltage grading system and is therefore under significantly less stress. Indeed, with a carefully designed downconductor in accordance with the invention, it is possible that the high voltage termination may require a third or less of the nominal high voltage hold-off voltage. An advantage of the upper termination being under significantly less electrical stress is that existing designs of terminations can be used with higher safety factors and more certainty, or alternatively a significantly cheaper upper termination can be designed and used.

The resistive, semi-conductive layer may be formed from a polymeric material, such as polyethylene, nylon or PVC, which has been impregnated with an electrically conductive material, for example carbon black. However, other types of resistive, semi-conductive materials may be used to form the resistive, semi- conductive layer. Preferably, the resistivity of the semi-conductive layer measured at 25 °C falls substantially within the range from 0.01 to 20.0 ohm metres, and more preferably from 0.1 to 0.5 ohm metres.

The conductive layer surrounding the resistive, semi-conductive layer may comprise the outermost layer of the downconductor except at the upper end of the downconductor where the resistive, semi-conductive layer is exposed. Alternatively, the conductive layer may itself be surrounded by an optional semi- conductive protective sheath. The conductive layer may be formed of copper, aluminium or other conductive metallic material. This conductive layer is preferably used to stabilise the surge impedance of the downconductor which is determined in conjunction with the other layers, particularly the inner electrical conductor and the insulating layer. In a preferred embodiment of a downconductor in accordance with the present invention, the surge impedance may be in the range from about 3 to about 20 ohms, resulting in minimised voltages when the downconductor carries large lightning currents.

BRIEF DESCRIPTION OF THE DRAWINGS:

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic view of one possible layup of a downconductor in accordance with the invention; Figure 2 shows the layers of the downconductor of Figure 1 in cross sectional view;

Figure 3 is a schematic diagram of a lightning protection system incorporating the downconductor of Figure 1 installed on a typical structure;

Figure 4 is a schematic diagram of a lightning protection system similar to that of Figure 3;

Figure 5 is a schematic diagram of a modified lightning protection system; and

Figure 6 is a schematic diagram of another lightning protection system which may incorporate a downconductor in accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS:

The downconductor (9) shown in Figures 1 and 2 comprises several layers (1) to (8), including an optional strain component (1) used for mechanical strength when lifting or handling, for example of kevlar or steel or similar high strength material, surrounded by an electrical inert filler material (2). The precise form of the optional strain component is not critical to this invention and various types of components are well known in the art. For example, it may be centrally located or comprise an annular ring with filler either side, or a series of wires or similar.

The purpose of the filler is to correctly position and dimension the main current carrying conductor (3) at an appropriate diameter and to have low inductance, as is well known in the art. For reasons of cost, weight, and flexibility, the filler is generally a type of non conductive plastic, but a conductive material would not cause the principles of this invention to be voided.

The main electrical conductor (3) is manufactured from copper or aluminium or other metallic conductor material. The dimensions and cross sectional area are important design parameters. In particular the outer dimension strongly affects the inductance per unit length of the downconductor, and the maximum electric stress occurring on its surface. The cross sectional area affects the ohmic heating caused by passing the large currents associated with a lightning strike. The exact layup of main conductor (3) is not important, for example it may be stranded, helical, braided, or tubular or some other form in construction.

One embodiment of the invention has the outer diameter of the main conductor (3) being about 18mm and the cross sectional area of copper forming the conductor being about 30mm², this being sufficient metallic material to carry the entire lightning current of large strikes without excessive ohmic heating as determined by the action integral or specific energy of the lightning waveshape, for examples to values published in international standards like IEC 1024-1 or IEC1312-1 on lightning protection. In other embodiments, cross-sectional areas of from about 30 mm² to about 75 mm² may be used to comply with standards requirements in different countries.

Outside the main current carrying conductor may be positioned an optional thin layer of conductive or partially conductive wrapping (4) which reduces local stress concentrations of individual strands or imperfections of the main conductive layer (3). One embodiment has this layer being a 0.2mm thick semi-conductive plastic material, however other thickness and materials are equally possible to achieve the purpose of stress smoothing.

The main insulating layer (5) has a high dielectric constant in the range from about 2 to about 6 to give low characteristic impedance of the downconductor. Typically this layer will be from about 1mm to about 25mm thick. One preferred embodiment uses a 1.5mm thick layer of low density polyethylene with a permittivity of approximately 2.8.

A functional layer (6), called the "inner sheath", is an important design feature and is comprised of partly conducting polyethylene, nylon or PVC or other material having a resistivity measured at 25 degrees Celsius typically in range from 0.01 to 20.0 ohm metres, preferably 0.1 to 0.5 ohm metres, which when combined with its thickness gives a resistance per metre length of downconductor of from about 100 to about 100,000 ohms, or more preferably from about 3,000 to about 100,000 ohms. A person skilled in the art may optimise the thickness to achieve such a lineal resistance per metre of downconductor using materials with a resistivity chosen from the range suggested herein. One preferred embodiment has a thickness of 0.125mm with a lineal resistance per metre of about 28,000 ohms and is made of nylon material impregnated with a high percentage, typically over 5%, of carbon black.

Next is a thin copper, aluminium or other metallic or other highly conductive material layer (7) that acts to determine in conjunction with other layers, particularly layers (3) and (5) the surge impedance of the overall downconductor. Layer (7) stabilises the surge impedance of the downconductor. It is not designed to be a return conductor according to coaxial cable theory and practice, however, due to capacitive and other leakage through the various layers, some small return currents may flow, especially near the ground. It typically has a cross-sectional area of 0.5 to 20mm², preferably 3 to 10mm². One preferred embodiment has a 5mm² cross-sectional area and is made of copper strands wound in a long helix.

The conductive layer (7) only carries significant current during the risetime of a typical lightning strike. During the longer decay time of a typical lightning strike when most ohmic heating occurs, the rate of current change with time is small, so there is very little current flow, thereby allowing the layer to have smaller cross section than the main current carrying layer (3). Layer (7) may be tubular, stranded, braid, foil, tape, or other construction, or alternatively may be formed from one or more conductive wires running longitudinally along the downconductor or wound helically. Layer (8) is an optional overall weather and mechanical protective sheath whose dimensions are chosen principally for mechanical properties and cost. It may be the same material as layer (6) for economy in manufacturing practice, or some other material, but must be semi-conductive in nature, with a resistivity measured at 25 degrees Celsius typically in the range from about 0.01 to about 20 ohm metres. Typically this layer is from about 0.1 to about 20mm thick, preferably from about 1.0 to about 5.0mm thick. A preferred embodiment uses a 1.5mm thickness of a material such as Union Carbide DHDA-7707 Black 55 semi- conductive shielding compound. The outermost layer would normally, but not essentially, be solar ultraviolet light resistive because of the outdoor installation of the downconductor in many practical situations.

Other wrappings and layers may be optionally added for reasons of manufacturing practice or utility provided they do not affect the overall function or principle of the downconductor as described herein.

Figure 3 illustrates use on a typical structure (13) showing how the downconductor (9) fits into an overall lightning protection system comprising a lightning collector of direct strike capture apparatus (10), mechanical support apparatus (12), upper termination apparatus (11) for ensuring the current leaves the collection apparatus and enters the downconductor, an earthing system (15) which is installed in the ground (16), and a lower termination (14) between downconductor (9) and earthing system (15).

Figure 4 similarly illustrates the system, but also shows that the downconductor (9) need not be straight, but can be bent to fit natural features on the structure (13). The ability to route the lightning discharge safely to ground around and away from critical electronic apparatus in a building or tower is an important advantage of this invention. An important feature of the application of the downconductor (9) in a lightning protection system is that at the top nearest the air terminal or lightning apparatus (10), the strike collection system, layers (7) and (8) are omitted, stripped away or otherwise removed so that functional resistive, semi-conductive layer (6) is exposed. The length of said layer removal is typically at least 0.1 metres, up to a practical limit of around 5 metres. This layer removal is conducive to effective voltage grading (reducing it) along the length of the downconductor and across the insulation layer (5) and semi-conductive layer (6).

In some cases, the downconductor (9) may be installed inside a non-conductive cylinder or support mast (12) that supports a lightning rod above the top of a structure for at least the length where such layer removal has occurred. Figure 5 illustrates such a system in which the upper part of the downconductor (9) and the upper termination (11) are installed inside the support mast (12).

Figure 6 shows a modified application whereby a downconductor (19) is used to connect a lightning rod (17) via an upper termination (18) to the structural steel (20) or reinforcing mesh in a building, routing the current away from important electrical equipment (21) and people (22). Human safety is enhanced and the electrical equipment has reduced the risk of being damaged by sideflashes.

In most cases, the termination between the lightning collection system and the downconductor is through a high voltage upper termination (11). This grades the voltages and ensures practically all the current enters the downconductor. The upper termination (11) will be designed to fit the actual dimensions of the embodiment chosen. However the novel feature of voltage grading caused by the design and dimensioning and material selection of the downconductor enables standard industry high voltage termination kits or materials to be considered, as for example manufactured by the 3M company. By way of example, another important advantage can be illustrated by the embodiment dimensioned herein. A downconductor with electrically conductive layer (3) made of electrical grade copper and having an outer diameter of 18mm, an insulation layer (5) made of low density polyethylene material with dielectric constant approximately 2.8 and outer diameter 20mm, a resistive, semi-conductive functional layer (6) made of nylon tape impregnated with carbon black with a thickness of 0.125 mm, a conductive layer (7) being copper tape with a thickness of 0.1mm and an outer layer (8) of semi-conductive polyethylene material of thickness 1.5mm has a surge impedance of the order of 7 ohms. This is a very low figure which means voltages are minimised when the downconductor carries large lightning currents.

Another feature of this embodiment is that after about the first five metres (the length over which layer (6) is exposed) , the surge impedance is mostly independent of length of downconductor, thereby enabling the downconductor to be used on very tall buildings and structures without additional risk of flashover.

During the risetime of the lightning current pulse, the voltage grading formed by the exposed length of layer (6) reduces the stress across the insulation layer (5) by a factor of 4 or more, to give an apparent impedance under rapid risetime conditions of less than 2.8 ohms, measured between items (3) and (6) at the top. The high voltage termination being connected HV to (3) and "ground" side to (6). Thus the voltage withstand requirement of the HV termination is minimised. Such low impedances are unknown in the prior art.

It is well known that currents injected into unmatched terminated cables can cause voltage oscillations as travelling voltages waves firstly reverse polarity then reinforce each other. A novel feature of this invention is that the construction and application of the downconductor as an excessively lossy or leaky transmission line ensures that such voltage oscillations induced by the rapid risetime of the input current pulse are fully damped or nearly so. Furthermore, during the fall or decay time of lightning, by nature of damping effect of the resistive, semi-conductive functional layer (6), transient oscillations occurring as result of fast transient excitations are eliminated so only minimal voltages occur in the downconductor during this relatively slow decay time.

The main differences between the embodiment (9) of this invention described above and the triaxial format downconductor used since 1987 are that:

(i) the triaxial format downconductor had an additional layer of metallic conductor between the insulating layer corresponding to (5) and the semi- conductive layer corresponding to layer (6), and

(ii) the outer semi-conductive layer was a low dielectric constant high dielectric strength insulator.

Thus, the triaxial format downconductor did not have a damping or lossy line system, nor did it have voltage grading for the HV termination. It would have to withstand the full effect of the surge impedance (around 18.5 ohms in the practical embodiment used since 1987) and provided no damping of the line oscillations.

The present invention dispenses with a conducting layer between layers (5) and (6), and very significantly, layer (6) is the functional high resistance layer giving a lossy transmission line characteristic along the whole length of the downconductor, and especially at the top region over which the layers (7) and (8) have been removed, the layer (6) gives a very high loss and also gives voltage grading for the purpose of minimising the voltage difference to which the HV termination is subjected.

USE OF THE INVENTION:

The downconductor can be used in conjunction with almost all practically direct strike lightning collection devices. These include patented and proprietary air terminals, Franklin rods whether sharp pointed or rounded, traditional ornamental shapes and weather vanes, horizontal meshes, metallic structural elements or components, and metallic building features like structural reinforcing, structural beams and columns, flagpoles and handrails. In addition to buildings, the downconductor is also applicable to free standing structures such as towers, and moving structures or vehicles such as cranes, ships, trucks and the like.

The principal purpose of the downconductor is to bring current to a safe place by a safe route for ultimate injection to earth. It may be lower terminated in other metallic structural elements for conductive reinforcing or it may go all the way to ground. It does not have to be installed in a straight vertical run, and provided mechanical bending radius limitations are met, it can be routed indirectly.

This ability choose the route is a key application and advantage for using the downconductor because it limits or reduces or minimises chances of flashover or sideflashes to the building components. The downconductor can be routed away from the electrical cables and equipment thereby reducing the lightning currents that would otherwise be induced into sensitive components and reducing magnetic fields in regions where it is desired to protect electronic components. Alternatively, the route can be chosen to minimise flashover risk to people, for example, away from high traffic areas.

A further novel use of the principle of where calculations based on international standards show it may be a difficult, expensive, or impossible situation to prevent sideflashes or metallic items inside buildings, a modified version of this downconductor system will offer significant advantages. In such cases, layers (7) and (8) would be stripped back for many metres and there would be exposed the high resistance conductive layer (6). The combination of stress grading longitudinally and through the remaining layers and the damping of high frequency components will reduce voltages and risk of flashover. Since modifications within the spirit and the scope of the invention may be readily effected by persons skilled in the art, it is to be understood that the invention is not limited to the particular embodiments described, by way of example, hereinabove.

Claims

1. A lightning downconductor having an upper end adapted for connection to a lightning collector and a lower end adapted for connection to ground comprising: an inner electrical conductor; an insulating layer surrounding the inner electrical conductor; a resistive, semi-conductive layer surrounding the insulating layer; and a conductive layer surrounding the resistive, semi-conductive layer, wherein the conductive layer is omitted or removed from a length at said upper end of the downconductor to expose the resistive, semi-conductive layer as an outer layer of the downconductor so as to provide voltage grading along the length of the downconductor at said upper end.

2. A lightning downconductor according to claim 1 wherein the resistive, semi-conductive layer is formed from a polymeric material impregnated with an electrically conductive material.

3. A lighting downconductor according to claim 2 wherein the polymeric material comprises polyethylene, nylon or poly vinyl chloride (PVC).

4. A lightning downconductor according to claim 2 or claim 3 wherein the polymeric material is impregnated with carbon black.

5. A lightning downconductor according to any one of claims 1 to 4 wherein the resistive, semi-conductive layer has a resistivity falling substantially within the range from about 0.01 ohm metre to about 20 ohm metres when measured at 25°C.

6. A lightning downconductor according to claim 5 wherein the resistivity of the resistive, semi-conductive layer falls, substantially within the range from about 0.1 ohm metres to about 0.5 ohm metres when measured at 25°C.

7. A lightning conductor according to any one of the preceding claims wherein the resistive, semi-conductive layer has a resistance falling substantially within the range from about 100 ohms per metre to about 100,000 ohms per metre.

8. A lightning conductor according to claim 7 wherein the resistive, semi-conductive layer has a resistance falling substantially within the range from about 3,000 ohms per metre to about 100,000 ohms per meter

9. A lightning conductor according to claim 8 wherein the resistive, semi-conductive layer has a resistance per metre of about 28,000 ohms.

10. A lightning conductor according to any one of the preceding claims wherein the resistive, semi-conductive layer is exposed at said upper end for a length of the downconductor of at least 0.1 metres.

11. A lightning downconductor according to claim 10 wherein the resistive, semi-conductive layer is exposed at said upper end for a length of up to about five metres.

12. A lightning downconductor according to any one of the preceding claims wherein the inner electrical conductor is of annular form surrounding a filler.

13. A lightning downconductor according to claim 12 wherein the filler includes a strain component of high strength material.

14. A lightning downconductor according to claim 13 wherein the strain component is formed from steel or kevlar.

15. A lightning downconductor according to any one of the preceding claims wherein the inner conductor is formed from copper or aluminium.

16. A lightning downconductor according to any one of the preceding claims further including an at least partially conductive layer provided between the inner conductor and said surrounding insulating layer to reduce or smooth stress concentrations in said inner conductor.

17. A lightning downconductor according to claim 16 wherein said at least partially conductive layer comprises a layer of semi-conductive plastic material.

18. A lightning downconductor according to any one of the preceding claims wherein the insulating layer has a high dielectric constant.

19. A lightning downconductor accordingly to claim 18 wherein the insulating layer has a dielectric constant of at least 2.0.

20. A lightning downconductor according to claim 19 wherein the insulating layer has a dielectric constant of up to about 6.0.

21. A lightning downconductor according to any one of claims 18 to 20 wherein the insulating layer has a thickness falling substantially within the range from about 1mm to about 25mm.

22. A lightning conductor according to any one of the preceding claims wherein the conductive layer surrounding the resistive semi-conductive layer is formed from a metallic material or other highly conductive material and stabilises the surge impedance of the downconductor.

23. A lightning downconductor according to claim 22 wherein the conductive layer has a cross-sectional area falling substantially within the range from about 0.5mm³ to 20mm³.

24. A lightning downconductor according to claim 23 wherein the cross- sectional area of the conductive layer falls substantially with the range from 3 to

10mm³.

25. A lightning down conductor according to any one of claims 22 to 24 wherein the conductive layer is formed from copper strands wound in a helix around the insulating layer.

26. A lightning downconductor according to any one of the preceding claims wherein the conductive layer is surrounded by a protective sheath.

27. A lightning downconductor according to claim 26 wherein the protective sheath is formed from the same material as the insulating layer.

28. A lightning protection system comprising a lightning collector, an upper termination device connected to the lightning collector and to the upper end of a downconductor in accordance with any one of the preceding claims, and an earthing system connected to the lower end of the downconductor.

29. A lightning protection system comprising a lightning collector, an upper termination device connected to the lightning collector and a downconductor connected between the upper termination device and an earthing system, wherein the downconductor comprises an inner electrical conductor, an insulating layer surrounding the inner electrical conductor, a resistive semi-conductive layer surrounding the insulating layer, and a conductive layer surrounding the resistive semi-conductive layer, wherein a length of conductive layer is omitted or removed to expose the resistive semi-conductive layer as an outer layer of the downconductor at the upper end of the downconductor adjacent the upper termination device so as to provide voltage grading along the length of the downconductor at said upper end.

30. A lighting protection system according to claim 28 or claim 29 wherein the resistive semi-conductive layer is exposed for a length of at least about 0.1 metres.

31. A lightning protection system according to claim 30 wherein the resistive, semi-conductive layer is exposed for a length of up to about 5 metres.

32. A lightning protection system according to any one of claims 28 to 31 wherein the exposed resistive, semi-conductive upper and of the downconductor and the upper termination device are provided inside a non-conductive support for the lightning collector.