Lightning Retardant Cable

This invention relates to electrical cable. More particularly, it relates to electrical cable which retards lightning so that the cable is not substantially affected by the lightning and, in the case of communication cable, the communication signal on a signal conductor within the cable is not substantially affected, as well as its associated equipment.

While this invention is applicable to both power and communication cable, most of the detailed discussion herein will focus on communication cable used in conjunction with an antenna.

As used herein, the term antenna includes television and radio antenna, satellite dishes and other devices which receive electromagnetic signals. A major problem associated with an antenna is caused by lightning striking the antenna. Often the high current associated with the lightning will travel through the communication cable which is attached between the antenna and electronic equipment. This current will damage the electronic equipment.

According to The Lightning Book, by Peter E. Viemeister, self-induction in a conductor may occur during a lightning strike. This occurs because lightning currents may rise at a rate of about 15,000 amperes in a millionth of a second. For a straight conductor with the usual cross section, this surging current can produce nearly 6,000 volts per foot of wire, which is enough to jump an insulated gap to a nearby conductor, such as the center conductor, in a coaxial cable.

Currently lightning protection of cable is more focused on the installation of cable within a system. The National Electric Code attempts to insure a proper path for lightning to discharge, thus reducing the damage of equipment connected to the end of the cable. The cable in and of itself offers little or no protection from electric fields or magnetic fields associated with the lightning strike. Even though electrical codes provide suggestions on installing and grounding equipment, their primary focus is providing a straight path to ground for lightning to discharge and eliminating the differences of potential between the two items.

FIG. 1 is an example of a home TV antenna installation according to the National Electric Code. If lightning were to strike antenna 10, half of the charge would be on ground wire 12 which is attached to the mast 14 of the antenna, and the other half would be on the coaxial cable's outer shield 16 which is connected to the antenna terminals 18. Theoretically, the current on coaxial cable 16 would travel to antenna discharging unit 20 and then through grounding conductor 22. The center conductor or signal conductor of the coaxial cable, however, is unprotected, which means that damage to the electronics in the receiver and other components within the home is likely. Furthermore, the longer the lead-in wire, the greater the problem. As lightning strikes this antenna 10 and discharges to ground, a large electric field is set up along the coaxial lead-in wire 16 and ground wire 12. At right angles to this electric field is an exceptionally strong magnetic field which surrounds all of the cable.

In addition, lightning follows the straightest, closest and best path to ground. Any sharp bends, twists or turns of the ground wire sets up resistance to the quick discharge. See Page 201 of The Lightning Book, referred to above. This resistance usually causes the discharge to jump off the ground wire with the bend and into a path of least resistance.

It is one object of this invention to provide an improved lightning retardant cable.

It is another object to provide a lightning retardant cable which deals with both electric and magnetic fields caused by lightning.

In accordance with one form of this invention there is provided a lightning retardant cable which includes at least one internal conductor. The internal conductor may be a signal conductor or a power conductor. A signal conductor conducts a signal containing information. A power conductor conducts current for operating devices and equipment.

A choke conductor is provided. The choke conductor is wound about the internal conductor in the shape of a spiral. The choke conductor is not in contact with the internal conductor. The choke conductor presents a high impedance to the electrical current caused by lightning when the lightning strikes near the cable.

Preferably, the internal conductor is made of metal for conducting electrical signals or current, although the internal conductor may be an optical fiber.

It is also preferred that a spiraled shield be placed underneath the choke conductor. The spiraled shield is also wound about the internal conductor, but in an opposite direction to the choke conductor. The adjacent windings of the shield are not in electrical contact with one another and act as another choke. Preferably, 90.degree. angles are formed at the crossing points between the choke conductor and the shield.

The choke conductor dissipates the electric field caused by the lightning strike. The shield performs two functions. It acts as a choke in the opposite direction of the choke conductor and thus enhancing the cancellation process and it acts as a Faraday Cage to greatly reduce the associated magnetic field.

It is also preferred that one side of the shield be insulated so that when the shield is wound about the cable a winding is not in electrical contact with the previous or next winding. This forms a choke shield.

It is also preferred that an overall outer jacket be provided for the cable and that a ground conductor be attached to the outer jacket.

The subject matter which is regarded as the invention is set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof may be better understood in reference to the accompanying drawings in which:

FIG. 1 is a simplified electrical diagram showing a prior art antenna signal transmission and grounding system;

FIG. 2 is a simplified electrical diagram showing the antenna signal transmission and grounding system of the subject invention;

FIG. 3 is also a simplified electrical diagram showing the antenna signal transmission and grounding system of the subject invention;

FIG. 4 is a side elevational view of the lightning retardant cable of the subject invention;

FIG. 5 is a side elevational view of an alternative embodiment of the lightning retardant cable of the subject invention;

FIG. 6 is a side elevation view of another alternative embodiment of the lightning retardant cable of the subject invention;

FIG. 7 is a side elevational view of yet another alternative embodiment of the lightning retardant cable of the subject invention;

FIG. 8 is a cross sectional view of the spiraled shield of FIGS. 5, 6 and 7;

FIG. 9 is a side elevational view of another alternative embodiment of the lightning retardant cable of the subject invention for a power application .

Referring now more particularly to FIG. 3 which relates to an embodiment of the invention where the lightning retardant cable is a communication cable, there is provided antenna signal transmission and grounding system 24 for grounding antenna 10. As previously indicated, antenna 10 may also be a satellite dish or another device for receiving signals from the air. System 24 includes lightning retardant cable 26, which is the cable of the subject invention and will be described in more detail below. Lightning retardant cable 26 is attached to antenna 10 at connector lead box 28. Cable 26 is also connected to standard antenna discharge unit 30. A typical antenna discharge unit 30 is a Tru Spec commercially available from C Z Labs. A coaxial cable 32 is connected to the discharge unit 30 and to electronic equipment (not shown).

A ground wire 34 connects the antenna discharge unit 30 to ground clamps 36 and 38. Ground clamp 38 is, in turn, connected to ground rod 39. In addition, the antenna mast 40 is connected to ground clamp 38 through ground wire 42.

FIG. 2 is similar to FIG. 3, but illustrates some of the details of cable 26. In the communication cable embodiment of this invention, cable 26 is preferably a coaxial cable, although, cable 26 could be a fiber optic cable or twin lead cable. A communication cable must include at least one signal conductor. In the preferred communication cable embodiment of this invention, however, cable 26 is a coaxial cable. FIG. 2 illustrates the center conductor 44. Center conductor 44 is the signal conductor and is connected to terminal box 46 attached to the mast of the antenna 10. Signal conductor 44 is connected through antenna discharge unit 30 to coaxial cable 32. Spiraled choke conductor 56 surrounds signal conductor 44 and is connected to antenna discharge unit 30 which, in turn, is connected to ground conductor 34. Cable 26 will be discussed in more detail below.

FIG. 4 shows lightning retardant cable 26 having signal center conductor 44 which is surrounded by foam dielectric 50. A standard coaxial cable shield 52 surrounds the dielectric 50. Insulated jacket 54 surrounds shield 52. A choke conductor 56 is wound about outer jacket 54 in a spiraled fashion. An overall outer insulated jacket may be placed over the cable to provide protection for the cable. The choke conductor 56 should be large enough to handle the high currents caused by lightning without melting. Choke conductor 56 should be at least 17 gauge and preferably is 10 gauge. Preferably the choke conductor is made of copper. If the choke conductor is made of a bundle of round copper wires, the bundle should be equivalent to at least 17 gauge wire or larger.

Referring now to FIG. 2, if lightning strikes antenna 10, the energy of that strike would normally be split, that is, one-half would follow ground wire 42 and the other half would follow cable 26 to ground rod 39. However, since cable 26 forms an electrical choke due to spiraled choke conductor 56, that is, conductor 56 actually chokes out the flow of current due to its high impedance to lightning current which has a very fast rise time, the majority of the surge follows ground wire 42 to ground and does not follow cable 26 to ground. One-half of the energy from the strike that would start down cable 26 after a lightning strike would quickly be cancelled out by the action of the choke. Each time the choke conductor 56 is twisted around the cable, it causes the electric field generated by the lightning to interact upon itself, thus blocking the flow of current.

As with any electrical discharge, there is an electric field, as well as a magnetic field at right angles to the electric field. Lightning causes a tremendously large magnetic field due to the huge discharge of electric current. FIG. 5 shows an alternative embodiment of the lightning retardant cable of the subject invention which includes a special shield to block the magnetic component of the lightning discharge, thus acting as a Faraday Cage.

In FIG. 5 there is provided a center signal conductor 44, dielectric 50, standard coaxial cable shield 52 and coaxial cable jacket 54. A substantially flat spiraled wrapped shield 58 is wound over the top of coaxial cable jacket 54.

As shown by a cross section of the spiraled shield 58 in FIG. 8, the shield includes a conductive top metal portion 60 which is insulated by plastic insulation 62 on the bottom. Thus the shield may be spiraled upon itself without causing an electrical short. Metal portion 60 of shield 58 is preferably made of aluminum or copper. Shield 58 is commercially available.

Choke conductor 56 is spiraled over the top of shield 58 in the opposite direction to the spiral of shield 58. Preferably, both shield 58 and choke conductor 56 are spiraled at 45.degree. angles with respect to signal conductor 44. Thus the shield and the choke conductor cross at 90.degree. angles. Alternatively, the spirals for both the choke conductor and the shield could be adjusted to various angles to maximize inductance depending on the desired effect.

In the embodiment of FIG. 5, choke conductor 56 is in electrical contact with the metallic portion 60 of shield 58. However, in the embodiment of FIG. 6, an insulated jacket 64 is provided between spiraled shield 58 and choke conductor 56 and a small drain wire 61 is placed in contact with shield 58 between shield 58 and jacket 64. The drain wire 61 enables one to conveniently terminate the shield. In the design shown in FIGS. 5 through 8, both electric and magnetic fields are addressed. The electric field is addressed by the spiraled choke conductor 56 which, as indicated above, functions as an electrical choke. The magnetic field is addressed by the spiraled shield 58, which acts as a Faraday Cage. Also, the spiraled shield acts as a flat choke in the opposite direction of the spiraled electrical choke 56, thus enhancing the cancellation effect. Therefore, shield 58 has two functions.

As indicated above, preferably, the shield 58 is preferably at a 45.degree. angle with respect to center transmission signal conductor 44 and is spiraled in counterclockwise wrap. The choke conductor 56 is preferably also at a 45.degree. angle with respect to center conductor 44, but is spiraled in the opposite direction around the shield 58, i.e., clockwise. The directions in which the choke conductor and signal conductor are wound could be reversed. The result is a 90.degree. angle between the magnetic shield and the electric choke.

Referring now more particularly to FIG. 7, for ease of installation, a ground wire 66 may be made as a component of the cable 24. Ground wire 66 is attached to the outer jacket 65 of the cable and is embedded in plastic which forms part of the extruded jacket 65. The ground wire 66 runs the length of the cable. The ground wire is set apart from the main cable so that it may easily be detached and attached to a grounding rod.

The cable shown in FIG. 5 has been tested in the laboratory and in the field. The results show a substantial improvement over the prior art.

The detailed description above primarily discusses communication cable applications of the invention. FIG. 9 shows a lightning retardant cable 19 of the subject invention for power applications. Internal conductor 70 and 72 are power conducts which are normally heavier gauge than communication conductions. Often a gravel conductor (not shown) is placed adjacent to the power conductors. Conductors 70 and 72 are covered by insulated jacket 74. Choke conductor 56 is spiraled about jacket 74 in the same fashion as shown and described in reference to FIG. 4. In addition, the shield arrangement shown in FIGS. 5, 6 and 7 may also be used in power cable applications.

From the foregoing description of the preferred embodiments of the invention, it will be apparent that many modifications may be made therein. It will be understood, however, that the embodiments of the invention are exemplifications of the invention only and that the invention is not limited thereto. It is to be understood therefore that it is intended in the appended claims to cover all modifications as fall within the true spirit and scope of the invention.