Electrostatic Hand Gun Cable

Abstract

An improved high voltage cable for usein electrostatic hand guns and includes a semiconductive stranded fiber glass core within a semiconductive shield surrounded by body of insulative material, a flexible conductive sheath in an outer protective covering; the semiconductive core and semiconductive shield provide resistance of not less than 20,000 and not more than 300,000 ohms per foot of cable length.

Description

The present invention related to a high voltage cable for use in connecting electrostatic spray coating devices with a separate source of high voltage.

Electrostatic coating systems frequently use voltages on the order of 20,000-100,000 volts of direct current. Voltage of this magnitude places great stress upon electrical insulation that is used to isolate it. Being exposed to such stress, high voltage insulation can fail permitting electrical discharge of the high voltage through the body of the insulation or across its surface. In addition, such high voltages can break down surrounding air and initiate a spark that can travel through air to adjacent bodies at lower potential. Electrical isolation of such high voltages is therefore a problem.

Electrostatic coating systems are frequently used with coating materials that have a flammable residue or that produce flammable vapors. Spraying operations are most frequently conducted in a spray booth to confine such flammable materials and to remove them safely from the interior of an industrial facility. The sprayers of electrostatic coating systems are thus most frequently used inside a spray booth. The high voltage source, however, is generally located outside of and at least 20 feet from the spraying area. The electrostatic charge to be applied to the spray particles must be carried from the high voltage source to the electrostatic spray device over a distance on the order of 20 feet.

Since the invention of U.S. Pat. No. 3,048,498, hand manipulated electrostatic spray guns have been commercially possible. Commercial embodiments of this invention have included electrostatic hand guns having insulating barrels containing resistors with resistances on the order of 40-1000 megohms to prevent objectionable electrical discharges from the spray gun. Such electrostatic hand guns were connected by a high voltage cable to the high voltage source outside the spraying area. To permit the electrostatic hand gun to be easily manipulated by the operator so that he could direct the spray at all surfaces of complexly-shaped articles, the high voltage cable had to be flexible. The achievement of flexibility, however, was not always consistent with the requirement that the electrical insulation of the high voltage cable withstand the tendency of high voltage to cause insulation failure.

In such commercial electrostatic hand gun systems, coaxial high voltage cables were used. The high voltage conductor was a metallic wire at the center of the high voltage cable. Surrounding the wire was a body of insulating material, generally a polyethylene. Surrounding the polyethylene insulation was the grounded metallic braid to act as a barrier to any electric discharge which might occur in the event of an insulation failure of the dielectric material, and to prevent such an electric discharge from occurring to personnel who may be using the electrostatic hand gun. For convenience, a protective covering of rubber or plastic was provided as the outermost covering of the cable.

Many attempts were made to improve upon such high voltage cables. Several attempts were made to incorporate into the high voltage conductor at the center of a coaxial high voltage cable, the entire resistance that was otherwise used within the barrel of the electrostatic hand gun. In essence, this approach was an attempt to stretch the high resistance from within the barrel throughout the length of the cable extending to high voltage source. In addition to eliminating a high voltage connection between the resistor and the high voltage cable within the electrostatic spray gun itself, this approach had the advantage of limiting the energy available in any electric discharge occurring in the high voltage cable because of the breakdown of the surrounding dielectric material. British Patent Specification 1,051,184 and U.S. Pat. No. 3,167,255 both disclose such an approach. Because of the technical difficulties in manufacturing such a cable, however, it never became a commercial reality. In addition, high voltage cables for electrostatic systems were manufactured having a central conductor made of a plurality of short but rigid high resistance segments that extended throughout the length of the cable (see U.S. Pat. No. 3,348,186). Cables of this latter type thus included many high voltage electrical connections at the junctions of the short high resistance segments. Furthermore, such cables included necessarily, a plurality of high voltage conductor surface discontinuities at these connections at which nonuniformly high potential gradients could occur. This plurality of connections of surface discontinuities was undesirable and accentuated the tendency of high voltage to break down the electrical insulation of the cable.

Our invention is an electrical cable used to connect an electrostatic hand spray gun to a separate high voltage source. Such a cable is generally on the order of at least 20 feet, although somewhat longer and shorter lengths of cable may be used. It includes a high voltage conductor which is smooth throughout its length and has a uniform distributed resistance of at least 0.5 to about 7.5 megohms equally distributed along its length. Preferably, the cable is thirty feet in length and has a total resistance of five megohms, or about 167,000 ohms per foot of cable length. The cable is small, having an overall diameter of 7/16 of an inch. This cable solves the difficulties of providing a cable that can be reliably manufactured for commercial use and provides the flexibility needed for manipulatable hand guns, with the ability to withstand the tendency of high voltage to break down its electrical insulation.

FIG. 1 is a diagrammatic sketch of an electrostatic hand gun system including such a cable.

FIG. 2 is a cross-sectional view of the cable.

FIG. 1 shows an electrostatic hand gun in use in a spraying area. Electrostatic hand gun 10 is connected to the high voltage source 11 by 25 feet of high voltage cable 12. One such electrostatic hand gun includes within its barrel portion a high voltage resistor 13 having a resistance on the order of 160 megohms. The high voltage for such a hand gun produces 60 kilovolts of direct current.

The high voltage cable 12 which carries electrostatic charge from the high voltage source to the hand gun as shown in FIG. 2, includes as the high voltage conductor 14 stranded conductive fiber glass. The central conductor is comprised of many fibers each having a very small diameter. When stranded, they produce a central high voltage conductor having a diameter of 0.015 inches. This stranded fiber glass conductor is treated to provide a resistance on the order of 20,000 to 300,000 ohms per foot. Immediately surrounding the stranded fiber glass conductor is a corona shield 15 of conductive polyethylene, consisting of a homogeneous mixture of polyethylene with a small percentage of carbon black. The conductive polyethylene minimizes variations of potential gradient which might occur at the surface of the high voltage conductor because of gas trapped during the manufacturing process or surface discontinuities. At the boundary between the corona shield and the surrounding insulation, the corona shield is typically 0.050 inches in diameter. The combined resistance of the semiconductive stranded fiber glass conductor and the semiconductive corona shield should preferably measure not less than 20,000 nor more than 300,000 ohms per foot of cable length.

surrounding the corona shield 15 is the high voltage cable insulation 16. The insulation is a high grade polyethylene that isolates the high voltage charge at its center. Such a polyethylene is typically DFD 60005 NT (Natural), manufactured by Union Carbide. Surrounding the insulation is a conductive braid 17. Such a braid is comprised of 160 strands of 30-gauge copper alloy wire braided together around the insulation. Surrounding the braid itself is a sheath 18 typically 0.030 of an inch thick polyurethane. Total diameter of such a cable structure can be 7/16 inch.

Because of the improved conductive fiber glass high voltage conductor, this cable can be easily manufactured. The fiber glass center conductor provides strength and flexibility needed to manufacture the cable. Furthermore, its smooth outer surface is relatively free of surface imperfections, which can accentuate the tendency of the high voltage to break down electrical insulation. The fiber glass provides a chemically and dimensionally stable medium and permits a conductivity which can be controlled within the limits needed for a comercially satisfactory product. Sufficient resistance can be incorporated into the central conductor so that in the event of an insulation failure, the energy stored throughout the cable by the high voltage electric charge present on the high voltage conductor is dissipated by the resistance of the high voltage conductor as the electric charge travels down the cable to the point of the insulation failure. Thus, the energy available in an electrical discharge between the high voltage conductor and the conductive braid may be limited by the action of the resistance of the high voltage conductor. Because of the reliability in achieving stable high resistance values to the high voltage conductor, it is possible to obtain a cable where the energy available in the event of an insulation failure is below the level necessary to ignite most combustible vapors and residues encountered in spray finishing operations. Incorporation of such a cable into an electrostatic hand gun system thus improves the safety of the system.

The specific cable described above is typical of those used in electrostatic coating systems. However, this invention is capable of incorporation into cables other than that specifically described above.

Claims

We claim:

1. An electrical cable used to connect an electrostatic spray gun to a separate high voltage source, said cable having a stranded fiber glass core within a semiconductive shield surrounded by a body of insulative material, a flexible conductive sheath in an outer protective covering; the improvement which consists of the core and the shield together having a resistance of not less than 20,000 and not more than 300,000 ohms per foot of cable length the improvement which consists of the core being semiconductive and with semiconductive shield providing a resistance of not less than 20,000 and not more than 300,000 ohms per foot of cable length.