1. Field of the Invention
The invention relates to a cable adapted for high voltages applications. The cable can be used with fixed apparatus which are either permanently installed or temporarily installed at a given location, such as office equipment, machinery, home appliances, etc. Such apparatuses may use or produce high voltages, in which case some parts of the apparatuses can generate high-voltage noise. The present invention more particularly concerns electrical cables for the high-voltage circuits used in the those parts susceptible to generation of high-voltage noise.
2. Description of Background Information
Known electrical cables for high-voltage circuits may be classified into two categories. The first category includes a cable system in which copper-conductor cables are used in a general manner, but in which downstream portions employ cables which contain a ferrite core portion in order to suppress noise (hereinafter designated "prior art I"). The second category includes a cable system which uses reinforced cables made of aramid fiber, glass fiber, etc., the surface of which is covered with conductive carbon to make the cable conducting. With this type of cable, noise is suppressed by increasing the impedance of the carbon portion of the conductive cables (hereinafter designated "prior art II").
It is also known that improved high-voltage breakdown resistance can be achieved by twisting together a plurality of conductive wires 1 to form a cable suitable for high-voltage circuits (FIG. 1). With this cable, the surface of the twisted conductive wires 1 is made uniformly smooth, so that the electrical voltage is prevented from concentrating on particular points. To this end, the twisted conductive wires 1 are coated with an electrically conductive resin 2 through an extrusion process, and are then provided with an insulating coating 3 (hereinafter designated "prior art III").
With this "prior art III", a material having a good high voltage breakdown resistance and a good extrudability, such as low-density polyethylene (LDPE) or crosslinked LDPE, may be used as the insulating coating 3. Currently, it is required that office or home appliances must be non-flammable. As pure polyethylene resins are flammable, flame retarders are usually added to these resins to meet the requirement for non-flammability.
With a cable for high-voltage circuits which includes a ferrite core portion ("prior art I"), it is difficult to suppress noise over a broad frequency spectrum. Therefore, additional means have to be adopted for effective noise suppression. However, these additional means involve extra costs, due to the supplementary manufacturing steps they require.
When a conductive cable is prepared by coating carbon around a reinforcing thread through a baking process ("prior art II"), the impedance may be set to a high level in order to remove high-voltage noise. However, the resulting conductive cable has a structure which does not form inductance elements, and therefore noise cannot be suppressed efficiently.
With "prior art III", the electrically conductive resin 2 will become thermally deteriorated after a long-term use, and form fine cracks on its surface. High-voltage fields will then tend to concentrate at these cracks. When a high voltage is charged in this state, dielectric breakdowns may occur, and the conductive wire 1 can then no longer serve as a high-voltage cable.
In addition, the end portions of the electrical cable must be prepared for high-voltage circuits by connecting metal terminals thereto. In the case of "prior art III", the connections established during this preparation process can sometimes be made through the electrically conductive resin 2, which causes impedance fluctuations. The impedance may also vary after prolonged use, owing to the deterioration of electrically conductive resin 2. Moreover, the grip for holding the terminals may be weakened, with the high-voltage resistance subsequently being deteriorated.
Moreover, when a low-density polyethylene is used, as in the case with "prior art III", the resulting electrical cable deforms at high temperatures. This may lead to some cable characteristics, such as behavior during the so-called "high-voltage cutting-through test", which deviates from the standards adopted by Underwriters' Laboratories Inc. (UL Standards) in active use in the United States. In such a case, a flame retarder can be added to make the cable more fireproof. However, such an additive lowers the cable's voltage breakdown resistance. A solution would be to maintain the breakdown resistance by making the insulating coating thicker. However, such a measure would be at the expense of the cable's plasticity, the resulting electrical cable for high-voltage circuits then becomes less flexible.