Capacitance exists from every surface in a cable to all other surfaces in and around cable. An understanding of the relative amount of capacitance to be expected in a cable is helpful in specifying cable and designing terminal equipment for the cable to be used. Although capacitance in a cable may have negligible effect on DC or 60 cycle AC cable circuits used for power or control, it does affect higher frequency AC voltages.
Specify quantity, gauge, stranding and insulation type for the conductors. Allowance of 10% spares is a common practice. Preferably suggest an agency specification for the wires. (i.e.: UL, CSA, MIL and their proper type or style no.) You may need a MIL reference to preferred wire types and jacket materials in various applications and environments.
Fillers are used in lieu of a wire to fill space in a cable bundle so that it can retain its controlled shape and geometry. Their use promotes roundness and uniform flexibility.
Preferable fillers are plastic rods, tapes, or fibers, having physical properties similar to those of the wire insulation. Round glass-fiber braid can be used for high temperature applications. Fibrillated polypropylene is commonly used; cotton or jute is used less frequently.
Cables within the Main Cable
Individually cabled groups of wires within the cable may be referred to as sub-cables. The constructional details of sub-cables should be just as complete as the main cable in which they are used. (i.e.: wire details, cabling, coding, shields, jacketing.) Sub-cabling permits neat and simple formation of branch legs from the main cable and can be used to aid in identifying wire groups.
Subcables may be individually jacketed. Jackets may be solid colors, or may be color-striped over solid colors.
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The purpose of helical twist used in cabling the wires into a round bundle is twofold. It holds the wires in a unit, and it will act to neutralize the tension and compression forces which occur in the wires each time the cable is bent or flexed. Since most cables use stranded wires the cableforming must be done without untwisting the strands of the individual insulated stranded wires. This requires very special machinery, not just a drill motor.
Suggested wording: "Cabling shall be done on a tubular or planetary-type helical cabling machine in such a fashion that no residual twist is introduced into the individual cable members."
The axial distance per turn is called the lay length. The lay length should be between 8 to 16 times the pitch diameter of that layer of wires. (Pitch diameter is measured from center-to-center of diametrically opposed wires in a given layer.)
Although the rotation direction may be the same for all layers of wires opposite directions of rotation for each layer in a cable is usually preferred and is called contra-helical construction.
Overall RFI Shielding - Over the cabled bundle of wires
Metal-Foil-Faced Tape RFI Shield
The lowest-cost, lightest and least effective shield method. Can be applied single or double-sided, in single or multiple layers. Usually applied in contact with an uninsulated "drain" wire used for connection of shield. Usually made with aluminum or copper metal foil glued to a thermoplastic polyester (Mylar) backing, the most common thickness' of material are 0.00035" of metal foil and 0.0005"of backing tape, plus perhaps 0.00015" of glue for finished thickness of 0.001 " . NWC normally uses a heavier backing (0.001") for a total thickness of 0.0015" to avoid shielding degradation. Shield performance will degrade if cable handling causes tape to deform, or stretch so that metal begins to flake or crack. Cable flexibility is reduced slightly when a foil shield is added.
Braided RFI Shields
The most common and perhaps the most durable cable shield is a braided tubular shield made of small copper wires. Wire sizes range from #38 to #32 AWG. A braid is the standard coaxial cable outer conductor. It may be applied in one or more layers. In conjunction with metallic tape or foils, performance becomes very good. Braiding is a continuous operation, any desired length can be made. A braid is machine-formed directly onto the cable as contra-helical interlocking spirals of groups of fine wires. The effectiveness of the shield braid depends on how well it covers up the surface of the cable. A minimum coverage requirement of 90% of the cable surface is common. Machinery limitations make coverage in excess of 96% impractical for a single-layer braid. For braids that must "push-back" or swell easily, the braid angle should be specified. We recommend angles between 20 and 38 degrees for that service. Various methods of shielding can be combined for optimum RFI protection. See "Shielding Effectiveness" elsewhere in this section.
Individually cabled groups of wires within the cable may be referred to as sub-cables. The choice of outer cable covering material is usually a compromise between cost, agency recognition/ratings, flexibility and durability. A chart of relative properties is shown in Table A. A thin-wall jacket is usually most flexible; however it can be prone to wrinkle at tight bends, and often shows more "wireform" of the underlying cable surface. For plastic jacketed cables, typical wall thickness is 10% of the unjacketed diameter or 0.010" minimum. Cable jacket walls below 0.010" are not recommended. Maximum walls rarely need to exceed 0.125" in plastics, or 0.250" in rubbers. Some agencies (UL, CSA) have their own preferred wall sizes which must be met to obtain their sanction for their stated use.
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Cable sheath plastic compounds can be pigmented nearly any color, and are often color-matched to customer requirements. However, nearly all materials will discolor in sunlight or ultraviolet. Use of carbon-black pigment is very common as it readily makes the jackets opaque to UV and sunlight. Best to supply your own color sample for matching if not using black. Color samples preferably should be of the same material as the jacket compound. Specify if color must be correct for both fluorescent and daylight.
Details of cabling design are available elsewhere in this section under "Designer's Guide on Cable Geometry."
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