Wires and cables are generally subject to abuse. The type and amount of abuse depends on how and where they are installed and the manner in which they are used. Cables buried directly in the ground must resist moisture, chemical action, and abrasion. Wires installed in buildings must be protected against mechanical injury and overloading. Wires strung on cross-arms on poles must be kept far enough apart so that the wires do not touch. Snow, ice, and strong winds make it necessary to use conductors having high tensile strength and substantial frame structures.
Generally, except for overhead transmission lines, wires or cables are protected by some form of covering. The covering may be some type of insulator like rubber or plastic. Over this, additional layers of fibrous braid or tape may be used and then covered with a finish or saturated with a protective coating. If the wire or cable is installed where it is likely to receive rough treatment, a metallic coat should be added.
The materials used to make up the conductor protection for a wire or cable are grouped into one of two categories: nonmetallic or metallic.
The category of nonmetallic protective coverings is divided into three areas. These areas are (1) according to the material used as the covering, (2) according to the saturant in which the covering was impregnated, and (3) according to the external finish on the wire or cable. These three areas reflect three different methods of protecting the wire or cable. These methods allow some wire or cable to be classified under more than one category. Most of the time, however, the wire or cable will be classified based upon the material used as the covering regardless of whether or not a saturant or finish is applied.
types of nonmetallic materials are used to protect wires and cables.
Fibrous braid is by far the most common and will be discussed first.
Fibrous braid is used extensively as a protective covering for cables. This braid is woven over the insulation to form a continuous covering without joints (the figure below). The braid is generally saturated with asphalt, paint, or varnish to give added protection against moisture, flame, weathering, oil, or acid. Additionally, the outside braid is often given a finish of stearin pitch and mica flakes, paint, wax, lacquer, or varnish depending on the environment where the cable is to be used.
The most common type of fibrous braid is woven from
light, standard, or heavy cotton yarn. Cotton yarn comes in different
colors, which allows color-coding of the individual conductors. Cotton
will not withstand all the possible environments in which a cable may be
laid. Other materials currently being used to make fibrous braid are
glazed cotton, seine twine or hawser cord, hemp, paper and cotton, jute,
asbestos, silk, rayon, and fibrous glass. The choice of which material
to use depends on the strength needed in the cable as well as how
resistant it must be to its environment.
Fibrous Tape Fibrous tape coverings are frequently used as a part of the protective covering of cables. The material of tape coverings is made into the tape before application to the cable. The material in yarns for braid covering is woven into fabric during the application to the cable. When tape covering is used, it is wrapped helically around the cable with each turn overlapping the previous turn.
most common types of fibrous tape are rubber-filled cloth tape and a
combination of cotton cloth and rubber. Except for duct tape, tape
covering is never used as the outer covering on a cable. Tape coverings
are used directly over the insulation of individual conductors and for
the inner covering over the assembled conductors of a multiconductor
cable. Frequently, tape coverings are used under the sheath of a
lead-sheathed cable. Duct tape, which is made of heavy canvas webbing
saturated with an asphalt compound, is often used over a lead-sheathed
cable for protection against corrosion and mechanical injury.
Woven covers, commonly called loom,
are used when exceptional abrasion-resistant qualities are required.
These covers are composed of thick, heavy, long-fibered cotton yarns
woven around the cable in a circular loom, much like that used on a fire
hose. They are not braids, although braid covering are also woven; they
are designated differently.
Rubber and Synthetic Coverings
synthetic coverings are not standardized. Different manufactures have
their own special compounds designated by individual trade names. These
compounds are different from the rubber compounds used to insulate
cable. These compounds have been perfected not for insulation qualities
but for resistance to abrasion, moisture, oil, gasoline, acids, earth
solutions, and alkalies. None of these coverings will provide protection
against all types of exposure. Each covering has its own particular
limitations and qualifications.
Jute and Asphalt Coverings
Jute and asphalt
coverings are commonly used as a cushion between cable insulation and
metallic armor. Frequently, they are also used as a corrosive-resistant
covering over a lead sheath or metallic armor. Jute and asphalt
coverings consist of asphalt-impregnated jute yarn heli-wrapped around
the cable or of alternate layers of asphalt-impregnated jute yarn. These
coverings serve as a weatherproofing.
Unspun Felted Cotton
Unspun felted cotton is commonly used only in special classes of service. It is made as a solid felted covering for a cable.