To be useful and safe, electric current must be forced to flow only where it is needed. It must be "channeled" from the power source to a useful load. In general, current-carrying conductors must not be allowed to come in contact with one another, their supporting hardware, or personnel working near them. To accomplish this, conductors are coated or wrapped with various materials. These materials have such a high resistance that they are, for all practical purposes, nonconductors. Nonconductors are generally referred to as "insulators" or "insulating material."
Only the necessary minimum amount of insulation is applied to any particular type of conductor designed to do a particular job. This is done because of several factors. The expense, stiffening effect, and a variety of physical and electrical conditions under which the conductors are operated must be taken into account. Therefore, there are a variety of insulated conductors available to meet the requirements of any job.
Two fundamental properties of insulating materials (that is, rubber, glass, asbestos, or plastic) are insulation resistance and dielectric strength. These are two entirely different and distinct properties.
Insulation resistance is the resistance to current leakage through the insulation materials. Insulation resistance can be measured with a megger without damaging the insulation. Information so obtained is a useful guide in appraising the general condition of insulation. Clean, dry insulation having cracks or other faults may show a high value of insulation resistance but would not be suitable for use.
Dielectric strength is the ability of an insulator to withstand potential difference. It is usually expressed in terms of the voltage at which the insulation fails because of the electrostatic stress. Maximum dielectric strength values can be measured only by raising the voltage of a TEST SAMPLE until the insulation breaks down.
Q.18 Compare the resistance of a conductor to that of an insulator.
TYPES OF INSULATION
The insulating materials discussed in the next paragraphs are commonly used in Navy electrical and electronic equipment.
One of the most common types of insulation is rubber. The voltage that may be applied to a rubber-covered conductor is dependent on the thickness and the quality of the rubber covering. Other factors being equal, the thicker the insulation, the higher may be the applied voltage. Rubber insulation is normally used for low- or medium-range voltage. Figure 1-7 shows two types of rubber-covered wire. One is a two-conductor cable in which each stranded conductor is covered with rubber insulation; the other is a single, solid conductor. In each case, the rubber serves the same purpose: to confine the current to its conductor.
Figure 1-7. - Rubber insulation.
Referring to the enlarged cross-sectional view in figure 1-7, note that a thin coating of tin separates the copper conductor from the rubber insulation. If the thin coating of tin were not used, a chemical action would take place and the rubber would become soft and gummy where it makes contact with the copper. When small, solid, or stranded conductors are used, a winding of cotton threads is applied between the conductors and the rubber insulation.
CODE-GRADED RUBBER. - Code-graded rubber is the standard that the National Electrical Code (NEC) has adopted as the minimum requirements for rubber insulation as specified by Underwriters' Laboratories. In this code system, the letter R indicates the use of a rubber insulator. Type R signifies that the wire is rubber coated.
The NEC codes Type RH and Type RHH signify a rubber heat-resistant compound. Type RW signifies a rubber moisture-resistant compound. A Type RHW signifies a rubber heat- and moisture-resistant compound. Type RHW is approved for use in wet or dry locations at a maximum conductor temperature of 75°C. Neoprene, a low-voltage compound, is the one exception to Type RHW. Although not a rubber compound, neoprene meets the requirements of Underwriters' Laboratories and was designated Type RHW.
LATEX RUBBER. - Latex rubber is a high-grade compound consisting of 90 percent unmilled grainless rubber. There are two designations for this type of insulation: Type RUH and Type RUW. Type RUH (rubber unmilled heat-resistant) is used in dry locations when the conductor temperature does not exceed 75°C. Type RUW (rubber unmilled moisture-resistant) is used in wet locations when the conductor does not exceed 60°C.
SILICONE. - Silicone is a rubber compound that does not carry the "R" designator for many of its applications. An example of this is Type SA (silicone-asbestos). In Type SA, the insulator around the conductor is silicone rubber, but the outer covering must consist of heavy glass, asbestos-glass, or asbestos braiding treated with a heat, flame, and moisture-resistant compound.