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
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.
Compare the resistance of a conductor to that of an insulator.
State two fundamental properties of insulating materials.
Define insulation resistance.
Define dielectric strength.
How is the dielectric strength of an insulator determined?
TYPES OF INSULATION
The insulating materials discussed in the next paragraphs are commonly used in Navy electrical and
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.