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Page Title: Factors Affecting Insulation Resistance
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If corresponding portions of a circuit are
current to flow between the conductor and the sheath
in 10 feet of cable, and the total amount of current
energized from the forward and aft IC switchboards,
the suffix letters F and A are added to the ends of wire
flowing in all of them would be 10 times as great as
markings to indicate the switchboard from which the
that which would flow if the cable were only 1 foot
wire originated.
long. Therefore, to establish a common unit of
comparison, cable-insulation resistance should be
All terminals in a circuit that may be connected
expressed in megohms (or ohms) per foot of length.
without a break (in the electrical sense) should be
This is determined by multiplying the measured
assigned the same wire marking. A fuse, switch, or
insulation resistance of the cable by its total length in
instrument is considered a break in the circuit and
requires a change in the wire marking.
When measured insulation resistance is converted
Signal contacts should be connected to the
to insulation resistance per foot, the total length of
positive (single-letter connection) in the instruments.
cable to be used is equal to the length of the cable
The section-wire markings for bell or visual signal
sheath for single-conductor cable and for multiple-
circuits should be assigned the next higher number
conductor cable in which each conductor is used in one
after assignment of numbers to secondary windings of
leg of a circuit. For example, in an LSTSGA cable with
all synchro receivers in the instruments. For example,
a cable sheath of 100 feet in which the three
in an instrument containing two synchro receivers the
conductors are phases A, B, and C of a 3-phase power
signal circuits should be assigned section wires No. 7,
circuit, the total length of the cable is 100 feet, not 300
8, and so on.
feet. The reason for this is that each conductor is
measured separately. If this cable is connected, either
in series or parallel, to a similar cable that has a sheath
length of 400 feet, the total length is 500 feet. As
The purpose of cable maintenance is to keep the
another example, 200 feet of type LSMSCA cable
cable insulation resistance high. Cables should be kept
(7-conductor cable) connected to 200 feet of
clean and dry, and protected from mechanical damage,
LSMSCA-24 cable (24-conductor cable) represents a
oil, and salt water.
total cable length of 400 feet.
The purpose of insulation on electrical cables and
TYPE OF CABLE.-- Insulation resistance will
equipment is to (1) isolate current-carrying conduc-
vary considerably with the nature of the insulating
tors from metallic and structural parts and (2) insulate
materials employed and the construction of the cable.
points of unequal potential on conductors from each
Therefore, it is possible to determine the condition of
other. The resistance of such insulation should be
a cable by its insulation resistance measurements only
sufficiently high to result in negligible current flow
when they are considered in relation to the typical
through or over its surface.
characteristics of the particular type of cable. A
Resistance Test Record Card, NAVSEA 531-1 (fig.
Factors Affecting Insulation Resistance
2-43), should be used to determine if the measured
insulation resistance values are above the minimum
Factors that affect cable insulation resistance
acceptable values.
measurements are length, type, temperature, and the
equipment connected in the circuit. Each of these
TEMPERATURE OF CABLE.-- It is important
to maintain the operating temperature of electrical
factors must be evaluated to reliably determine the
condition of the cable from the measurements
equipment within their designed values to avoid
premature failure of insulation. Temperatures only
slightly in excess of designed values may produce
LENGTH  OF  CABLE.--  The  insulation
gradual deterioration, which, though not immediately
resistance of a length of cable is the resultant of a
apparent, shortens the life of the insulation. Therefore,
number of small individual leakage paths or
the temperature of the cable must be considered with
resistances between the conductor and the cable
the insulation resistance measurements. Consult
sheath. These leakage paths are distributed along the
NSTM, chapter 300, for the proper procedures for
cable. Hence, the longer the cable, the greater the
measuring the temperature of a cable.
number of leakage paths and the lower the insulation
resistance. For example, if one leakage path exists in
each foot of cable, them will be 10 such paths for
tion resistance measurements are made with

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