Factors Affecting Insulation Resistance

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

feet.

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

MAINTENANCE OF CABLES

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

obtained.

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

EQUIPMENT CONNECTED.-- When insula-

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

2-38