Cable Repair

where insulation resistance must be checked at a much

telephone circuits, insulation resistance readings of

lower potential. The megohmmeter operates on

50,000 ohms is the acceptable minimum. For short

cable runs, minimum insulation resistance should be

internal batteries. When circuits or components under

well above 50,000 ohms.

test contain a large electrical capacity, the meg-

ohmmeter READ button must be depressed for a

IC cables should be tested with an insulation

sufficient time to allow its capacitor to charge before

resistance measuring instrument (Megger). If a

a steady reading is obtained. The test voltage applied

Megger is not available, consult NSTM, chapter 300,

by the megohmmeter to an unknown resistance is

for alternate methods of testing the insulation

a p p r o x i m a t e l y 50 volts when resistances of

resistance.

approximately 10 megohms are measured and slightly

To ground test a multiconductor IC cable with a

greater than this when higher resistances are

Megger, proceed as follows:

measured.

1. Check to see that the cable armor is grounded

by measuring between the cable armor and the metal

CABLE REPAIR

structure of the ship; normally, grounding has been

accomplished by cable straps. If a zero reading is not

A cable repair is the restoration of the cable armor

obtained, ground the cable armor.

or the outermost sheath or both. Cable repair may be

made by ship's force. However, cable repair should be

2. Select one conductor to be tested, and connect

according to NAVSEA standard methods drawing

all other conductors in the cable together. Ground them

803-5001027, unless standard methods cannot be

with temporary wires or jumpers.

applied.

3. Measure the resistance of the conductor being

tested to ground. Apply test voltage until a constant

CABLE SPLICING

reading is obtained. Crank hand-driven generator-type

Meggers for at least 30 seconds to ensure a steady

A cable splice is the restoration on any part of a

reading.

cable that cannot be restored by a cable repair. Cable

4. Repeat steps 2 and 3 as necessary to test each

splices should be according to NAVSEA standard

conductor to ground.

methods drawing 803-5001027, unless standard

methods cannot be applied. Cable splices should not

A reading equal to or above the accepted

minimum for the cable concerned indicates that the

be made by ship's force except in an emergency. When

conductor under test is satisfactory. A reading below

such splices are made, they should be replaced at the

the accepted minimum indicates that the insulation

earliest opportunity by a continuous length of cable or

resistance of the conductor under test to ground or

by an approved splice installed by a repair activity.

from one or more of the grounded conductors or both

is low. The grounded conductors must then be

CABLE INSTALLATION

disconnected from ground, and each conductor tested

individually to isolate the low-reading conductor(s).

The job of installing cable may be performed by

IC personnel whenever necessary to repair damage or

An alternate method of ground testing multi-

to accomplish authorized ship alterations

conductor cables is to connect all conductors together

(SHIPALTs). Before work is begun on a new cable

and measure the insulation resistance from all

conductors to ground simultaneously. If this reading

installation, cableway plans should be available. If

is equal to or above the accepted minimum, no other

repairs to a damaged section of installed cable are to

reading need be taken. If the reading is below the

be effected, information on the original installation

accepted minimum, the conductors must be separated

can be obtained from the plans of the ship's electrical

and tested individually to isolate the low-reading

system, which are normally on file in the engineering

conductor(s).

department office (legroom) aboard ship. If a

SHIPALT is to be accomplished, applicable plans not

When checking insulation resistance on circuits

already on board can be obtained from the naval

where semiconductor control devices are involved, the

shipyard listed on the authorization for the SHIPALT

500-volt dc Megger cannot be used. An electron tube

at the planning yard for the ship.

megohmmeter is used on circuits and components

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