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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
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:
1. Check to see that the cable armor is grounded
by measuring between the cable armor and the metal
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.
3. Measure the resistance of the conductor being
tested to ground. Apply test voltage until a constant
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
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
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
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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