MIL-DTL-24643C
a. Specimens incorporating one or more individually shielded insulated conductor transmission lines: one
transmission line shall be selected for measurement. One conductor of this transmission line shall be electrically
connected to one terminal of the megohmmeter, and the shield and all other conductors (if any) of this transmission
line shall be electrically connected to the other megohmmeter terminal. The resistance measurement shall then be
performed as specified. In an identical manner, the resistance between each remaining conductor in this
transmission line (if any) and the short circuited combination of all other conductors (if any), plus the shield in this
transmission line shall be measured this, and in an identical manner, resistance measurements shall be made for all
remaining transmission lines within the specimen. (Additional measurements may be required, see c and d.)
b. Specimens incorporating multiple insulated conductors without individual shields: one insulated conductor
shall be selected and electrically connected to one terminal of the megohmmeter. All other specimen conductors,
including the overall shield (if any) shall be electrically connected to the other megohmmeter terminal. The
resistance measurement shall then be performed as specified. In a similar manner, the resistance between each
remaining conductor within the specimen and the short circuited combination of all other conductors plus the overall
shield (if any) shall be measured. If the specimen incorporates an overall shield, then an additional resistance
measurement shall be made between the shield and the short circuited combination of all remaining conductors. If
the specimen does not incorporate an overall shield or armor over the jacket the test shall be made after placing the
reel of cable in water for a period of not less than 1 hour. (Additional measurements may be required, see d.)
c. Specimens incorporating one or more individually shielded insulated conductor transmission lines (these
measurements shall be required only when shield to shield measurements are specified, see 3.1): one shield shall be
selected and electrically connected to one terminal of the megohmmeter. Other specimen shields shall be
electrically connected to the other megohmmeter terminal, and the resistance measurement shall then be performed
as specified. In a similar manner, the resistance between each remaining shield within the specimen and the short-
circuited combination of all other shields shall be measured.
d. Specimens incorporating double overall shields: one shield shall be electrically connected to one terminal
of the megohmmeter and the other shield shall be connected to the other terminal. The resistance measurement shall
be performed as specified. (Additional measurements required, see a).
e. Specimens incorporating single or conductors without shield or armor shall be tested to water. The
specimens shall be submerged for not less than 1 hour, except for its extreme ends (which shall remain exposed to
air), in the water tank. The single conductors are to be electrically connected to one terminal of the megohmmeter.
For other specimens, the overall outermost shield (if any) or if there is no overall shield, the individual shields that
are outermost in the specimen core shall be electrically connected to one terminal of the megohmmeter. The other
megohmmeter terminal shall be electrically connected to the water surrounding the specimen (as by means of a bare
metal wire which is both connected to the megohmmeter terminal and suspended within the water surrounding the
specimen). The resistance measurement shall be performed as specified. The temperature of the water surrounding
the specimen shall also be measured and shall be used to determine F in the following formula:
The lowest value of resistance measured in accordance with each of a, b, c, and d, as applicable, shall be
modified to include the effects of specimen length and temperature by using the following formula:
Insulation resistance, in megohms 1000 feet = RLF/1000
Where:
R
=
Lowest value of resistance measured in accordance with a, b, c, or d, as applicable, in
megohms.
L
=
Measured specimen length, in feet; measurement accuracy shall be within ±2 percent.
F
=
An ad-hoc number, which shall accurately modify the measured resistance R to that value
of resistance which would be measured if the specimen were at a uniform temperature of
15.5 °C. This number shall be determined by the specimen manufacturer via ad hoc testing
for the specific insulation or jacketing material separating the conductors between which
the resistance measurement was made, and shall be normalized to unity for a material
temperature of 15.5 °C.
4.9.5.5 Observation. Specimen failure shall be construed if any calculated value of insulation resistance is less
than that specified (see 3.1).
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