MIL-DTL-24640C
4.8.3.4 Observation. Specimen failure shall be construed if the specimen exhibits a characteristic impedance
other than that specified (see 3.1).
4.8.4 Conductor resistance. This test shall determine whether or not conductors within the completed cable are
fabricated from the specified material.
4.8.4.1 Procedure. Conductor resistance shall be tested in accordance with ASTM B193 and shall be corrected
to 25 °C.
4.8.4.2 Observation. Specimen failure shall be construed if any specimen conductor exhibits a conductor
resistance greater than that specified (see 3.1).
4.8.5 Insulation resistance. This test shall determine the ability of completed cable to transmit DC without
exhibiting unacceptable current flow through insulation or jacketing material, either between conductors, or between
single conductor cable and any conducting substance which may be in contact with the surface of the cable during in
service use. The voltage withstand test (see 4.8.6) shall be performed prior to the insulation resistance test.
4.8.5.1 Specimen. The conductor insulation resistance test shall be performed on all conductors which have a
nominal circular mil area of less than 18 AWG; for conductors which have a nominal circular mil area of 18 AWG
or greater, the conductor insulation resistance test shall be performed on conductors selected in accordance with
table XIX.
TABLE XIX. Number of conductors tested for insulation resistance.
No. of insulated conductors in
No. of insulated conductors
cable
selected for test
1 through 4
All
5 through 20
4
21 through 30
5
31 through 42
6
43 through 56
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4.8.5.2 Specimen. The specimen shall be that which was previously used to perform the voltage withstand test
(see 4.8.6.1).
4.8.5.3 Special apparatus. Apparatus shall include the following:
a. A megohmmeter, megohm bridge, or other suitable instrument which shall use a DC test voltage of not less
than 200 V nor greater than 500 V, which shall support the requirements as specified in 4.8.5.4, and which shall
make measurements within an accuracy of ±10 percent.
b. Water tank (may not be required, see 4.8.5.4.d which shall be of a size to contain the specimen, and which
shall be suitable for the use as specified in 4.8.5.4.d.
4.8.5.4 Procedure. The specimen shall be maintained at a constant temperature, within ±3 °C, for a period of
not less than 5 hours immediately prior to testing. This temperature shall be noted and shall be used to calculate F
(see formula below) when measurements are made in accordance with (a), (b) or (c) below. The megohmmeter shall
then be electrically connected to the specimen as specified [see (a), (b), (c), and (d) below]. For each connection,
the test voltage from the megohmmeter shall be continuously applied to the specimen for a period of 3 minutes, or
until a steady reading is obtained immediately following which the resistance reading of the megohmmeter shall be
noted. Care shall be taken during each such measurement to assure that any electrical jumper leads connecting the
specimen to the megohmmeter shall be spaced sufficiently far from each other and from any common conducting
surface that measurement accuracy shall not be impaired.
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