MIL-DTL-24640C
g. Component shield to shield. Used for cable types incorporating two or more individually shielded
components (these measurements shall be required only when component shield to shield measurements are
specified, see 3.1): one component shield shall be selected and electrically connected to one terminal of the
megohmmeter. All other specimen component shields shall be electrically connected to the other megohmmeter
terminal, and the insulation resistance test shall then be performed as specified. In a similar manner, the insulation
resistance test shall be performed between each remaining component shields within the specimen and the short
circuited combination of all other component shields.
h. Component shield to shield and overall shield. Used for cable types incorporating two or more individually
shielded components and an overall shield (these measurements shall be required only when component shield to
shield measurements are specified; see 3.1): one component shield shall be selected and electrically connected to
one terminal of the megohmmeter. All other specimen component shields and the overall shield shall be electrically
connected to the other megohmmeter terminal, and the insulation resistance test shall then be performed as
specified. In a similar manner, the insulation resistance test shall be performed between each remaining component
shields within the specimen and the short circuited combination of all other component shields.
i. Overall shield to shield. Used for cable types incorporating two or more overall shields (electrically
separate) (these measurements shall be required only when overall shield to overall shield measurements are
specified; see 3.1): the inner overall shield shall be selected and electrically connected to one terminal of the
megohmmeter. The other overall shield shall be electrically connected to the other megohmmeter terminal, and the
insulation resistance test shall then be performed as specified.
The lowest value of resistance measured in accordance with each of (a), (b), (c), (d), (e), (f), (g), (h), and (i), 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), (d), (e), (f), (g), (h),
and (i), as applicable, in megohms.
Measured specimen length, in feet; measurement accuracy shall be within ±2 percent.
L
=
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.
4.8.5.5 Observation. Specimen failure shall be construed if any calculated value of insulation resistance is less
than that specified (see 3.1).
4.8.6 Voltage withstand. This test shall determine the ability of completed cable to withstand overvoltages
without exhibiting electrical discharge through insulation or jacketing material, either between conductors, or
between these conductors and any conducting material which may be in contact with the surface of the cable during
in service use.
4.8.6.1 Specimen. The specimen shall consist of a unit of product as specified in 6.7.4.1.
4.8.6.2 Special apparatus. Apparatus shall include the following:
a.
An adjustable voltage source, which shall be in accordance with either of the following, as appropriate (see
(1) An AC voltage source which shall produce a single phase, approximately sinusoidal voltage under all
test conditions, which shall have a frequency of not more than 100 Hertz, and which can be adjusted to the specified
rms magnitude (see 3.1). This source shall have the capacity to provide an effective power of not less than 5 kVA,
and shall incorporate an overload relay which shall reduce the source voltage to zero whenever an applied load (that
is, the specimen, see 4.8.6.3) attempts to draw electrical current to exceed this effective power capacity.
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