MIL-DTL-24643C
b. Current source. A 3-phase, 60 Hz adjustable voltage source shall be used with three current transformers,
as shown on figure 4, to provide three adjustable, single-phase test currents as specified. The secondary winding of
each current transformer shall be isolated from electrical ground. Bus bar conducting test current without generating
significant amounts of heat shall be used to convey the test currents from the current transformers to the specimen
conductors.
c. Electrical metering. Instrumentation shall be provided, as shown on figure 4, such that the test voltage and
current applied to each specimen conductor shall be measured with an error of not more than ±2 percent.
d. Heating chamber. A heating chamber shall be provided for supporting the requirements as specified in
4.8.2.3. This chamber shall have interior finishes and dimensions which discourages the reflection of radiant heat
(generated by the test currents) onto the specimen.
e. Thermocouple devices. Four thermocouples and an associated temperature indicating device shall be
provided for the use and temperatures as specified in 4.8.2.3. Temperatures shall be measured with an error of not
more than ±1 °C.
4.8.2.3 Procedure. The specimen shall be formed into a single, horizontal loop within the heating chamber (to
permit free circulation of convection air currents). Air temperature within the chamber shall be maintained at
30±2 °C during the test (except in the immediate vicinity of the specimen) and means shall be provided to prevent
forced air drafts from directly striking the specimen. The four thermocouples shall be placed on the top of the
specimen, near its midpoint, and in firm contact with its jacket. These thermocouples shall be placed longitudinally
along the specimen jacket at 2±1/16 inch intervals. On unarmored specimens, firm jacket contact can be assured by
the use of a length of string with a weight on each end as this can be hung over the jacket after forming a single
wrapping around the jacket and over the thermocouple bead. On armored specimens, firm jacket contact can be
assured by opening a small space between the jacket and an armor carrier (as with an ice pick), and inserting the
thermocouple bead within this space, afterwards pulling the armor carrier longitudinally to press the thermocouple
bead tightly against the jacket. Under no circumstances shall the jacket be cut or shall tapes be used to hold the
thermocouple beads since these may cause premature specimen failure due to elevated jacket temperatures (see
4.8.2.4). Two thermocouples shall be used to measure ambient temperature within the chamber. The test voltages
and currents shall be simultaneously applied to the specimen as shown on figure 4. If the specimen is armored, an
additional test voltage shall be applied by grounding the armor, as shown on figure 4. This shall effectively apply an
additional voltage which is approximately equal to the test voltage divided by the square root of 3 between the armor
and the specimen conductors. The test currents and voltages shall remain continuously applied to the specimen for a
period of not less than 7 hours, unless specimen failure (see 4.8.2.4) occurs prior to the end of this period, then the
test shall be terminated prematurely. The following notes shall apply for specimens with more than 3 conductors:
a. Unarmored 4 conductor specimens. The green insulated conductor shall be isolated from all test voltages
and currents.
b. Armored 4 conductor specimens. The green insulated conductor shall be electrically connected to the
armor during the test.
c. Six conductor specimens (armored or unarmored). The radially opposed conductors within the core of the
specimen shall be electrically connected, at both specimen ends, to effectively form three conductors during the test.
4.8.2.4 Observations. Any of the following shall constitute specimen failure, if occurring prior to the
expiration of the 7 hour minimum test period:
a.
The blowing of any fuse or the tripping of any overload relay.
b.
Any thermocouple indication of the jacket temperature in excess of 135 °C.
c.
Jacket sagging on an unarmored specimen.
d. Jacket exudation through the armor of an armored specimen. (Jacket bulging, without actual flow or
cutting, is not considered to be exudation.)
4.8.3 Bending endurance. The ability of completed cable to withstand repeated smooth and continuous,
reversing bending motion while subjected to a specified temperature shall be determined.
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