MIL-DTL-24640C
4.7.8.1 Specimen. The specimen shall consist of a 48±6 inch length of completed cable. The jacket and
blocking materials shall be removed for a distance of 3 inches on each end of the sample. Sufficient insulation shall
be removed from one end of each conductor for making electrical connections as follows:
a. Specimens with one conductor shall be tested with single-phase voltage between the conductor and a
copper braid installed around the specimen's jacket and providing no greater than 50 percent coverage. The braid
shall be attached to ground.
b.
Specimens with two conductors shall be tested using two phases of the three-phase source.
c.
Specimens with three conductors shall be tested with each conductor connected to a separate phase.
d. Specimens with four conductors shall have physically opposite conductors and be connected together in
pairs to form two effective electrical circuits.
e. Specimens with six conductors shall have physically opposite conductors and be connected together in
pairs to form three effective electrical circuits.
f. Specimens with seven or more conductors shall have conductors connected together to form three effective
electrical conductors. Connections shall be made such that physically adjacent conductors shall be members of
different effective electrical conductors to the maximum practicable extent and such that the number of conductors
constituting each effective electrical conductor does not differ by more than one.
4.7.8.2 Special apparatus. Apparatus shall include the following:
a. A metered and adjustable source of natural gas or other flammable gas and an adjustable source of air (see
figures 4, 5, and 6), able to support the requirements as specified in 4.7.8.3. The gas and air support shall be made
available as a mixture through a single conduit.
b. A ribbon burner, which accepts gas and air from the gas and air supply via a single, ½-inch nominal pipe
set vertically into its underside, which thoroughly blends the gas and air within an internal mixing chamber, which
allows the blended gas and air mixture to exit for burning via a multiplicity of holes in its top which shall be
fabricated in accordance with figures 4 and 5. The mixing chamber, fabricated from number 16 United States
Standard Gauge (USSG) steel, shall be horizontally centered within the burner and shall be airtight except for the
specified holes on its two sides and through the inside of the ½-inch nominal pipe. The burner exterior shall be
fabricated from number 14 USSG steel and shall be airtight except for the specified holes on its top and the specified
holes in the mixing chamber.
c. A three-phase, 60 Hertz voltage source, as shown in figure 6, shall provide a separate, single-phase test
voltage through a normal blow fuse or overload relay to each specimen conductor. This voltage source shall be in
accordance with the following:
(1) Specimens rated for 1 kV which incorporate conductors with cross sections of less than 14,000 circular
mils and all specimens rated for less than 1 kV. The test voltage shall be 120±5 V rms between conductors. Fuses
or overload relays shall each have a maximum rating of 1 A.
(2) The secondary winding voltage rating of any constituent voltage step up transformer shall be not less
than the required test voltage. Each phase of the voltage source shall have the capacity to supply the specimen with
a current which shall be not less than twice the fuse and overload relay rating.
d. A chamber enclosing the horizontally mounted burner and the suspended specimen (see 4.7.8.3). This
enclosure shall incorporate an exhaust fan at its top and shall incorporate apertures for admitting air along its sides
adjacent to its base. The exhaust fan shall be operated to produce no more suction than is necessary to carry off
smoke and gases. The enclosure shall be designed to both minimize horizontal air drafts to the greatest practicable
extent and to allow free circulation of convection air currents generated during the gas flame test.
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