MIL-DTL-24643C
4.8.14.2 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 8, 9, and 10), able to support the requirements as specified in 4.8.14.3. 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 8, 9, and 10. The mixing chamber, fabricated from number 16 United States
Standard Gage (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 Hz voltage source, as shown on figure 11, 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 incorporates 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 Vrms between conductors.
Fuses or overload relays shall each have a maximum rating of 1 A.
(2) Specimens rated for 1 kV which incorporates conductors with cross-sections of 14,000 circular mils or
more, the test voltage shall be 450±10 Vrms between conductors. Fuses or overload relays shall each have a
maximum rating of 1 A.
(3) Specimens rated for 5 kV, the test voltage shall be 4160±100 Vrms between conductors. Fuses or
overload relays shall each have a maximum rating of ¼ A.
The secondary winding voltage rating of any constituent voltage step up transformer shall be not less then
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 4.8.14.3). 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.
4.8.14.3 Procedure. The burner shall be mounted within the chamber, such that the holes in its top are in
common horizontal plane. The specimen shall be suspended within the enclosure, such that it is parallel to the
burner, horizontally centered directly above the burner and spaced 1⅞± inches vertically above the top of the
burner surface. Specimen suspension shall be accomplished in a manner (such as by wire loops) which shall not
significantly shield the specimen from flame and which shall prevent any portion of the specimen from dropping
closer than ¾ inch to the top surface of the burner during specimen burning. The gas air supply shall then be
attached to the burner and the gas adjusted to provide a flow rate of not less than that calculated by the following
formula:
Gas flow rate, in cubic feet per hour = 1387 (T+273)/PN
Where:
T
=
Measured ambient temperature in the vicinity of the burner, in °C. Measurement accuracy
shall be within ±0.5 °C.
Local atmospheric pressure, in lb/in2 absolute. Measurement accuracy shall be within ±2
P
=
percent.
Gross heating value, in BTU/ft3, of the gas at an atmospheric pressure of 14.69 lb/in2
N
=
absolute and a temperature of 23 °C, this value shall be accurate in ±2 percent.
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