MIL-DTL-24643C
Conductor temperature, in °C = (234.5 + t) (R/r) - 234.5
Where:
R
=
Measured conductor resistance, in ohms during heat aging.
t
=
Measured room temperature, in °C, to which the specimen is exposed prior to heat aging.
Temperature t shall be measured in the immediate vicinity of the specimen; measurement
accuracy shall be within ±0.5 °C.
r
=
Measured conductor resistance, in ohms, when the specimen is exposed to room
temperature it shall be held constant, within ±0.5 °C for a period of not less than two hours
prior to this measurement.
The specimen shall next be placed into the oven such that a length of not less than the sum of 12 inches plus 20
times the maximum specified overall cable diameter (see 3.1) remains straight and horizontal and such that the
remainder of the specimen is formed into the largest practicable horizontal coil (to permit free circulation of
convection air currents). The current source shall be connected between the two end conductors of the single series
circuit within the specimen. The specimen shall then be subjected to heat aging; the air temperature within the oven
shall be raised to 50±3 °C and the current source adjusted to produce a conductor temperature of 125±5 °C. These
temperatures shall be maintained for a continuous period of not less than 400 hours. The current source shall not be
shut off during heat aging, except for momentary shut off for taking resistance measurements when sub-method a(2)
herein is used. Following heat aging, the specimen shall be allowed to cool throughout to room temperature. The
conductor temperature shall then be recorded. A recording which differs from room temperature by more than 5 °C
shall be considered to indicate failure of the conductor measurement means. If any such failure should occur, the
heat aging test shall be considered to be invalid and shall be repeated using a different specimen removed from the
same cable. (A possible advance indication of such a failure and the need to retest may be the need to make
prominent adjustments of conductor current in order to maintain indicated conductor temperature during heat aging.)
Following heat aging, the specimen shall be subjected a second time to the insulation resistance and voltage-
withstand tests as specified in 4.9.5 and 4.9.8, respectively. A sample, of length not less than 12 inches plus 20
times the maximum specified overall cable diameter (see 3.1), shall then be removed from that portion of the
specimen which had been kept straight during the heat aging. This sample shall be subjected to the bending
procedure as specified in 4.8.6.5. A portion of the remainder of the specimen shall be subjected to the cable filler
removability test as specified in 4.8.7.
b. Prior to heat aging, hot air oven method specimen one shall be subjected to the insulation resistance and
voltage withstand tests as specified in 4.9.5 and 4.9.8, respectively. The voltage withstand test shall use the voltages
specified (see 3.1). Both specimens shall then be placed within the oven; specimen number one shall be formed into
the largest practicable horizontal coil within the oven and specimen number two shall be positioned such that it is
both straight and horizontal. Both specimens shall be simultaneously subjected to heat aging. The air temperature
within the oven shall be raised to 125±3 °C. This temperature shall be maintained for a continuous period of not
less than 400 hours. Following heat aging both specimens shall be allowed to cool to room temperature. Specimen
one shall then be subjected a second time to the insulation resistance and voltage withstand tests as specified in 4.9.5
and 4.9.8, respectively. A portion of specimen one shall next be subjected to the cable filler removability test as
specified in 4.8.7. Specimen two shall be subjected to the bending procedure as specified in 4.8.6.5.
4.8.6.4 Observation. Any of the following shall constitute specimen failure:
a.
The falling away of any material from either end of any specimen during heat-aging.
b.
Jacket sagging on any unarmored specimen.
c. Jacket exudation through the armor of any armored specimen. (Jacket bulging, without actual flow or
cutting, is not considered to be exudation.)
d. Specimen (specimen one if the hot air oven method is used) fails either of the insulation resistance tests
(see 4.9.5).
Specimen (specimen one if the hot air oven method is used) fails either of the voltage withstand tests (see
e.
Specimen (specimen one if the hot air oven method is used) fails the cable filler removability tests (see
f.
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