MIL-DTL-24640C
b. The mandrel (required for either heat aging method) shall be rigid, smooth cylinder or partial cylinder, with
a continuous curved surface of not less than 180 degrees, of suitable length and construction for the specimen
bending as specified in 4.7.2.5. The mandrel diameter shall be approximately 12 times the specified maximum
overall cable diameter (see 3.1).
Voltage withstand test apparatus (required for either heat aging method) shall be as specified in 4.8.6.2
c.
d.
Insulation resistance test apparatus (required for either heat aging method) shall be as specified in 4.8.5.3.
e. Thermocouple device (required for the current overload if submethod 4.7.2.3.a.(1) is used) and an
associated temperature indicating device shall be provided for the use and temperatures specified in 4.7.2.3.a.
Temperatures shall be measured with an error of not more than ±1 °C.
f. Current source (required for current overload method only) shall be an adjustable source of either DC or 60
Hertz single-phase AC, for maintaining specimen conductors at an elevated temperature as specified in 4.7.2.3.a. A
means shall be provided for measuring the current produced by this source with an error of not more than
±5 percent.
g. Resistance meter (required for the current overload method if submethod 4.7.2.3.a is used) shall be
provided for measuring the resistance of a single specimen conductor at room temperature and at elevated
temperatures, as specified in 4.7.2.3.a. The meter shall exhibit a measurement error of not more than ±1 percent.
4.7.2.3 Procedure. Select and perform either of the two heat aging methods, the current overload method or the
hot air method, specified in (a) or (b) herein, respectively. The oven air temperature (in both methods), conductor
temperature, and current (in the current overload method only) shall be recorded at intervals of not more than 15
minutes during the first hour after attaining the heat aging temperature, at intervals of not more than 1 hour for the
following 5 hours and not less than twice daily thereafter for the duration of the heat aging. The interval between
the consecutive temperature recordings shall not exceed 17 hours at any time during heat aging.
a. Prior to heat aging, the current overload method specimen shall be subjected to the insulation resistance and
voltage withstand test in accordance with 4.8.5 and 4.8.6, respectively. The voltage withstand test shall use the
specified voltages (see 3.1). Following these, specimen conductor ends shall be electrically interconnected such that
a single series electrical circuit is formed in the conductors. Provision shall then be made to measure specimen
conductor temperature, using either of the following submethods.
(1) The smallest practicable "V" shaped cut shall be made along the specimen at a point approximately
midway between the specimen ends, the thermocouple shall be placed beneath the resulting triangular tab. The
thermocouple shall be positioned such that it is in firm contact with any single specimen conductor layer. (For
specimens with only a single layer of conductors, any specimen conductor may be used.) The thermocouple shall be
used with its associated temperature indicating device to measure specimen conductor temperature during heat
aging.
(2) The resistance meter shall be connected to measure the resistance between the ends of any single
specimen conductor in the innermost conductor layer. (For specimens with only a single layer of conductors, any
specimen conductor may be used.) The meter leads shall be positioned on the conductor extremities such that all
conductor interconnections are excluded from resistance measurements. The conductor temperature shall then be
determined by using the following formula:
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 t. t shall be held
constant, within ±0.5 °C for a period of not less than 2 hours prior to this measurement.
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