MIL-R-19365E
4.7.6 Insulation resistance (see 3.11). Resistors shall be tested in accordance with method 302 of MIL-STD-202. The following details shall apply:
a. Test condition letter: - A.
b. Special preparations: As specified in 4.7.5a.
c. Points of measurement: As specified in 4.7.5d with the positive lead connected to the winding and the negative lead to the mounting hardware.
4.7.7 Power rating (see 3.12). The adjustable terminal shall be positioned at one end as close to the fixed end terminal as possible while still engaging the bare resistance wire. The resistors shall be mounted horizontally with the exposed portion facing downward. Total resistance shall be mounted as specified in 4.7.1.1. The resistors shall be supported without mounting hardware by means of their terminals in free space and still air (see note for table IV) at
an ambient temperature of 25OC +5OC, -0OC (see 4.7.7.1). If the power rating is measured at any ambient temperature other than 25OC, a power rating correction factor with reference to 25OC +5OC, -0OC ambient
temperature shall be applied in accordance with the derating curve shown on figure 1. Rated wattage from an ac
supply at commercial line frequency and waveform shall be applied to the fixed end terminals until thermal stability has been reached. During the test, added precaution shall be taken to determine the possibility of intermittent open circuiting in the winding. The hotspot surface temperature shall be determined by the use of a thermocouple. (see
4.7.7.2). Measurements of resistance shall be made not less than 2 hours after removal of the load potential.
4.7.7.1 Measurement of ambient temperature. The ambient temperature shall be measured on a level with, and at a point 12 inches from, the nearest resistor under test.
4.7.7.2 Thermocouple. The thermocouple junction shall rest on the hotspot, and the thermocouple wires shall
hang down on opposite sides of the resistor. Precaution shall be taken that the thermocouple wire does not touch the exposed portion of the resistor. Weights of at least 2 ounces on each wire lead shall provide the pressure of the
thermocouple junction against the surface of the resistor under test. The thermocouple shall be composed of wire
.010 inch (0.25 mm) in diameter (AWG size 30), or smaller. The thermocouple wire shall not extend more than .125 inch (3.18 mm) beyond the junction. A negligible difference of potential may exist between the thermocouple and the resistor winding so as to prevent a possible dielectric failure of the coating.
4.7.8 Thermal shock (see 3.13). Resistors shall be mounted on a rack of low heat conducting material. Following a measurement of total resistance (see 4.7.1.1), rated wattage from an ac supply at commercial line frequency and waveform shall be applied until thermal stability has been reached. The power shall then be removed and, within 8 second to 12 second, the resistors shall be subjected to an air temperature of -55OC ±5OC for a period of not less
than 15 minutes nor more than 30 minutes. Total resistance shall again be measured not less than 2 hours after final exposure. Resistors shall then be examined for evidence of mechanical damage and change in any part of the resistors.
4.7.9 Short time overload (see 3.14). Resistors shall be mounted by their normal mounting means. Following a measurement of total resistance (see 4.7.1.1), a potential from an ac supply at commercial line frequency and waveform which will result in 10 times the rated wattage (see 3.1) shall be applied to the resistors for 5 seconds. In no case shall this voltage exceed 6,000 volts. Total resistance shall again be measured after the resistors have cooled to room temperature. Resistors shall then be examined for evidence of mechanical damage, arcing, burning, and charring.
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