UL 482 Standard For Portable Sun/Heat Lamps

23.13 At a point on the surface of a coil where the temperature is affected by an external source of heat, the temperature rise measured by means of a thermocouple may be higher by the following amount than the maximum indicated in Table 23.1: provided that the temperature rise of the coil by the change-of-resistance method is not more than specified in Table 23.1.

23.14 The temperature rise of a winding by the change-of-resistance method is to be calculated from the formula (windings are to be at room temperature at the start of the test): 

in which [delta]t is the temperature rise, R is the resistance of the coil at the end of the test, r is the resistance of the coil at the beginning of the test, t(1) is the room temperature (*)C at the beginning of the test, t(2) is the room temperature (*)C at the end of the test, and k is 234.5 for copper, 225.0 for electrical conductor grade (EC) aluminum. Values of the constant for other grades must be determined.

23.15 If it is necessary to de-energize the winding before measuring R, the value of R at shutdown may be determined by taking several resistance measurements at short intervals, beginning as quickly as possible after the instant of shutdown. A curve of the resistance values and the time may be plotted and extrapolated to give the value of R at shutdown.