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37.1.2 A protective device shall not operate during the normal temperature test.
37.1.3 With reference to 37.1.1, the load shall be subdivided between the output circuits so that at least
one circuit of each rating is loaded to 80 percent of its rating and the remainder of the load is distributed
equally among the other circuits.
37.1.4 A converter or inverter designed for mounting or support in more than one position shall be tested
in a manner representing the most severe conditions.
37.1.4 effective April 13, 1993
37.1.5 Unless investigated and found acceptable – see 6.13 – a supporting means formed of soft rubber
or rubberlike material is to be removed prior to the temperature test. If the supporting means has a metal
insert, such as a screw or rivet, the test is to be conducted with the converter supported by the metal
insert. At the request of the manufacturer, the test may be conducted without any means of support.
37.1.6 Coil and winding temperatures are to be measured by thermocouples located on exposed
surfaces, except the resistance method may be used for a coil that is inaccessible for mounting of these
devices such as a coil:
a) Immersed in a sealing compound;
b) Wrapped with thermal insulation such as asbestos; or
c) Wrapped with more than two layers of material such as cotton, paper, or rayon more than 1/32
inch (0.8 mm) thick.
In an alternating-current motor, the thermocouple is to be mounted on the integrally-applied insulation of
the coil wire.
37.1.7 In using the resistance method, the windings are to be at room temperature at the start of the test.
The temperature of a winding is to be calculated from the formula:
in which:
Tc is the temperature of the winding at the end of the test in °C;
R is resistance of the winding at the end of the test in ohms;
r is resistance of the winding at the beginning of the test in ohms;
t1 is room temperature at the beginning of the test in °C; and
k is 234.5 for copper, 225.0 for electrical conductor grade (EC) aluminum. Values of the constant
k for other grades must be determined.
37.1.7 effective April 13, 1993
DECEMBER 14, 1993 UL 458 53
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