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NOT AUTHORIZED FOR FURTHER REPRODUCTION OR
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49.3 Transformer burnout
49.3.1 A resistive load that will cause the input to draw three times the current obtained during normal
operation – see 33.1 – is to be connected directly to the transformer secondary winding with the converter
or inverter connected to 100 percent of the maximum test voltage specified in Table 31.1. The transformer
is to be operated continuously:
a) Until ultimate conditions are observed;
b) For 7 hours if cycling of an automatically reset protector occurs; or
c) For 50 cycles of resetting a manually reset protector.
For a transformer having a center-tapped secondary, a single load shall be connected across the
secondary winding that results in the greatest potential. During this test all dead metal parts of the
enclosure of the converter or inverter are to be connected directly to ground, the converter or inverter is
to rest on a soft-pine surface covered with white tissue paper, and a double layer of cheesecloth is to be
draped over the converter or inverter.
Exception: A nonmetallic enclosure with exposed dead metal parts that are not likely to become
energized need not be grounded.
49.3.1 revised January 27, 1995
49.3.2 A ferroresonant transformer is to be tested as described in 49.3.1, except that a resistive load that
will draw the maximum power input – see Section 33, Power Input Test – is to be connected directly to
the transformer secondary winding, with the converter or inverter connected to 106 percent of the
maximum test voltage specified in Table 31.1.
49.3.2 effective April 13, 1993
49.3.2.1 A switch mode transformer is to be tested as described in 49.3.1 except the output of the
transformer is to be connected to a resistive load that will draw maximum current without causing
shutdown due to operation of circuitry or overcurrent protective devices provided as part of the product.
This condition is to be held to the position immediately before foldback.
49.3.2.1 added January 27, 1995
49.3.3 For the test required by 49.3.1, primary overcurrent protective devices, and secondary manually
or automatically reset protectors integral with the transformer and connected in the secondary winding, are
to remain in the circuit.
49.3.4 An open switch or other open contact device used within an enclosure that has openings in the
bottom shall operate for 50 cycles with the secondary output short-circuited or carrying an equivalent
current. There shall be no emission of flame or molten metal.
OCTOBER 15, 2004UL 45868
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