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47.10.4 The abnormal tests may be conducted with a protective device built into the transformer or with
an external protective device connected in either the primary or secondary circuit, or both. A protective
device that is relied upon to open the circuit as a result of an abnormal test is to be one that has been
investigated and found to be acceptable for the purpose.
47.10.4 effective April 13, 1993
47.10.5 For the purpose of these requirements, each secondary winding tap other than a center tap and
each primary winding tap designed to supply power to a load are considered to be the equivalent of a
secondary winding.
47.10.5 effective April 13, 1993
47.10.6 For the sequence of tests described in 47.10.1, if an abnormal operation test continues for 15
days without a winding or a protective device opening, the remaining tests need not be conducted. For
example, if the test described in 47.10.1(a) continues for 15 days, the tests described in (b) and (c) need
not be conducted.
47.10.6 effective April 13, 1993
47.10.7 To determine whether a transformer complies with the requirement in 47.10.1, one sample is to
be subjected to each condition described in 47.10.1(a) – (c). For a transformer that employs more than
one secondary winding, each of the secondary windings is to be loaded for each condition specified in
47.10.1 with the other windings loaded to rated current. The test conditions are to be as described in
47.10.8 – 47.10.13.
47.10.7 effective April 13, 1993
47.10.8 To determine the short-circuit current value for conducting the tests described in 47.10.1(b), the
transformer is to be at room temperature at the beginning of the measurement, and the short-circuit
current is to be measured approximately 1 min after the voltage is applied to the primary winding. A
protective device outside the transformer is to be short-circuited during the measurement of the
short-circuit current. If the line fuse or transformer winding opens within 1 minute after the application of
the primary voltage, the short-circuit current is considered to be that value recorded just before the line
fuse or winding opens. The short-circuit current of any one winding is to be measured with the other
secondary windings open-circuited.
47.10.8 effective April 13, 1993
47.10.9 For the loading conditions, a variable resistor is to be connected across the secondary winding.
The tests described in 47.10.1(a) – (c) are to be continued for 15 days unless a winding of the transformer
or a protective device opens in a shorter time. In conducting the tests described in 47.10.1(b), the variable
resistance load is to be adjusted to the required value as quickly as possible and readjusted, if necessary,
1 minute after voltage is applied to the primary winding.
Exception: For a switch-mode transformer, the load is to be connected to the output of the power supply
connected to the transformer.
47.10.9 effective April 13, 1993
47.10.10 If short-circuiting the secondary winding causes one of the windings to open before 15 days,
then the next test in the sequence described in 47.10.1(b) that continues for 15 days is to have a variable
load resistor reduced to zero impedance at the end of the 15 days.
47.10.10 effective April 13, 1993
DECEMBER 14, 1993 UL 458 65
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