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47.9 Blanketing test
47.9.1 A converter or inverter that incorporates standoffs is to be subjected to a blanketing test. A double
layer of blanket – see 47.9.3 – is to be loosely draped over the top and drawn in as close as possible
against all sides of the converter or inverter. The blanket may fall or sag between any standoffs that are
provided on the converter or inverter. The test is to be continued until the overload protective device
opens – see 20.2.1. If an automatically reset protector is provided, the test is to be continued for 15 days.
A manually reset protector is to be operated for 50 cycles of operation, with the protective device being
reset as rapidly as possible. The protector shall be operative upon completion of the test.
Exception: The test may be conducted for 1 cycle if protection is provided by a fuse that is not accessible
without the use of a tool.
47.9.1 effective April 13, 1993
47.9.2 Accessible secondary-circuit overcurrent-protective devices shall be shunted during the test, if
they would open the circuit prior to opening of an internal protective device.
47.9.3 With reference to 47.9.1, the blanket is to be 100 percent unbleached plain weave napped cotton
flannelette weighing 4.40 ounces per square yard (148.4 grams per square meter).
47.10 Flanged bobbin transformer abnormal test
47.10.1 A flanged bobbin transformer required to be tested as provided in (c) of the Exception to 17.2.3
– also see 17.2.4 – shall operate for 15 days with the secondary winding or windings loaded to the
conditions described in (a) – (c). A risk of fire or electric shock shall not result from:
a) Short-circuiting the secondary winding;
b) Loading the secondary winding to a current equal to the maximum normal current plus X
percent of the difference between the short-circuit current and the rated current – where X equals
75, 50, 25, 20, 15, 10 and 5, respectively; and
c) Loading the secondary winding to the maximum normal current.
Exception: A flanged bobbin transformer used in a circuit where isolation is not required or where the
secondary circuit does not extend out of the converter or inverter – see 17.1.1 – need not be subjected
to this test.
47.10.1 effective April 13, 1993
47.10.2 A risk of fire as described in 47.10.1 is considered to exist if the cheesecloth or tissue paper
glows or flames or the protective device, if provided, opens.
47.10.2 effective April 13, 1993
47.10.3 While still in a heated condition from the test described in 47.10.1, a transformer shall withstand
the dielectric voltage withstand test described in 39.1(a)(1). The dielectric voltage withstand test potential
is to be applied to the transformer approximately 1 minute after completion of the test described in
47.10.1.
47.10.3 effective April 13, 1993
DECEMBER 14, 1993UL 45864
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