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17.2.3 A flanged bobbin-wound transformer shall be constructed so as to maintain physical separation
between the primary and secondary windings. Physical separation may be accomplished by employing a
3-flange bobbin for winding the primary and secondary windings adjacent to each other. As an alternative,
a telescoping bobbin construction, with each section containing an individual winding may be used where
the primary winding is wound over the secondary winding, or the secondary winding is wound over the
primary winding. The bobbin insulation shall comply with Notes a, b, c or d of Table 17.1.
Exception: A 2-flange bobbin having the primary winding wound over the secondary winding, or the
secondary winding wound over the primary winding with the primary winding insulated from the secondary
winding by means of tape insulation is acceptable if:
a) The tape insulation complies with Note a or c of Table 17.1,
b) The tape insulation provides a continuous overlap on the bobbin flanges,
c) The transformer complies with the tests described in the flanged bobbin transformer
abnormal operation test as specified in 47.10.1 – 47.10.13.
d) The transformer complies with the induced potential tests specified in 40.1 – 40.4.
17.2.3 revised October 15, 2004
17.2.4 With reference to (c) of the Exception to 17.2.3, the flanged bobbin transformer abnormal
operation test is not required if the transformer is supplied from a low-voltage, limited energy circuit in
accordance with 2.14 or a limited energy circuit in accordance with 2.12.
OCTOBER 15, 2004UL 45832
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