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16 Insulating Materials
16.1 An insulating washer, a bushing, or the like, and a base or a support for mounting a live part, shall
be moisture-resistant material that will not be adversely affected by the temperatures and stresses to
which it will be subjected under conditions of use.
16.2 Insulating material is to be judged with respect to its acceptability for the particular application.
Materials such as mica, ceramic, and some molded compounds are usually acceptable for the sole
support of live parts. If an investigation is necessary to determine whether a material is acceptable,
consideration is to be given to:
a) Its mechanical strength, resistance to ignition, dielectric strength, insulation resistance, and
heat-resistance qualities in both the aged and unaged conditions;
b) The degree to which it is enclosed; and
c) Any other feature affecting the risk of fire, electric shock, or injury to persons.
All factors are to be considered with respect to conditions of service.
17 Transformers
Section 17 effective April 13, 1993
17.1 General
17.1.1 A transformer used to supply a Class 2 circuit, that extends from the converter or inverter to a
remote panel, status panel or the like shall have its primary windings electrically isolated from all other
windings and shall be constructed as specified in 17.1.2 – 17.2.3 so that there is no electrical connection
– under normal and overload conditions – between the primary and secondary windings, between the
primary winding and the core, or between separate adjacent secondary windings, if such connection may
result in a risk of fire or electric shock.
17.1.2 A transformer coil, unless inherently moisture resistant, shall be treated with an insulating varnish,
and baked; or otherwise impregnated to exclude moisture or acid vapor. Film-coated magnet wire is
considered moisture resistant.
17.1.3 A thermal cutoff or other device employed to reduce the risk of fire or electric shock due to
overheating of a transformer during abnormal operation shall comply with the requirements applicable to
such a device in addition to the applicable requirements in this standard. For example, a thermal cutoff
shall comply with the applicable requirements in this standard and those in the Standard for Thermal
Cutoffs for Use in Electrical Appliances and Components, UL 1020.
17.2 Coil insulation
17.2.1 A transformer winding, including the start, all taps, finish, and crossover leads up to the point
where insulated leads, if used, are provided, shall be constructed as specified in Table 17.1.
17.2.2 Insulating material, such as outer-wrap and crossover-lead insulation, employed to reduce the risk
of live parts from becoming accessible through openings in the outer enclosure in accordance with
Accessibility of Uninsulated Live Parts, Film-Coated Wire, and Moving Parts, Section 8, shall comply with
Note a or c of Table 17.1.
DECEMBER 14, 1993 UL 458 31
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