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NOT AUTHORIZED FOR FURTHER REPRODUCTION OR
DISTRIBUTION WITHOUT PERMISSION FROM UL
28.1.2 With reference to Exception No. 1 to 28.1.1 and with reference to UL 840, reduced through-air
spacings are based on the use of insulating materials that resist arc tracking and the degree of conductive
pollution present in the environment of use. They are applicable to converters or inverters in which the
transient voltages are known and are controlled by a transient suppressive device and to converters or
inverters that withstand an impulse withstand voltage test. The spacing requirements of UL 840 shall not
be used for field-wiring terminals and spacings to a dead metal enclosure.
28.1.2 effective April 13, 1993
28.1.3 A converter or inverter shall be considered overvoltage category II and/or overvoltage category I
as defined in UL 840.
28.1.3 effective April 13, 1993
28.1.4 In order to apply Clearance B (controlled overvoltage) clearances as defined in UL 840, controlled
overvoltage shall be achieved by providing an overvoltage device or system as an integral part of the
product.
28.1.4 effective April 13, 1993
28.1.5 All printed-wiring boards are considered to have a minimum comparative tracking index of 100
without further investigation.
28.1.5 effective April 13, 1993
28.1.6 With reference to Exception No. 1 to 28.1.1 and the pollution degree levels described in UL 840,
pollution degree 2 is considered applicable to converters or inverters employing protected environment
enclosure as described in Section 7, Enclosures Used for Protected Environments. Pollution degree 3 is
considered applicable to converters or inverters intended for use in a general environment.
28.1.6 effective April 13, 1993
DECEMBER 14, 1993UL 45838
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