ANSI UL 458 - Insulation Barriers

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UL COPYRIGHTED MATERIAL –

NOT AUTHORIZED FOR FURTHER REPRODUCTION OR

DISTRIBUTION WITHOUT PERMISSION FROM UL

28.1.14 Spacings between uninsulated live parts of different potential and between such parts and dead

metal that are grounded in service are not speciﬁed for parts of isolated limited-energy circuits in

accordance with 2.12. Spacings in circuits that exceed 30 volts (42.4 peak) are to be investigated by the

applicable dielectric voltage-withstand test speciﬁed in Section 39, Dielectric Voltage-Withstand Test.

Exception: A meter complying with the requirements in the Standard for Electrical Analog Instruments –

Panelboard Type, UL 1437, need not be subjected to a dielectric voltage-withstand test.

28.1.14 revised September 11, 1998

28.1.15 All uninsulated live parts connected to different circuits shall be spaced from one another as if

they were parts of opposite polarity and shall be judged on the basis of the highest voltage involved.

28.1.16 The acceptability of spacings between live and dead metal parts connected to the enclosure

within an electrical instrument shall be determined by a dielectric voltage-withstand test in accordance with

39.1.

28.2 Insulation barriers

28.2.1 An insulating liner or barrier of material such as vulcanized ﬁber or thermoplastic employed in lieu

of required spacings mentioned in Exception No. 1 to 28.1.1 shall not be less than 0.028 inch (0.71 mm)

thick and shall be so located or of such material that it will not be adversely affected by arcing.

Exception No. 1: Vulcanized ﬁber not less than 0.013 inch (0.33 mm) thick may be used in conjunction

with an air spacing of not less than 50 percent of the minimum acceptable through-air spacing and

between a heat sink and a metal mounting surface, including the enclosure, of battery-charging circuits

rated 50 volts rms or less.

Exception No. 2: Mica not less than 0.006 inch (0.152 mm) thick may be used as insulation between a

heat sink and a live case of a semiconductor device.

Exception No. 3: An insulating material having a thickness less than that speciﬁed in 28.2.1 may be used

if, upon investigation, it is found to be acceptable for the particular application.

28.2.1 effective April 13, 1993

28.2.2 A wrap of thermoplastic tape, complying with the requirements in the Standard for Insulating Tape,

UL 510, may be used in lieu of required spacings if all the following conditions are met:

a) The wrap is no less than 0.013 inch (0.33 mm) thick, is applied in two or more layers, and is

used with no less than half the required through air spacing.

b) The wrap is no less than 0.028 inch (0.71 mm) thick when used with less than half the required

through-air spacing.

c) Its temperature rating is no less than the maximum temperature observed during the

Temperature Test, Section 37.

d) The tape is not subject to compression.

e) The tape is not wrapped over a sharp edge.

28.2.2 effective April 13, 1993

DECEMBER 14, 1993UL 45842