ANSI UL 458 - Spacings; General

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27.2 A resistor, capacitor, inductor, or other part that is mounted on a printed-circuit board to form a

printed-circuit assembly shall be secured so that it cannot be displaced to result in a risk of electric shock

or ﬁre by a force likely to be exerted on it during assembly, normal operation, or servicing of the converter

or inverter.

27.2 effective April 13, 1993

27.3 Consideration is to be given to a barrier or partition that is part of the converter assembly and that

provides mechanical protection and electrical insulation for a component connected to a printed-circuit

board.

28 Spacings

28.1 General

28.1.1 General environment spacings shall be as speciﬁed in Table 28.1. Protected environment

spacings – see 7.1 – 7.5 – shall be as speciﬁed in Table 28.2. For the purpose of this requirement, a

general environment is considered to be an environment other than a protected environment.

Exception No. 1: Converters or inverters complying with the requirements in the Standard for Insulation

Coordination Including Clearances and Creepage Distances for Electrical Equipment, UL 840, may have

spacings less than that speciﬁed in Tables 28.1 and 28.2. See 28.1.2 and 28.1.3.

Exception No. 2: Spacings may be as provided in 28.2.1 if liners and barriers are used.

Exception No. 3: Spacings may be as provided in 28.1.2, between adjacent foils on printed wiring boards

provided with a conformal coating complying with the requirements in the Standard for Polymeric Materials

– Use in Electrical Equipment Evaluations, UL 746C. See 28.1.2.

Exception No. 4: On printed wiring boards having a ﬂammability classiﬁcation of 94V-0 and constructed

from a base material having a minimum Comparative Tracking Index (CTI) rating of 100 and 175 volts for

protected and general environments respectively, spacings (other than to ground, between primary and

secondary circuits, and at ﬁeld wiring terminals) are not speciﬁed between traces of different potential

connected in the same circuit if the spacings comply with the requirements in Section 48, or an analysis

of the circuit indicates that no more than 12.5 milliamperes of current will ﬂow between short-circuited

traces having reduced spacings.

Exception No. 5: For multilayer printed wiring boards, the minimum spacing between adjacent internal foils

of opposite polarity and between an internal foil and a plated-through hole is 1/32 inch. If these foils are

in circuit described in 13.3 – 13.9, no spacing is speciﬁed.

Exception No. 6: The spacing requirements in Tables 28.1 and 28.2 may not apply to inherent spacings

of a component such as a switch, lampholder, or a motor. Such spacings are to comply with the

requirements for the component in question if the spacings are less than the values speciﬁed in Tables

28.1 and 28.2. Spacings from such components to another component and to the enclosure shall comply

with the appropriate spacings speciﬁed in this Standard.

Exception No. 7: Spacings between adjacent terminals of a power switching semiconductor device

including the connection points of the terminals of the device are not speciﬁed.

28.1.1 effective April 13, 1993

DECEMBER 14, 1993 UL 458 37