Note that references are to IEC 60950 Third Edition 1999-04.
Where the references in the Second Edition differ, they appear in parentheses.
REQUIREMENTS The materials used for the construction of the enclosure
must be such that the enclosure complies with the mechanical strength and flammability requirements of the standard. The flammability requirements of the enclosure depend on whether it is regarded as a fire enclosure. Whether a fire
enclosure is necessary is defined in clause 4.7.2 (4.4.5) of the standard and is dependent on the types of components within the enclosure. In the majority of cases, the enclosure
does need to be considered a fire enclosure
. Exceptions are listed in clause 4.7.2.2 (4.4.5.2).Enclosures made of metals, ceramic materials and heat-tempered, wired or laminated glass are considered to
comply with the flammability requirements for fire enclosures. Plastic materials of
flammability class HB or better must be used for enclosures which are not considered fire enclosures. Small external decorative parts, which would
contribute negligible fuel to a fire, are exempt.Plastic fire enclosures for moveable equipment weighing up to 18 kg must have flammability class
V-1 or better or must pass the flammability test of clause A.2.Plastic fire enclosures for moveable equipment weighing more than 18 kg must have flammability class 5V or better
or must pass the flammability test of clause A.1Clause 4.7.3.6 (4.4.4) gives more stringent requirements for parts of enclosures which are located within 13 mm of
arcing parts, or parts that are liable to ignite the enclosure in normal or abnormal operation. For the US, combustible materials having an exposed area of greater than 0.9 m2
or a single dimension greater than 1.8 m must be previously evaluated to UL 723 with a flame spread rating of 50 or less for computer room applications,
or 200 or less for other applications. These limits refer to an exposed surface area on a single unbroken section. If two sides of a single piece are exposed, only the larger side is
to be considered in computing the area. Previously evaluated plastics can be found in the UL Recognized Plastic Directory.
DESIGN TIP The most straightforward route to compliance is to use a plastic material which has a UL approval to the appropriate flammability class. The UL approval is generally also
accepted by European Test Houses. Materials which are not UL approved will probably need to be tested within the framework of your product's test.
REFERENCES Complete requirements are given in IEC 60950, clauses 4.6.2 and 4.7 (4.4.5, 4.4.6 and 4.4.7).
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