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Note that references are to IEC 60950 Third Edition 1999-04. Where the references in the Second Edition differ, they appear in parentheses.

REQUIREMENTS

This table grades the various flammability classes by their levels of flame resistance.

As a default, all materials used in the construction of a product must have a flammability class of V-2 or better.

Small components mounted on printed circuit boards of flammability class V-1 or better are exempt from flammability requirements.

Components within a metal enclosure of 0.06 cu.m. volume or less, and having no ventilation openings are exempt from flammability requirements.

Components, which are separated by at least 13 mm of air from electrical parts which are likely to cause ignition under fault conditions, have reduced requirements.

PVC, TFE, PTFE, FEP and neoprene insulation on wiring is exempt.

Certain other exemptions or reduced requirements are given in clauses 4.7.3.3, 4.7.3.4 and 4.7.3.5 (4.4.3.3, 4.4.3.4, 4.4.3.5, 4.4.3.6 and 4.4.4) of the standard.

The flammability requirements of an enclosure depend on whether it is regarded as a fire enclosure . A fire enclosure is necessary as 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.1

Clause 4.7.3.2 (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.

DESIGN TIPS

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.

General design principles for limiting fire risk include:

• Limit the energy available that can cause high temperatures and the risk of fire, e.g. by the use of fuses.
• Limit as far as possible the amount of combustible material in the product.
• Use materials with suitable flammability ratings.
• Limit the spread of fire by the use of a fire enclosure, and suitable mechanical design of the enclosure .

Complete requirements are given in IEC 60950, clauses 4.7.2 and 4.7.3 (4.4.3.2, 4.4.3.3, 4.4.3.4, 4.4.3.5, 4.4.3.6 and 4.4.4).