GAMBICA’S Controlgear Group
Technical committee has welcomed
publication of IEC 61439, a new
series of international standards for
low-voltage switchgear and
controlgear assemblies, which will be
implemented in the UK as BS EN
61439. GAMBICA warns, however,
that manufacturers working to the
new standard still need to exercise
skill and caution in relation to the
ratings they adopt for components
used in their assemblies.

The nominal current rating of
circuit breakers and other components
relate to tests carried out in ‘free air’.

The component is mounted on an
open framework with air space all
around it, which helps to dissipate
excess heat, very different from the
conditions in which the component
will operate. As a result, substantial
derating may be needed to ensure safe
operation and to comply with the
temperature rise requirements of IEC
61439. Nominal component ratings
should be considered only as a starting
point for making sound engineering
decisions on PSC design and
construction.

Reputable manufacturers will be
aware of these issues, and will willingly
undertake testing to determine
ratings that are appropriate to the use
of particular components in their
enclosure systems. Alternatively, the
manufacturers may choose to adopt
another approach permitted by IEC
61439, which relies on calculations
that incorporate generous safety
margins.