Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 170)

terminal have a relative short open length over which heat can be dissipated and they mutually
heat each other in the duct.
6.1.4.7 Operating frequency and harmonics
All normal technical data and tests relate to the normal supply frequency of 50/60 Hz. At higher
frequencies additional losses occur that adversely affect the loss balance or reduce the load
capacity of the devices. See Section
2.4.3 .
6.1.4.8 Mounting devices side-by-side
In real-life switchgear assemblies, the switchgear devices are very often placed in rows side-by-
side. Circulation of the ambient air between the devices is then not possible and as a conse-
quence the rate of cooling of devices in comparison to the standard test conditions is reduced
(see
Fig. 6.1-5 ). Where this results in an inadmissible temperature rise, then a reduction in load
capacity will occur.
In practice, adjacent devices are frequently not loaded at the same time or the devices are
operated with currents that are well below the conventional thermal current in open air (I
th
). In
such cases, adjacent placement of devices is permissible with respect to temperature-rise.
Care is required when operating adjacent devices close to the I
th
and in case of a combination
of adverse factors with respect to the heating as described above. In such cases, spacing
between the devices is recommended in order to reduce mutual heating. Often instructions are
included in the manufacturer’s information – for example with the dimensional drawings
(catalog, packaging, application recommendations). To avoid hotspots, circulation of the air in
switchgear assemblies is advantageous.
6.1.4.9 Mounting position
In manufacturers documentation there are specifications with respect to the permissible
mounting positions and in the case of installations differing from the normal positions the
corresponding influence on the operational parameters. With respect to heating effects it should
also be remembered that heat dissipation within the devices is not evenly distributed, but is
concentrated on specific components, for example, the bimetal strips of circuit breakers or
motor protection relays. With a mounting position that differs from normal, the mutual effect on
adjacent devices can also change.
6.1.5 Thermal imaging cameras
Thermal imaging cameras are increasingly being used to examine heat formation in switchgear
assemblies. They are a very useful tool for recognizing critical spots, but there is also a certain
risk of incorrect interpretation of measurements. Thus an apparently high measured surface
temperature can be caused by radiation from hot internal components, although in fact – for
example when measured with thermocouples – the hot-spot is actually below the surface
.
Incorrect measurements are possible due to the different emission factors of the various
materials used. It is a good idea to get the advice of a competent specialist if apparently
excessive high temperatures are measured.
LVSAM-WP001A-EN-P - April 2009 6-6