Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 167)

The real application conditions often differ from test conditions. Devices are usually closely
mounted next to each other and connected with short conductors. Often the conductors of
several circuits are routed closely together so that they compound the heating effect. In addition
the devices are usually installed in a housing, the interior of which reaches temperature above
the external ambient. It should be noted that the normal ambient temperature range for the
devices is identical with the normal ambient temperature range for switchgear assemblies.
Current source
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The practical conditions with respect to the connections (example on right) differ considerably from the
arrangements at type testing.
It is the responsibility of the manufacturer of a switchgear assembly or of a control system to
ensure that the prescribed temperature limits are respected under practical conditions. Special
attention should be paid to devices that are operated close to the thermal continuous current, in
particular to circuit breakers and thermal overload relays. At utilization categories such as AC-3
and AC-4, where the switching capacity of the devices is the most significant selection consid-
eration, heating by the continuous operational current is usually less critical.
6.1.3 Verification of temperature-rise
For control systems and switchgear assemblies, verification of temperature-rise is generally
required. This can be based on measurements (for example with series-produced devices and
systems and modular equipment), or on calculation, or be derived from measured systems. For
calculating the temperature-rise of switchgear assemblies, IEC 60890 provides a method that is
relatively straightforward for determining the over-temperature in an enclosure if the heat
dissipation inside is known. In addition some conditions must be fulfilled, for example a largely
uniform distribution of heat sources in the cabinet.
Rockwell Automation has created a very useful tool for calculating the temperature-rise in
enclosures and switching cabinets in the form of TRCS (Temperature Rise Calculation Soft-
ware) based on IEC 60890. TRCS also enables very efficient determination of heat dissipation
by components and conductors in the assembly on the basis of their operational data. Experi-
ence shows that the contribution of conductors to temperature-rise is significant (up to 50 %), if
they are operated close to the rated current and hence to the rated insulation temperature (for
example 70 °C).
6.1.4 Important aspects regarding device temperature rise;
Recommendations
6.1.4.1 Rated current
For many low-voltage components (for example circuit breakers, load switches, contactors,
fuses, conductors), ohmic losses are the main sources of losses. They are proportional to the
square of the operational current. The r.m.s. value is definitive. Under variable load conditions
(for example intermittent operation) the r.m.s. value can be averaged over time, if the cycle time
is shorter that the heating time constants of devices. In the power range up to around 40 A the
permissible integration time (= cycle time) is around 15 … 20 minutes.
LVSAM-WP001A-EN-P - April 2009 6-3