Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 43)

- Rated service short-circuit interrupting capacity I
CS
:
I
CS
values are usually lower than the values for I
CU
. Circuit breakers that have been switch-
ing-off at the level of the service short-circuit breaking capacity continue to be serviceable
afterward. In plants in which interruptions to operations must be kept as short as possible,
product selection should be carried-out based on I
CS
.
- Breaking capacity of fuses
The same applies to fuses as to circuit breakers with respect to the I
CU
: at the given rated
operational voltage, the rated breaking capacity must be at least as large as the prospective
short-circuit current at the site of installation.
2.3.5 Thermal protection
Compliance with the permissible operational temperatures of electrical equipment is both a vital
safety factor as well as critical regarding its effective life span. The rate of ageing of plastics
increases exponentially with their operational temperature.
For all electrical equipment, limiting values for the load currents are defined, compliance with
which should be ensured by suitable protective devices and measures (fuses, overload relays,
temperature sensors).
2.3.5.1 Ambient temperature
Electrical equipment is designed for operation in defined temperature ranges. The upper
temperature limit is of special importance, because practically all electrical equipment dissipates
power and hence produces heat. The selection of the devices must consider the device’s
ambient temperature and the permitted load at this temperature.
The normal ambient temperature range under IEC 60947, IEC 60439 and IEC 60204 is –5 °C to
+40 °C with a 24-hour average that does not exceed +35 °C. It should be noted that the rated
values of the current load capacity unless otherwise stated are related to an ambient tempera-
ture of +40 °C. With other (higher) temperatures the loads should be reduced in accordance
with manufacturer specifications or larger devices should be chosen. For industrial switchgear,
loading specifications are often provided for an ambient temperature of +55 or +60 °C.
The lower limit of the operational temperature may be critical with electronic devices and it
should be assured by provision of heating that temperature does not fall below. In conjunction
with moisture (freezing), low temperatures can also adversely affect the operability of electro-
mechanical devices.
Rated operational values designated as “open” apply for devices used in free air, while values
designated “enclosed” apply for devices installed in an enclosure of small size specified by the
manufacturer. The reference ambient temperature for “open” is the temperature of the ambient
air of the device, even if this is installed in a box or cabinet. The reference ambient temperature
for “enclosed” is the air-temperature of the housing environment. The ambient temperature of
the device in the housing is higher because of the effect of its own heat dissipation. In practice
this means for example that for a contactor “open” at 60 °C I
th
= 20 A will be stated and “en-
closed” at 40 °C the same value, because due to heating in the housing the contactor is
subjected to the same immediate ambient temperature of 60 °C. At 40 °C “open” the same
contactor can for example conduct 25 A.
In switchgear, in which the temperature in the cabinet (see Software TRCS) is calculated or
measured, the data for “open”, that is in the immediate device environment (microclimate)
should be taken into account when selecting the devices. It should be ensured by temperature
monitoring and cooling measures that the actual temperature does not exceed the reference
value on which the component selection is based.
2.3.5.2 Operational overcurrents, heavy-duty starting
Operational overcurrents occur especially when motors are started. Switchgear such as
contactors or load switches should be rated so that it can cope with the regularly occurring
overcurrents without difficulty, assuming it has been selected in accordance with the corre-
sponding utilization category. Motor starts that cause normal motor protection relays of trip
LVSAM-WP001A-EN-P - April 2009 2-15