Rockwell Automation Low-Voltage Switchgear and Controlgear User Manual
Page 138
voltages below 400 V are thereby uncritical). Use at for example 690 V may therefore only be
possible with reduced switching capacity. The performance data for the specified operational
voltage should be respected.
Circuit breakers must be capable to control the largest possible short-circuit current at the point
of installation at the given operational voltage. Intrinsically short-circuit proof circuit breakers
(Section
) can be used in supplies of any magnitude of short-circuit current, as their
internal impedance limits the short-circuit current to the switching capacity of the switch (or
below). If the switching capacity of the circuit breaker is smaller than required, then a backup
protection must be provided (fuse or circuit breaker connected in series). The required switching
capacity must be ensured in conjunction with the backup protection device. The sizing of the
backup protection can be obtained from the product documentation.
Ultimate switching capacity and service switching capacity
IEC 60947-2 makes distinction between the rated ultimate short-circuit breaking capacity I
CU
and the rated service short-circuit breaking capacity I
CS
:
- Rated ultimate short-circuit breaking capacity I
CU
The test sequence is O-t-CO
Circuit breakers that have operated at the level of the ultimate short-circuit breaking capacity
are only limited serviceable afterward. There may be changes in the overload tripping char-
acteristic and increased temperature rises as a consequence of erosion of contact material.
- Rated service short-circuit breaking capacity I
CS
The test sequence is O-t-CO-t-CO
Circuit breakers that have operated at the level of the service short-circuit breaking capacity
are further serviceable afterward.
O
breaking the short-circuit from the closed state
t time
interval
CO switching onto the short-circuit followed by breaking it
Copyright © IEC, Geneva, Switzerland. www.iec.ch
The ratings of circuit breakers for I
CU
are usually higher than for I
CS
. The majority of circuit
breakers is therefore (for cost reasons) selected according to I
CU
. In plants in which down-time
must be kept as short as possible, product selection should be based on I
CS
.
After a short-circuit has been broken, it is generally recommended to examine the device to
make sure it is fully functional.
Let-through values
The essential quality attributes with respect to good short-circuit protection are the let-through
values (
). The magnitude of the cut-off current and let-through energy in relation to the
prospective short-circuit current I
cp
provide information about the quality of current limitation by
the switch. They show the extent to which downstream devices such as contactors or switches
are stressed in the event of a short-circuit. The let-through values directly affect the sizing of
these series-connected devices – for example short-circuit coordination type 2 without oversized
contactors – and determine the constructional design of the installation.
LVSAM-WP001A-EN-P - April 2009
4-30