Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 139)

Fig. 4.2-5
Max. cut-off current and max. forward (let-through) energy of strongly current limiting circuit breakers at a
rated operational voltage of 415 V
Life span of circuit breakers
IEC 60947-2 defines the number of switching operations that a circuit breaker has to perform
without load, at normal load, at overload or with a short-circuit. The values vary between two
breaks (O-t-CO) for the rated ultimate short-circuit breaking capacity and a couple of thousand
operations for purely mechanical switching without load.
The electrical life span (contact life span) of a circuit breaker like with contactors depends on the
size of the current to be broken. Small currents in the order of the rated current or the tripping
range of thermally delayed overload releases have a much smaller effect on the contact life than
short-circuit currents of the magnitude of the breaking capacity. The contacts may be so eroded
even after exposure to just a few high short-circuit currents that replacement of the circuit
breaker is required. The short-circuit currents that arise in practice are usually well below the
calculated maximum values and the switching capacity of the switches deployed. They therefore
cause less contact erosion.
Operational switching
In the lower power range, circuit breakers are also used to manually operate smaller – fre-
quently mobile – equipment and devices (for example milling machines, circular saws, sub-
mersible pumps). The electrical life of the switches is rarely used to the full at the low number of
operations typical in these applications. The circuit breakers with motor protection characteristic
replace the combination fuse, motor protective device and load switch.
Auxiliary contacts and displays
Auxiliary contacts enable the functional integration of the protective device in the control system.
ON, OFF, overload and/or short-circuit tripping can be signaled with the aid of the appropriate
auxiliary contacts. These auxiliary switches can be mounted on or inserted in the circuit breaker
and are either connected to terminals or connectable via loose wire ends.
In addition to auxiliary switches, circuit breakers are often equipped with visual indicators of the
state of operation and also often for the tripped state and the cause of tripping. These are
valuable aids for diagnosis on site during commissioning and fault rectification.
Shunt-trip and undervoltage releases
Shunt-trip releases enable remote circuit breaking by means of a control signal, for example for
electrical interlocking.
The undervoltage release switches the circuit breaker OFF when the voltage falls below a
(usually fix) certain level of the applied voltage and is used for example for detecting voltage
outages. They are in particular used as safety components, for example to prevent automatic
LVSAM-WP001A-EN-P - April 2009 4-31