Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 137)

4.2.2.3.2 Electromagnetic overcurrent releases
In circuit breakers with motor protection characteristic overcurrents from a value of
10 ... 16 times the upper scale setting immediately cause the electromagnetic overcurrent
release to act. High efficiency motors may require higher magnetic trip levels (see
1.7.1.2.1 ).
The precise tripping value is either adjustable (matching for selectivity or various making current
peaks in case of transformer and generator protection) or is determined by the design. In circuit
breakers for plant and line protection the tripping zone is lower.
In small circuit breakers (usually < 100 A), the pole conductor is shaped in the form of a small
coil. If a high overcurrent flows through these coils, a force acts on the armature enclosed by the
coil. This armature unlocks the loaded switch latch that releases the stored spring energy and
hence opens the main contacts and disconnects the overcurrent.
Plunger for high current-limiting circuit breakers
Current-limiting circuit breakers limit the fault current and hence reduce the mechanical and
thermal stress in the event of a fault (see Section
4.1.3.3.2 ). Circuit breakers with rated currents
up to around 100 A are offered for the rapid disconnection of the short-circuit current with a
plunger system, that in the event of a short-circuit additionally forces the main contacts open
and hence supports extremely short break times (
Fig. 4.2-4 ).
An alternative to the plunger system at larger rated currents is the slot motor that opens the
contacts very fast, largely by means of electrodynamic forces.
The faster it breaks, the less energy has to be managed in the switch and the more compact the
circuit breaker can be. This means that this is prerequisite for circuit breakers to be built with
compact external dimensions.
Fig. 4.2-4
The contacts of a high current-limiting circuit breaker are forced open in the event of a short-circuit by a
plunger and the current is directed immediately to the arcing chambers. The circuit is such broken even
while the current is still rising.
4.2.2.3.3 Main contact system and switching capacity
The requirements on a circuit breaker main contact are a high making capacity, high breaking
capacity, low heat dissipation at operational current, low contact erosion, small inertia and
optimum shape for a favorable movement of the electric arc. The switching arc should quickly
be directed out from the area between the contact surfaces, cooled, divided, extended and thus
extinguished. The de-ion plates must form a functional unit with the main contact with respect to
shape and arrangement.
In order to optimally fulfill these high requirements, the very highest demands are placed on the
design and materials and not least on the simulation and testing techniques.
Contact systems are designed to produce optimum switching performance at the main rated
voltage. The number of de-ion plates is critical for the electric arc voltage during circuit breaking
and hence for the switching capacity and current limitation. For example a contact system
designed for 400 V has a reduced switching capacity at supply voltages above 400 V (supply
LVSAM-WP001A-EN-P - April 2009 4-29