Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 134)

pulled out of the contacts with this cover for replacement. This means that the circuit can be
made and broken under load.
A further development of the above is the “switch-disconnector-fuse” combination. To make the
replacement of fuse-links even safer, they are first isolated from the voltage on both sides. This
means that neither return voltages nor the direction of power supply have to be taken into
account by the user. For reasons of space economy, in most cases busbar designs are used.
4.2.2 Circuit breakers
4.2.2.1 Principle of operation and design
The circuit breaker is a mechanical switching device, capable of making, carrying and breaking
currents under normal circuit conditions and also making, carrying for a specified time and
breaking currents under specified abnormal conditions such as those of short-circuit (IEC
60947-1).
Circuit breakers have the capacity to break short-circuits. They are classified according to their
breaking capacity, their design and their capability to limit short-circuit currents. They are
classified according to the following groups:
ƒ Circuit breakers that clear at current zero
ƒ Current-limiting circuit breakers
The devices of both groups can be further subdivided according to design:
ƒ Miniature Circuit Breakers (MCB)
Single pole or modular multipole circuit breakers for up to around 100 A rated current for line
protection with or without residual current release for installation applications
ƒ Moulded Case Circuit Breakers (MCCB)
Circuit breakers with a housing of insulating material that forms an integral part of the circuit
breaker (rated currents typically up to around 1600 A)
ƒ Air Circuit Breakers (ACB)
Large installation switches with open design (rated currents typically 300 … > 3000 A)
4.2.2.2 Standards, functions and utilization categories
4.2.2.2.1 Standards
The standards that are applicable for circuit breakers are
ƒ IEC 60947 – 1 (Low-voltage Switchgear, General Requirements) and
ƒ IEC 60947 – 2 (Circuit Breakers).
For use in North America, the standards and approvals under UL 489 or CSA 22.2 apply.
Devices that are not approved under these standards are not recognized or approved as circuit
breakers in North America.
For circuit breakers with motor protection function IEC 60947-4-1 also applies. Such circuit
breakers can be used in North America under UL 508 under certain conditions (construction
type E). For details with respect to applications in North America, see UL-WP001A-EN-P [15].
4.2.2.2.2 Functions and utilization categories
A circuit breaker basically consists of an actuation device (manual or – optionally – remote
controlled), usually an (thermal or electronic) overcurrent release, an electromagnetic short-
circuit release, a tripping mechanism with spring storage (switch latch), the main contact system
and optional auxiliary contacts. The combination of these functions in one unit means that
installations can be made compact and that circuit breakers can be integrated in the automation
environment. Thus modern starter combinations consist of only two components – a circuit
breaker with motor protection characteristic and a contactor.
Circuit breakers offer functions such as:
ƒ Short-circuit
protection
ƒ Line, load (motor), plant protection
LVSAM-WP001A-EN-P - April 2009 4-26