Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 143)

4.2.3 Miniature Circuit Breakers MCB
4.2.3.1 Principle of operation and design
Miniature circuit breakers are primarily designed to protect cables and lines against overload
(thermal) and short-circuit (electromagnetic). They thus care for protecting this electrical
equipment against excessive temperature rises and destruction in the event of a short-circuit.
Miniature circuit breakers are used in distribution networks in homes and in industrial applica-
tions. They meet the requirements for different applications by various designs and with the aid
of a comprehensive range of accessories (for example auxiliary and signal contacts etc.).
The structural shape of all line protection switches is similar. Certain dimensions are defined by
the installation standards (in some cases national). The major differences lie in the widths (for
example 12.5 and 17.5 mm) or depths (for example 68 and 92.5 mm). The breaking capacity is
one of the factors that determine the size.
4.2.3.2 Standards, tripping characteristics and rated switching capacity
MCB’s are subject to international and national norms. The design and test requirements are
defined in the standard IEC 60898. For the various applications three trip characteristics B, C
and D are defined in IEC 60898 (
Fig. 4.2-9 ):
Fig. 4.2-9
The tripping characteristics B, C and D under IEC 60898 are distinguished by the trip level of the short-
circuit trigger
- Trip
characteristic B is the standard characteristic for wall outlet circuits in domestic and utility build
ings (I> ≥3 … 5*I
e
)
- Trip characteristic C is advantageous when using electrical equipment with higher inrush currents
as for example of lamps and motors (I> ≥5 … 10*I
e
)
- Trip characteristic D is adapted to electrical equipment that can produce strong current surges
such as transformers, electromagnetic valves or capacitors (I> ≥10 … 20*I
e
)
AC miniature circuit breakers are normally suitable for single- and three-phase supplies up to a
rated voltage of 240/415 V and AC-DC MCB’s additionally for direct voltage supplies up to rated
voltages of 125 V, 220 V or 440 V depending on the number of poles.
In addition to the quality of releasing according to the tripping characteristic, a key feature of
MCB’s is their rated switching capacity. They are assigned to switching capacity classes, which
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