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3
Selection and application
3-1 Selection check points
3-1-2 Selecting and MCCB ratings
(1) Rated ultimate short-circuit breaking capacity
(Icu)
A breaker must be selected that has a rated ultimate short-
circuit breaking capacity (Icu) higher than the short-circuit
current that passes through it. The short-circuit current will
vary with transformer capacity as well as with the impedance
between the load and the MCCB.
Since a breaker should protect the load-end terminal and
protect against failures that occur near that terminal, it should
have a breaking capacity that is higher than the short-circuit
current at the load-end terminal.
(2) Rated current
The rated current of an MCCB is the maximum current
that can be continuously flowed through the MCCB without
problems, and should be higher than the maximum load
current expected in the circuit.
Select an MCCB with a rating that can carry a load current,
including transient currents, such as motor starting current,
and that can protect the cable and equipment from the
overcurrent.
The load current must not exceed the derated current
value when the MCCB is derated according to the following
environmental factors.
1. Effects of ambient temperature
MCCB performance conforms to the standard operating
conditions stipulated in IEC 60947-2. (For further details,
see Table 4-1.)
When the ambient temperature exceeds standard operating
conditions (−5 to 40°C), you must select an MCCB that
allows less load current to pass through the breaker.
2. Difference between the nominal rated current of the load
equipment and its actual value
3. Increase in the load current resulting from supply voltage
variations
4. Frequency variations (including waveform distortion)
5. Other
(3) Rated frequency
MCCBs for AC application are rated for operation at both 50
and 60Hz. If these MCCBs are used in circuits having other
frequencies, their operating performance, current carrying
capability, or breaking characteristics may be altered, and prior
verification is required. (Refer to page 80 3-12.)
When MCCBs are to be used for DC circuits, it is important
to ensure that the MCCBs are marked with “Acceptable DC
circuits.” (Refer to page 81 3-13.)
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