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Chapter 12 Helpful information
There are three main types of MCBs:
Type B - trips between 3 and 5 times full load current.
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Type C - trips between 5 and 10 times full load current.
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Type D - trips between 10 and 20 times full load current.
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Standard current ratings
Circuit breakers are manufactured in standard sizes, using a system of preferred numbers to
cover a range of ratings. Miniature circuit breakers have a fixed trip setting; changing the
operating current value requires changing the whole circuit breaker. Larger circuit breakers can
have adjustable trip settings, allowing standardized elements to be applied but with a setting
intended to improve protection. For example, a circuit breaker with a 400 ampere "frame size"
might have its over-current detection set to operate at only 300 amperes, to protect a feeder
cable.
International Standards, IEC 60898-1 and European Standard EN 60898-1, define the rated
current I
of a circuit breaker for low voltage distribution applications as the maximum current
n
that the breaker is designed to carry continuously (at an ambient air temperature of 30 °C). The
commonly available preferred values for the rated current are 6 A, 10 A, 13 A, 16 A, 20 A, 25
A, 32 A, 40 A, 50 A, 63 A, 80 A, 100 A, and 125 A. The circuit breaker is labelled with the
rated current in amperes, but excluding the unit symbol, A. Instead, the ampere figure is
preceded by a letter, B, C, or D, which indicates the instantaneous tripping current - that is, the
minimum value of current that causes the circuit breaker to trip without intentional time delay
(i.e., in less than 100 ms), expressed in terms of I
For the purpose of this section, only Type D are explained.
200
.
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