what are ABCD curves in MCBs and how we observe that curves?
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Answer Posted / janet roadway
There is no A curve type within the IEC 60898 product
standard. The standard curves are B, C and D
Type B :
MCBs are set to trip when the current passing
through them is between 3 and 5 times the normal full load
current. The calibration temperature of this full load
current can vary from manufacturer but is generally in the
region of 30 - 40 degrees Celcius to allow for the heat
generated by the MCB within a distribution board.
Type C :
MCBs react more slowly, and are recommended for
applications involving inductive loads with high inrush
currents, such as fluorescent lighting installations. Type C
MCBs are set to trip at between 5 and 10 times the normal
full load current. Temperature as above
Type D :
MCBs are slower still, and are set to trip at between
10 and 20 times the normal full load current. They are
recommended only for circuits with very high inrush
currents, such as those feeding transformers and welding
machines. Temperature as above.
Some manufacturers produce their own specialist curves -
such as the ABB 'K' Curve which are designed for specific
applications such as tranformer feeds and motors. These
curves can be very useful.
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