MCBs provide overcurrent and short-circuit protection only and are unable to detect residual current (earth leakage current) unless it is large enough to be classed as an overload or short circuit.

The most common MCB characteristics are Types B, C and D. What is the difference between type B,C,D MCB?

In consumer unit applications it is rare to see anything other than Type B MCBs used, with the occasional use of Type C devices on high inrush circuits such as some lighting loads. Let’s examine the most common MCB characteristics in a little more detail:

Type B MCBs react quickly to overloads, and are built to trip when the current passing through them is between 3 and 4.5 times the normal full load current. They are suitable for protecting incandescent lighting and socket-outlet circuits in domestic and commercial environments, where there is little risk of current surges of a magnitude that could cause the MCB to trip.

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 built to trip at between 5 and 10 times the normal full load current.

Type D MCBs are slower still, and are set to trip at between 10 and 20 times normal full load current. They are recommended only for circuits with very high inrush currents, such as those feeding transformers and welding machines. Note, however, that MCBs with Type K characteristics may provide better protection in some applications of this type.