An MCB, which reacts instantaneously, would trip every time such a peak occurred, which would make it unusable. Fortunately, the thermal element in MCBs does not react instantaneously, as the bi-metal strip takes time to heat up. It is, therefore, hardly affected by short-term current peaks. By changing the design of the bimetal elements, MCB manufacturers can determine what size of peak current a particular MCB will ignore, and for what length of time. This relationship between current and tripping time is usually shown as a curve, known as the MCB's trip characteristic.
To avoid the need for users to work with the curves, BS EN 60898 defines several types of standard characteristic, the most important of which are Types B, C and D, all of which are included in Moeller FAZ range. In most cases, it's easy to choose one of these types to match the application in hand, and only in really specialised applications will users need to work with the full characteristic curves.
Type B MCBs react quickly to overloads, and are set 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 surges 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 set 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 the normal full load current. They are recommended only for circuits with very high inrush currents, such as those feeding transformers and welding machines.
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