MCB works by tripping when a circuit is overloaded or when a short circuit has occurred in the system, and has a current rating such as 6A, or 10A depending upon its intended use, i.e. for residential, commercial, industrial or public buildings.
There are 3 MCB types, Type B, Type C and Type D, and the speed at which they trip depends upon the level of overload, and is usually determined by a thermal device within the MCB.
All 3 MCB types use a magnetic fault protection, which trips the MCB within one tenth of a second when the overload reaches a set level.
- Type B trips between 3 and 5 time full load current;
- Type C trips between 5 and 10 times full load current; and
- Type D trips between 10 and 20 times full load current.
An MCB’s circuit rating is given in Kiloamps (KA), and this indicates the level of its ability to work. For example a domestic MCB would normally have a 6KA fault level, whereas one used in an industrial application may need a unit with a 10KA fault capability.
Type B devices are commonly used in domestic systems and light commercial applications where surges are low, for instance where inrush currents may come from a small number of fluorescent fittings.
However, unwanted tripping can occur due to high arcing currents, often due to poor quality lamps, and in this case either a higher rating B type MRC should be used, or a type C device may be more suitable instead. Again, the MCB type used should be selected in accordance with BS 7671.
Type C MCBs are most suitable for commercial and industrial use, where there are motors and perhaps a high number of fluorescent fittings which, when switched off together may cause a high inrush current. Whether an upgrade to a Type D is suitable should be decided according to operating times and consideration of Regulation 413–02–08.
Type D units are for more specialist industrial use, where current inrushes can be high, for example with X–ray machines and transformers. They may require a lower earth loop impedance (Zs) to achieve the operating times required.