A Guide to Miniature Circuit Breakers

Miniature Circuit Breakers (MCBs) are designed to protect the cable downstream of the device against overloads and short circuits, preventing damage to cables and equipment.
MCBs are installed in modern consumer units, and are a convenient alternative to fuses as they can be reset without having to replace wire with a simple flick of a switch or button.

There are 3 MCB types:

  • Type B
  • Type C
  • 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.
    An 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.
    MCB ratings relate to continuous service under specified installation conditions, although cables can carry higher currents for short periods without causing permanent damage.
    Selecting the right one should be done in accordance with BS 7671:Requirements for Electrical Installations (The IEE Regulations, Sixteenth Edition) and will ensure that the device will not give unwanted tripping.

    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 767

    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.

    Whichever MCB type is used, it is extremely dangerous to cut corners by using inferior quality devices, therefore they should only be bought from a reputable manufacturer.
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