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Basics of Miniature Circuit Breaker
Aug 28, 2015

Basics of Miniature Circuit Breaker

Miniature Circuit Breakers (MCBs) are designed to protect the cable downstream of the device against overloads and short circuits, preventing damage to cables and equipments.

The other low voltage circuit breaker types are : Molded Case Circuit Breakers, Motor Protection Circuit Breakers, Earth Leakage Circuit Breaker, and Air Circuit Breaker.

MCB Operation and characteristics

An MCB is a thermo-magnetic device, having two methods of circuit interruption. A thermal mechanism, usually a bi-metallic strip, provides protection against moderate overcurrent. The heating action of the current causes the bi-metallic strip to curve and break circuit contact. This method is complemented by a solenoid designed to respond to larger currents.

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.

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 miniature circuit breaker 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 and is generally used for lighting and domestic applications.
  • Type C trips between 5 and 10 times full load current and is generally used for industrial and commercial applications.
  • Type D trips between 10 and 20 times full load current and is generally used for specialist industrial use with high inrush currents.

(Inrush current is the current that a load draws when the supply is switched on.Values can range from being insignificant (a few times the normal current), 5 to 10 times normal current for iron core transformers (e.g. conventional ballast fluorescent luminaires) and up to 20 times normal current for much modern electronic equipment, including the power supplies found in user equipment.)

Breaking Capacity

The breaking capacity of an miniature circuit breaker is referred to as the current which it is able to interrupt which getting damaged or causing electric arc with unacceptable duration. It is measured in kA. The generally used breaking capacities are 6kA and 10kA.

The basic principle of working of a circuit breaker remains the same irrespective of the type of circuit breaker and can be understood here.

The user can choose good quality electrical breaker components from L&T, Siemens, Schneider, Hager, Legrand or other reputed brands.

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