Miniature Circuit Breakers - MCB Types and Characteristic Curves

MCBs or Miniature Circuit Breakers are electromechanical devices which protect an electric circuit from an overcurrent. The overcurrent, in an electrical circuit, may result from short circuit, overload or faulty design. 

An MCB is a better alternative to a Fuse since it does not require replacement once an overload is detected. Unlike fuse, an MCB can be easily operated and thus offers improved operational safety and greater convenience without incurring large operating cost. 

Miniature Circuit Breakers are used to protect lower current circuits and have the following Specifications 

Current rating - Amperes                                       Short Circuit Rating - Kilo Amperes (kA) Operating Characteristics - B, C or D Curves

Miniature Circuit Breakers are usually available in the range of 0.5A to 100A. An MCB's Short 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. 

Working Principle of MCB

There are two arrangement of operation of miniature circuit breaker. One due to thermal effect of over electric current and other due to electromagnetic effect of over current. The thermal operation of miniature circuit breaker is achieved with a bimetallic strip. 

Whenever continuous over electric current flows through MCB, the bimetallic strip is heated and deflects by bending. This deflection of bimetallic strip releases mechanical latch. As this mechanical latch is attached with operating mechanism, it causes to open the miniature circuit breaker contacts. 

But during short circuit condition, sudden rising of electric current, causes electromechanical displacement of plunger associated with tripping coil or solenoid of MCB. The plunger strikes the trip lever causing immediate release of latch mechanism consequently open the circuit breaker contacts. This was a simple explanation of miniature circuit breaker working principle.

Types of MCB

MCBs are of following types

1. Single Pole
2. 1 Pole (with N)
3. 2 Pole (DP)
4. 3 Pole (TP)
5. 3 Pole with N (TPN)
6. 4 Pole (4P)

 Characteristic Curves of MCB

Characteristic curve is the curve between release current and tripping time. MCB have Type B, Type C and Type D characteristic curves. They are shown below,

The classification of Type B, C or D is based on fault current rating at which magnetic operation occurs to provide short time protection (typically less than 100ms) against short circuits  It is important that equipment having high inrush currents should not cause the circuit-breaker to trip unnecessarily, and yet the device should trip in the event of a short-circuit current that could damage the circuit cables. 

Type B devices are generally suitable for domestic applications. They may also be used in light commercial applications where switching surges are low or non-existent. Tyne B devices are designed to trip at fault currents of 3-5 tomes rated current (In). For example a 104 device will trip at 30-50A. 

Type C devices are the normal choice for commercial and industrial applications where fluorescent lighting, motors etc. are in use. Type C devices are designed to trip at 5-10 times In (50-100A for a 10A device). 

Type D devices have more limited applications, normally in Industrial use where high inrush currents may be expected. Examples include large battery charging systems, winding motors. transformers. X-ray machines and some types of discharge lighting. Type D devices are designed to trip at 10-20 times In (100-200A for a 10A device). 

Normal cable ratings relate to continuous service under specified installation conditions. Cables will, of course, carry higher currents for a short time without suffering permanent damage. Type B and C circuit breakers can generally be selected to achieve tripping times that will protect the circuit conductors against normal surge currents in accordance with BS 7671. This is more difficult to achieve with Type D devices, which may require a lower earth loop impedance (Zs) to achieve tile operating times required by Regulation 413-02-08. 

Surge currents

Surge currents in domestic installations are generally low, so that a Type B device is adequate. For example Inrush currents associated with one or two fluorescent fittings. Or the compressor motor in a refrigerator/freezer are unlikely to cause unwanted tripping. Fluorescent and other discharge lamps produce surge currents and while one Or two fluorescent lamps are unlikely to cause a problem, the block switching of a number of fluorescent lamps In a shop, office or factory can produce substantial inrush currents. For this reason Type C devices are recommended for these applications. 

The magnitude of the surge current will depend on the lamp rating, starting system and type of control gear used in the luminaires. Reputable MCB manufacturers produce tables listing the number of fittings of a particular make and type that can be used with their devices. 

Unwanted Tripping

Sometimes failure of tungsten filament lamps can trip Type B circuit breakers in domestic and retail environments. This is caused by high arcing currents occurring at the time of failure and is generally associated with inferior quality lamps. If possible the user should be encouraged to use better quality lamps. If the problem persists then one of the measurement listed below should be considered. 

A Type C device may be substituted for a Type B device where unwanted tripping persists, especially in commercial applications. Alternatively it may be possible to use a higher rating Type B MCB say 10A rather than 6A. Whichever solution is adopted, the installation must be in accordance with BS 7671. 

A change from Type C to Type D devices should only be taken after careful consideration of the irstallation conditions, in particular the operating times required by Regulation. 

Other Considerations 

The importance of selecting circuit-breakers from reputable manufacturers cannot be over emphasised. Some imported products, claiming .to have a 6kA short-circuit capacity, had failed dramatically under test. In contrast the test procedures followed in British ASCTA (Association of Short Circuit Testing Authorities) laboratories are among the most suitable in the world. 

Type B devices should only be used in domestic situations where high inrush currents are unlikely and Type C devices should be used in all other situations.