What is MCB?
MCB or Miniature Circuit Breaker is an electromechanical device which protects an electric circuit from an overcurrent. The overcurrent, in an electrical circuit, may result from short circuit, overload or faulty design.
In short, MCB is a device for overload and short circuit protection. They are used in residential & commercial areas. Just like we spend the time to make a thorough check before buying appliances like washing machines or refrigerators, we must also research about MCBs.
An MCB is a better alternative to a Fuse since it does not require replacement once an overload is detected. Unlike a fuse, an MCB can be easily operated and thus offers improved operational safety and greater convenience without incurring a large operating cost. They 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 switchgear which is usually available in the range of 0.5A to 100A.
Their 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.
MCB’s are protective devices that are made to break the circuit in case of overload or Short circuit.
- Due to the thermal effect of over electric current
- Due to the electromagnetic effect of overcurrent.
The thermal operation of the 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 the operating mechanism, it causes to open the miniature circuit breaker contacts.
But during short circuit condition, sudden rising of electric current causes electro-mechanical 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.
This video explains the working principle of an MCB is simple words.
Tripping Mechanism of MCB
1. Thermal Trip Unit
The thermal trip unit protects against overload currents.
When there is an overload, the increased current flow heats the bimetal causing it to bend. As the bimetal bends it pulls the trip bar which opens the breaker’s contacts. The time required for the bimetal to bend and trip the breaker varies inversely with the current.
The Magnetic trip unit protects against a short circuit.
The magnetic trip unit is comprised of an electromagnet and an armature.
When there is a short circuit, a high magnitude of current passes through the coils creating a magnetic field that attracts the movable armature towards the fixed armature. The hammer trip is pushed against the movable contact and the contacts are opened.
Types of MCB based on Tripping Characteristics
MCBs are classified according to tripping over the range of fault current as follows:
- Type B MCB
- Type C MCB
- Type D MCB
- Type K MCB
- Type Z MCB
|Type||Tripping Current||Operating Time|
|Type B||3 To 5 time full load current||0.04 To 13 Sec|
|Type C||5 To 10 times full load current||0.04 To 5 Sec|
|Type D||10 To 20 times full load current||0.04 To 3 Sec|
|Type K||8 To 12 times full load current||<0.1 Sec|
|Type Z||2 To 3 times full load current||<0.1 Sec|
1. Type B MCB:
This type of MCB trips between 3 and 5 times full load current.
Type B devices are mainly used in residential applications or light commercial applications where connected loads are primarily lighting fixtures, domestic appliances with mainly resistive elements. Also used for computers and electronic equipment with very low inrush loads (PLC wiring).
The surge current levels in such cases are relatively low.
Function: protection and control of the circuits against overloads and short-circuits; protection for people and big length cables in TN and IT systems.
Applications: residential, commercial and industrial.
2. Type C MCB:
This is used in a commercial or industrial type of applications where there could be chances of higher values of short circuit currents in the circuit.
The connected loads are mainly inductive in nature (e.g. induction motors) or fluorescent lighting. Applications include small transformers, lighting, pilot devices, control circuits, and coils.
Function: protection and control of the circuits against overloads and short-circuits; protection for resistive and inductive loads with low inrush current.
Applications: residential, commercial and industrial.
3. Type D MCB:
This type of MCB trips between 10 and 20 times full load current.
These MCBs are used in specialty industrial/commercial uses where current inrush can be very high. Examples include transformers or X-ray machines, large winding motors etc.
D-curve devices are suitable for applications where high levels of inrush current are expected. The high magnetic trip point prevents nuisance tripping in high inductive applications such as motors, transformers, and power supplies.
Function: protection and control of the circuits against overloads and short-circuits; protection for circuits which supply loads with high inrush current at the circuit closing (LV/LV transformers, breakdown lamps).
Applications: residential, commercial and industrial.
4. Type K MCB:
Function: protection and control of the circuits like motors, transformer, and auxiliary circuits, against overloads and short-circuits.
Advantages: no nuisance tripping in the case of functional peak currents up to 8xIn, depending on the series; through its highly sensitive thermostatic bimetal trip, the K-type characteristic offers protection to damageable elements in the overcurrent range; it also provides the best protection to 2 cables and lines. Applications: commercial and industrial.
5. Type Z MCB:
Function: protection and control of the electronic circuits against weak and long duration overloads and short-circuits.
Applications: commercial and industrial.
All the above types of MCBs provide tripping protection within one-tenth of a second.
Types of MCB based on Number of Poles
Single Pole (SP) MCB:
A single pole MCB provides switching and protection only for one single phase of a circuit.
Double Pole (DP) MCB:
A two Pole MCB provides switching and protection both for a phase and the neutral.
Triple Pole (TP) MCB:
A triple/three phase MCB provides switching and protection only to three phases of the circuit and not to the neutral.
3 Pole with Neutral [TPN (3P+N) MCB]:
A TPN MCB has switching and protection to all three phases of the circuit and additionally Neutral is also part of the MCB as a separate pole.
However, Neutral pole is without any protection and can only be switched.
4 Pole (4P) MCB:
A 4 pole MCB is similar to TPN but additionally, it also has protective release for the neutral pole.
This MCB should be used in cases where there is a possibility of high neutral current flow through the circuit as in cases of an unbalanced circuit.
Selection of Right MCB
- Type B, C, and D are used for overcurrent protection of cables in accordance with IEC/EN 60898-1
- Type K for the protecting motors and transformers and simultaneous overcurrent protection of cables with overload tripping based on IEC/EN 60947-2
- Type Z for control circuits with high impedances, voltage converter circuits, and semi cable protection and simultaneous overcurrent protection of cables with overload tripping based on IEC/EN 60947-2
MCB Characteristic Curves (Type B,C & D)
The circuit breaker’s tripping curves consist of two parts:
- Tripping of overload protection (thermal tripping device): The higher the current, the shorter the tripping time
- Tripping of short-circuit protection (magnetic tripping device): If the current exceeds the threshold of this protection device, the breaking time is less than 10 milliseconds.
The second region is the response time of the magnetic trip which differentiates each characteristic and for which an identifying letter (Type B, C, D, K, Z) is assigned.
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.
The most important MCB characteristics are
- Type B characteristic curves.
- Type C characteristic curves.
- Type D characteristic curves.
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.
1. Type B Curve
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.
Type B devices are designed to trip at fault currents of 3-5 times rated current. For example, a 10A device will trip at 30-50A.
2. Type C Curve
Type C devices are designed to trip at 5-10 times In (50-100A for a 10A device).
3. Type D Curve
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 (100-200A for a 10A device).
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.
Sources of Surge currents
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.
Overcoming Unwanted Tripping of MCB
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 measurements listed below should be considered.
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 installation conditions, in particular, the operating times required by Regulation.
In contrast, the test procedures followed in British ASCTA (Association of Short Circuit Testing Authorities) laboratories are among the most suitable in the world.