Unlike fuses, which must be replaced when they open, a circuit breaker can be reset once the overcurrent condition has been corrected. Pushing the handle to the “OFF” position then back to the “ON” position restores the circuit. If a circuit reopens upon reset to the “ON” position, the circuit should be checked by a qualified electrician.
In the following illustration, an AC motor is connected through a circuit breaker to a voltage source. When the circuit breaker is closed, a complete path for current exists between the voltage source and the motor allowing the motor to run. Opening the circuit breaker breaks the path of current flow and the motor stops. The circuit breaker automatically opens when it senses a fault. After the fault has been cleared, the breaker can be closed, allowing the motor to operate.
Below CBs are low capacity Breakers: (6 Amp to 400 Amp)
1. Miniature Circuit Breaker (MCB)
MCB or Miniature Circuit Breaker is an automatically operated electromechanical device used for the protection of the circuit from overloading or short circuit. It breaks or opens the circuit when the current flowing through it exceeds its rated limit. MCB is used for the protection of low voltage circuit 240/415 v AC having a wide range of current ratings below 125V.
MCB doesn’t trip (switch off) instantly, instead there is a time delay between fault occurrence & the breaking of contacts. Generally, they are designed to have a time delay of less than 2.5 millisecond for short circuit & 2 sec to 2 min for overloading. It is to make sure the CB does not trip every time with a momentary surge or starting of inductive load due to high inrush current from such loads such as electrical motors.
MCB does not have adjustable trip characteristics. While the breaking mechanism could be either thermal or thermal-magnetic in operation. The thermal breaking mechanism is used in case of overloading while the magnetic breaking mechanism is used in case of short circuit.
MCB is enclosed in an insulating casing. The fixed & moving contacts made of copper or silver alloy connects with the two terminal for current supply. There is an arc chute consisting of multiple conducting plates called arc splitters that dissipate the arc energy. While the operating mechanism as discussed earlier is of two types i.e. thermal & magnetic.
The thermal tripping mechanism consists of a bimetallic strip (made from two different metals having different thermal expansion) usually made from steel & brass is used for breaking the circuit in case of overloading. When the current above-rated limit starts flowing through the metallic strip, it heats up & starts expanding due to which it bents & triggers the latch to separate the contacts.
The magnetic tripping mechanism consists of a coil or solenoid that produces a magnetic field when current flows through it. In case of a short circuit or very high current, the solenoid produces a strong magnetic field to pull the lever & separate the contacts.
2. Molded Case Circuit Breaker ( MCCB)
Molded Case Circuit breaker is an electromechanical circuit breaker having very high current ratings up to 2500 Amps. It is used in applications where the current ratings exceed the range of MCB (Miniature Circuit Breaker). It offers a thermal-magnetic tripping mechanism where the thermal mechanism is used for overloading & magnetic is used for short circuit conditions. It can interrupt current around 10k – 200k amps.
The best & most prominent part of MCCB is that its trip characteristics are adjustable in any current rating. MCB does not have such a feature. MCCB is suitable for applications where normal current is above 100 amps. They are installed in industries.
MCCB can have fixed or interchangeable trip unit. The trip unit is responsible for breaking the contacts upon meeting the fault condition
3. Earth Leakage Circuit Breaker (ELCB)
ELCB is a type of circuit breaker that breaks its circuit upon sensing of leakage current. Leakage current occurs due to insulation failure of the wiring & it can flow through a person’s body & cause electrical shock. Thus they are used for protection against electrical shock. They do not offer protection against overloading or short circuit. Therefore, they must be used in series with an MCB.
There are two types of ELCB;
- Voltage ELCB
- Current ELCB (aka RCCB)
Both types of ELCB detect the leakage current but their sensitivity & the level of protection they offer are different. Voltage ELCB was invented before current ELCB. Voltage ELCB is inferior to current ELCB. Therefore, to avoid confusion, the voltage ELCB is renamed ELCB while the current ELCB is renamed as RCCB.
Voltage ELCB
Voltage ELCB operates on voltage level between earth & the body of the equipment. Such ELCB has an extra terminal for earth connection which is directly connected to the load or equipment’s body. If the load’s body comes in contact with the live wire, it may cause electrical shock upon touching it.
A relay is connected in series with the earthed wire. This relay senses voltage difference between the body & earth. It trips the circuit breaker off if there is a substantial amount of current flow through the earth wire due to the potential difference.
However, ELCB cannot sense the current leakage if a person comes in contact with a live wire. Therefore, ELCB cannot offer protection for other types of leakage current.
4. Residual Current Circuit Breaker ( RCCB)
Current ELCB is generally known as RCD or RCCB. Residual Current Device (RCD) or Residual Current Circuit Breaker (RCCB) is a type of ELCB that breaks the circuit in case of leakage current. It helps in protection against electrical shock or downed line.
The current leakage occurs when the current flows in an unintended path. In normal conditions, the current flows into the load through a hot or live wire & flows out of the load through the neutral wire. The current leaks if the current flows out through the ground wire or through a person’s body connected with the ground.
RCCB works on the principle of Kirchhoff’s current law, according to which the amount of current entering the circuit must be equal to the amount of current leaving the circuit. It continuously monitors the current in the hot wire & neutral wire. The difference between these two currents is called residual current. When there is an imbalance in the circuit, the residual current will trip the circuit breaker.
The live & neutral wire goes through a zero-sequence current transformer (it is used for sensing an imbalance of current between the two wires). The live & neutral wire is used for current going into & out of the circuit respectively. Since the amount of current is same in both wires, their flux cancels each other. when the imbalance occurs due to any ground fault, the resultant flux induces a voltage in the current transformer which is connected with a relay that breaks the circuit.
Below CBs are high capacity Breakers: (800 Amp to 10000 Amp)
1. Air Circuit Breaker (ACB)
4. Sulphur Hexa Fluoride CB (SF6 CB)
5. Vacuum Circuit Breaker (VCB)