Residual Current Circuit Breaker

Residual Current Circuit Breaker (RCCB) or a ‘trip switch’ is a protection device used in electrical systems. RCCBs protect both humans and electrical appliances from electrical faults such as electric shock and current leakage. In this article, we’ll be discussing what RCCBs are, how they are made and how they function as electrical circuit protection devices.

Residual Current Circuit Breaker How it Works

Residual Current Circuit Breakers are also known as ‘trip switches’ and RCD (Residual Current Device). They belong to the category of electrical circuit protection devices. RCCDs can be found in every domestic and industrial power distribution circuit. They are available in single phase and three phase configurations and have multiple classes.

The simplest form of RCCB is the single-phase type. They have two input terminals and two output terminals in each unit. Similar to a MCB, there is also a switch that’s spring loaded to turn the device on and off. In addition to the switch, there is a ‘test’ button to test if the device is actually working. This button can be of different color like red, green, yellow, orange or blue depending on the manufacturer.

A typical human being can only tolerate upto 30mA of electric current before life-threatening injuries occur. If a human or any living being comes into contact with a live electrical connection and is grounded, they will get electrocuted and be seriously injured. Although not this severe, electric machinery also can be damaged due to electrical short circuits and current leakage.

A residual current circuit breaker works by the principle of Kirchhoffs’s Current Law (KCL). KCL states that the current flowing into a circuit must be equal to the current returning from the circuit. Similarly, the RCCB measures any significant changes in the current going through live (L) and neutral (N) terminals. In an ideal scenario, the difference should be very close to 0mA.

If there is a current leakage in an electrical equipment protected by the RCCB, the neutral current will be lower than live wire’s current because some amount of current will be grounded through a person or the machine’s earth connection. This difference is called the ‘residual current’. When this difference exceeds the RCCB’s tripping threshold (i.e. 20mA), it rapidly trips the switch and immediately breaks the connection and cuts power to the equipment. 

The tripping action happens instantly within a few milliseconds. Therefore, the person getting electrocuted can be protected from life threatening injuries. Although the electric shock cannot be prevented, the safety of the person is always guaranteed. When considering electrical appliances, if such a device has a current leakage fault that leaks current into the chassis, it can immediately trip the RCCB to prevent potential electric hazards.

Residual Current Circuit Breaker Circuit Diagram

Now that we know the operation of a residual current circuit breaker, let’s have an in-depth look at how an RCCB works.

Shown below is the internal construction of aa single-phase (two-pole) RCCB device.

Starting from the left, the input terminals are connected to the main electrical supply from the grid or the generator. This is switched using a two pole MCB to isolate the circuit when needed. The supply is then passed through 2-pole breaker contacts; one for live and one for neutral for total protection.

The live and neutral wires pass through a special coil that has one winding per each wire and another separate winding called ‘sensing coil’. Each end of these two primary coils are connected to the load. As discussed above, when current is passing though these coils, they create a magnetic field according to the Faraday’s Law of Electromagnetic Induction. On normal circumstances where there is no fault in the load circuit, these two magnetic fields cancel out each other.

When there is a fault condition and the live current is higher/lower than the neutral current, these two magnetic fields are not able to cancel out each other. This induces a voltage inside the ‘sensing coil’. If this voltage becomes too high (which means the fault current/residual current is too high), it activates a solenoid to break the contacts to cut off power to the load. Once tripped, it needs to be manually turned on after fixing the fault.

RCCBs also come equipped with an additional circuit that includes a button and a resistor. This is a circuit that allows testing of the device. When we press the button, it creates an artificial fault condition (residual current) and triggers the RCCB. If the device trips when the button is pressed, it confirms that the protection is in place.

RCCB Wiring

RCCBs are safety devices that are installed in electrical circuits. They are often installed at power distribution boards close to the source. The image shown below indicates the order of installation of an RCCB.

The RCCB is connected after the primary switch, which is a 2-pole MCB. The MCB acts as a main switch to completely isolate the circuit from the grid in cases such as repair and maintenance. The RCCB is placed next to protect the subcircuits. Single pole MCBs that provide over current protection to subcircuits are placed after the RCCB.

Classification of RCCB

RCCBs are commonly classified into two main types according to their terminal count. They are single-pole RCCB and 4-pole RCCBs.

  • Single-pole RCCB
    • Single-pole RCCBs are commonly used in domestic applications and small scale industrial applications where single phase loads are used. They have two terminals and two output terminals, live and neutral (L and N).
  • 4-pole RCCB
    • 4-pole RCCBs are used for three-phase power system protection. There are 4 input and output terminal sets for L1, L2, L3 and N wires in a three-phase system. These are commonly found in industrial power distribution circuits and some domestic power distribution circuits where the power usage is high.

In addition to these two physical configurations, there are few more types that are specially designed to fit for specific applications. These are defined in the IEC 60755: General safety requirements for residual current operated protective devices.

  • Type AC – general purpose
    • This type is suitable for alternating sinusoidal voltage systems. These are the most commonly used type of RCCBs in domestic and industrial applications.
  • Type A – Single phase, class I electronic loads
    • Type A RCCBs are most suitable for circuits that are operating on either pure sinewave or modified sinewave sources such as inverters. These are also used in circuits where the loads have diode and/or thyristor rectifier circuits such as VFDs (Variable Frequency Drive) motor controllers. Type A RCCBs can be used as a replacement for type A units.
  • Type F – newly introduced, special purpose
    • Type F RCCBs are specially made to be used in variable speed drive circuits such as motor controllers. These perform very well in high frequency, distorted current and voltage waveform circuits such as HVAC systems. Type F RCCBs are very sensitive to fault currents. These do not trip due to sudden inrush currents. Type F can replace Type A and AC RCCBs.
  • Type B – 3-phase systems, EV and PV systems
    • Type B residual current circuit breakers are designed to work in electric vehicle charging circuits, photovoltaic systems and other 3-phase rectified systems. These can detect sinusoidal AC, pulsating DC with multiple frequencies and even smooth DC residual currents. In addition to the waveform types, the characteristics are also defined under specific frequencies from 50Hz to 1kHz. Type B RCCBs comply with Type A, F and AC types and can be used as a replacement.

RCCB Advantages and Disadvantages

Residual Current Circuit Breakers are one of the most crucial electrical circuit protection devices. We see them in almost every electrical distribution board, protecting the users and the equipment from damages. Like any device, RCCBs also have disadvantages as well as advantages.

RCCB Advantages/Benefits

  • RCCBs are mainly used to provide electric shock protection to humans and other living beings.
  • In addition to electric shocks RCCBs can also protect humans and electrical equipment from earth faults such as current leaks.
  • RCCBs automatically trip when the fault current exceeds the rated sensitivity.
  • They are available in multiple sensitivities and rating types to suit for particular application.
  • RCCBs come in single phase (two-poles) or three-phase (four-poles) configurations. These arrangements allow us to completely isolate the circuit from the supply.
  • Most RCCBs contain additional filtering circuits which help to protect the equipment from voltage fluctuations.

RCCB Disadvantages

  • RCCBs do not have short circuit/overload protection. They completely rely on the residual current. If a load accidentally shorts, the return current will be the same as the current flowing out of the RCCB. This will not trigger the device protection.
  • Highly sensitive variants can be triggered by lightning strikes and voltage fluctuations.
  • RCCBs cannot protect against faulty electrical wiring (i.e. when live and neutral wires have been swapped in electrical outlet)
  • RCCB only provides protection against live-earth or neutral-earth shocks. It cannot protect the user from live-neutral shocks. This can be lethal.
  • Failure to select the correct type of RCCB for a particular application leads to reduced level of protection against electric shock.

Residual Current Circuit Breaker Price

Electrical protection devices are inherently expensive. They are sensitive devices that need to meet industry standards to ensure that the minimum required level of protection is guaranteed from each unit.

Residual Current Circuit Breakers’ prices start from $25-$30 for single-pole 25A types. As the current rating, number of poles and the sensitivity increases, an RCCB can cost you from a few hundred dollars to several thousand dollars. For example, the 125A ABB 4-pole RCCB costs a hefty $51730 at the time of this article being written.

Residual Current Circuit Breaker with Overcurrent Protection (RCBO)

As mentioned above, a typical RCCB is not able to detect over-current events. This is due to the working principle of the device. It solely relies on the difference between the output and the return current (residual current) to trigger the safety. If a load draw too much current, the RCCB cannot detect it as the return current is exactly the same as the output current from the device.

To prevent this, there can be two approaches. The usual practice is to use one RCCB and distribute the power through multiple MCBs. This way, the residual current protection is handled by the RCCB and overcurrent protection is provided by the MCB. However, a residual current event in any of the subcircuits causes the RCCB to trigger and shut off the entire system.

RCBOs are designed to work around this problem. They consist of an overcurrent protection circuit in addition to the general residual current protection circuit.

The image shown below is a 4-pole RCCBO. RCCBOs are distinguishable by their MCB-like appearance that has 4 connected poles. They almost looks like an MCB paired with an RCCB.

What Causes an RCCB to Trip?

RCCBs are designed to trip whenever there is a fault current. This current is called ‘residual current’. The device, as explained above, monitors the current traveling in Live(s) and Neutral wires. Under normal working conditions, the difference between the two currents must be near-zero.

Whenever there is a current leak; for example, a person who is grounded gets electrocuted, a small amount of current from exposed live wire travels through the person into the ground. This causes an imbalance in the elive and neutral currents in the RCCB. IF the difference is too high, the RCCB trips.

Apart from this primary functionality, events such as lightning, non-uniform loads such as highly inductive loads (electric motors, welding plants) can also trigger the RCCB inadvertently. This is unwanted and can be prevented by using the correct RCCB type suitable for the type of load.

The upgraded RCBOs can also trip due to overcurrent conditions. RCBOs can detect overcurrent/short circuit instances and trip the switch accordingly.

Conclusion

Residual current circuit breakers are very reliable safety devices that protect both equipment and human operators from hazardous electric shock. There are very advanced RCCBs in the market that even allow programming of parameters such as tripping current and tripping delay. Always use the suitable RCCB in your power distribution system to ensure optimum protection.

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