What is an RCD (Residual Current Device)? – RCB and RCCB

Residual Current Device or Residual Current Circuit Breaker – Construction, Working, Types, Rating and Applications

What is an RCD?

An RCD, which stands for Residual Current Device, is also known as a Residual Current Breaker (RCB) or Residual Current Circuit Breaker (RCCB). It is a safety device designed to protect against electric shock and hazardous fires.

The primary function of an RCD is to monitor the electrical current flowing in a circuit and quickly disconnect the power supply if it detects an imbalance current (leakage of current to ground) between the live and neutral conductors.

An RCD is essentially a current-operated ELCB and is commonly known as an RCCB, being more accurate and reliable in sensitivity during operation than voltage-operated ELCBs, which rely on a ground/earth rod.

An RCD with overcurrent protection forms an RCBO (Residual Current Breaker with Over-Current), providing protection against leakage current as well as overcurrent.

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Features and Characteristics of RCDs

• The most widely used RCDs in residential applications are 10mA to 30mA.
• RCCBs ranging from 100mA to 500mA are used in industrial and commercial applications.
• The tripping time is 40ms (for 10mA – Type 1) and 300ms (for 30mA – Type 2) RCDs (AS/NZS 3760:2010).
• RCDs are wired like generic MCBs, i.e., two incoming wires (Line and Neutral) from the power supply connects to the input terminals and both outgoing wires connected to the load circuit.
• RCDs and RCBOs are very effective against electric shocks and fires.
• An RCD trips and disconnects the circuit when it senses an earth fault current.
• If there is a fault current, such as leakage or earth fault current, the RCD will trip and disconnect the circuit from the power supply.
• For normal operation, the same amount of current should flow in both the Phase and Neutral wires.
• If the current in the Phase and Neutral is unequal, the RCD detects the imbalance current and disconnects the circuit within 300ms to 30ms.
• All circuits must be protected with an RCD if the earthing system is only connected to the earth rod and not to the main incoming supply system. In such a case, the MCB may not sense the fault current, and as a result, it will not trip to disconnect the circuit.

In short, a residual current device having the following operation features:

1. Sensitivity: RCDs are designed to detect very small differences in current, often in the range of milliamperes, to provide effective protection.
2. Fast Response: RCDs are quick-acting devices, disconnecting the power supply rapidly to minimize the risk of electric shock.
3. Trip Mechanism: When a fault is detected, the RCD uses a trip mechanism to open the circuit, preventing the flow of electricity and mitigating potential hazards.

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Tripping Current and Time of RCDs

Typical RCDs used in home applications (consumer units) have a rating of 30mA with a tripping time of 300ms.

Current in mA: According to the UK standard and BS regulations, the RCD used in a consumer unit will operate and interrupt the circuit when it senses a fault current up to 30mA or above. Below this range of leakage current, it will not trip to avoid the unwanted operation of the circuit.

Time in ms: According to BS EN 61008, the RCD (RCB and RCBO) must interrupt the circuit as soon as the leakage current surpasses the predefined value. The time frame of operation depends on the amount of imbalance current.

The time frame in seconds is given below based on the amount of fault current:

For example:

• • 1 x IΔn = 300ms
  • 2 x IΔn = 150ms
  • 5 x IΔn = 40ms

The “IΔn” indicates the tripping current. For instance, if the IΔn is 0.03A (30mA):

1 x 30mA = 300ms.

It means the RCD must trip the circuit within 300ms when a leakage earth fault current starts to flow in the RCD-connected circuit. Regardless of the case, it must comply with BS7671 for all RCDs (RCCBs & RCBOs).

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Construction of RCD (RCB or RCCB)

As shown in the figure illustrating the internal structure of an RCD, it contains a zero-sequence current transformer with three coils: the main coil, neutral coil, and search coil (the purposes of these coils are explained in the “operation section” below). There is a trip coil in the sense and tripping circuitry (mainly a relay) that senses the earth fault current from signals provided by the search coil and trips the circuit.

The test wire and test resistor shown in the figure are connected to the test pushbutton, which can be pressed to test if the circuit is working properly. The status indicator shows the ON and OFF operation of the breaker, while the operator handle can be used for manual ON/OFF operations.

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Working of an RCD

In a normal, balanced electrical circuit, the current flowing through the phase (live or hot) conductor should be equal to the current returning through the neutral conductor. However, if there is a fault, such as current leakage due to a damaged wire or an appliance malfunction, the RCD detects the difference in current and interrupts the circuit within milliseconds.

An RCD operates based on Kirchhoff’s Current Law, which states that the current entering a point should be equal to the current leaving that point. If the current in the phase (line) wire is not equal to the current returning in the neutral wire, it indicates residual current leakage flowing somewhere outside the circuit. The sense and tripping circuitry in the RCD then detect this residual current and break the circuit from the main supply.

The main (phase) coil, neutral coil, and search coil are wound around the toroidal core. Under normal conditions, when the current flowing in both the neutral and phase wires is equal, the resultant induced fluxes in the related coils cancel each other out. The search coil does nothing, and the circuit operates as normal.

In the case of an earth fault, an imbalance current starts to flow in the phase and neutral. Now, the flux induced in the phase and neutral coils is unequal. Consequently, the search coil detects this difference and sends a signal to the tripping circuit. In this way, the tripping coil activates and interrupts the circuit from the main power supply within seconds.

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Types of RCDs

Based on Class:

Residual Current Devices (RCDs) come in different types, each designed for specific applications and with varying characteristics. Here is a list of common types of RCDs based on classes:

1. Type AC (General-Purpose): Provides protection against AC residual currents. Suitable for general applications where most electrical loads are resistive or slightly inductive. They comply with IEC 61008/61009.
2. Type A (High Sensitivity): Offers protection against AC and pulsating DC residual currents having frequency waveforms of 1.000 Hz . Suitable for environments where electronic equipment or devices with electronic components are present. Keep in mind that they are not sensitive for 250A of impulse current (8/2 waveform). They also comply with IEC 61008/61009.
3. Type B (Selective Sensitivity): Provides protection against AC, pulsating DC, and smooth DC residual currents. It is more selective than Type A and is suitable for specific applications where a higher level of sensitivity is required. They comply with IEC 62423 but must be compliant according to IEC 61008/9.
4. Type F (Fast AC and Smooth DC): Combines features of Type A and Type B, offering protection against AC, pulsating DC, and smooth DC residual currents having waveforms of 50Hz and 60Hz. It is designed for use with devices that include frequency inverters and electronic ballasts. It is good to know that they are not sensitive for 3000A of impulse current (8/2 waveform).
5. Type B+ (Ultra-Fast AC and Smooth DC): Provides ultra-fast protection against AC and smooth DC residual currents. Ideal for applications with critical and high-risk equipment.
6. Type S (Selective): A more advanced and selective version that combines features of Type A and Type B RCDs. It is suitable for installations with mixed loads and diverse electrical devices.

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Based on Design and Application

1. Fixed (Din Rail Mounted):
   • Installed at a fixed location in the electrical distribution board or consumer unit.
   • Provides protection for a specific circuit or a group of circuits.
   • Typically used in permanent electrical installations in residential, commercial, and industrial settings.
   • Offers comprehensive protection against residual currents for a designated area.
   • They have a mark or a ‘Test’ pushbutton on the front panel, indicating it is a fixed RCD.
2. Socket-Outlet (Power Point):
   • Installed in individual socket outlets or incorporated into extension leads.
   • Provides localized protection for specific appliances or devices plugged into the protected socket.
   • Convenient for retrofitting and adding an extra layer of protection to specific outlets.
   • Commonly used in areas where portable electrical equipment is connected.
3. Portable Plug-in:
   • Designed for temporary or portable use and can be plugged into existing outlets.
   • Often referred to as plug-in RCDs or portable residual current devices.
   • Suitable for use with various electrical appliances and tools, providing on-the-go protection.
   • Frequently used in outdoor settings, construction sites, and for DIY projects.
   • Offers flexibility as it can be moved and used with different power sources.

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Applications of RCD

RCDs are commonly used in electrical installations in homes, workplaces, and outdoor settings where electrical equipment may come into contact with water. They are often installed in power distribution boards and consumer units or as standalone devices in power outlets.

In short, RCDs are used in residential, commercial, and industrial applications to provide protection against electric shock and hazardous fires in the case of earth faults, where leakage current flows in the connected circuit.

Keep in mind that RCDs only provide protection against earth fault current, not against short circuits. A better approach is to use them with an MCB or use a compact RCBO, with built-in protection against both short circuits and residual current leakage.

Bonus: Additional Premium Resources for RCDs Wiring Tutorials

1. How to Wire Single-Phase, 230V Consumer Unit with RCD? IEC, UK & EU
2. How to Wire 230V Dual Split Load Consumer Unit? – RCD+MCB
3. Wiring of the Distribution Board with RCD (Residual Current Devices)
4. How to Wire 1-Phase Split Load Consumer Unit? – RCD+RCBO
5. How to Wire an RCBO? Residual Current Breaker with Overcurrent
6. How to Wire a Garage Consumer Unit?

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