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Operation Principle of Residual Current Device (RCD)

What is Residual Current Device (RCD)?

An RCD is a device that is designed to provide protection against electrocution or electrical fires by cutting off the flow of electricity automatically or actuating an alarm when it senses a ‘leakage’ of electric current from a circuit.

To appreciate the importance of an RCD it is helpful to understand how much electrical energy it takes to kill a human being. The smallest fuse used in a normal electric plug is 3 Amps; it takes less than one-twentieth of that current to kill an adult in less than one-tenth of a second.

RCD Operation


The operation of an RCD can be understood by taking an analogy from the water flowing in a central heating system.

A leak may occur when the pipework is damaged or punctured. In the same way, a ‘leak’ of electricity can occur when the cable insulation in a circuit is faulty or damaged.

In a central heating system the ‘flow’ pipe takes the water from the boiler to the radiators; if the installation is sound the same amount of water will return to the boiler as in figure1. However, if there is a leak, there will be less water in the return pipe than in the flow pipe. If the system had flow detectors in the flow and return pipes, these could be coupled to a valve so that the valve closed when the rate of flow in the return pipe was less than that in the flow pipe as in Figure 2.

  • Figure-1: Healthy central heating circuit. The same amount of water flows in the ‘flow’ and ‘return’ pipes

  • Figure-2: If there is a leak, there will be less water in the ‘return’ pipe than in the ‘flow’ pipe. This could be used to trip a valve.

The rate of flow of water can be compared with the current in an electrical circuit and the water pressure can be compared with the voltage. When the line and neutral currents are equal, the RCD will not trip but when it senses that the neutral current is less than the line current it will trip.

In both cases, the leakage is detected without actually measuring the leak itself. It is the flow and return rates that are measured and compared. An RCD compares the line and neutral currents and switches off the electricity supply when they are no longer equal.

With an RCD the line (brown) and neutral (blue) conductors pass through the core of a sensitive current transformer, see Figure 3, the output of which is electrically connected to a tripping system. In a healthy installation, the current flows through the line conductor and returns through the neutral conductor and since these are equal and opposite the core remains balanced. However, when leakage of electric current occurs, as in Figure 4, the line and neutral currents are no longer equal; this results in an output from the transformer which is used to trip the RCD and disconnect the supply.

  • Figure-3: In an RCD, the line and neutral conductors of a circuit pass through a sensitive current transformer. If the line and neutral currents are equal and opposite, the core remains balanced

  • Figure-4: If there is an earth fault the neutral current will be lower than the line current. This imbalance produces an output from the current transformer which is used to trip the RCD and so break the circuit.

Operation Principle of RCD

The basic principle of operation of the RCD is shown in Figure 5. When the load is connected to the supply through the RCD, the line and neutral conductors are connected through primary windings on a toroidal transformer. In this arrangement, the secondary winding is used as a sensing coil and is electrically connected to a sensitive relay or solid-state switching device, the operation of which triggers the tripping mechanism. When the line and neutral currents are balanced, as in a healthy circuit, they produce equal and opposite magnetic fluxes in the transformer core with the result that there is no current generated in the sensing coil. (For this reason, the transformer is also known as a ‘core balance transformer’).

Figure-5: Schematic of an RCD

When the line and neutral currents are not balanced they create an out-of-balance flux. This will induce a current in the secondary winding which is used to operate the tripping mechanism. It is important to note that both the line and neutral conductors pass through the toroid.

A common cause of unwanted tripping is failure to connect the neutral through the RCD. RCDs work equally well on single-phase, three-phase or three-phase and neutral circuits, but when the neutral is distributed it is essential that it passes through the toroid.

Test Circuit

A test circuit is always incorporated in the RCD. Typically the operation of the test button connects a resistive load between the line conductor on the load-side of the RCD and the supply neutral.

The test circuit is designed to pass a current in excess of the tripping current of the RCD to simulate an out-of-balance condition. Operation of the test button verifies that the RCD is operational. It is important to note, therefore, that the test circuit does not check the circuit protective conductor or the condition of the earth electrode.

On all RCDs a label instructs the user to check the function of the RCD at regular intervals and to observe that the RCD trips instantly

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