RCD Function, Working Principle, Connection, Wiring Diagram

Hey, in this article we are going to see the RCD Wiring diagram and its connection procedure. RCD means Residual Current Device. It is an electrical protective device that protects electrical circuits and devices from some electrical faults such as leakage faults, electrical shock, current unbalance due to equipment failure, etc. It works on the principle of sensing residual current which is why it is called a residual current device. Nowadays, all domestic and commercial electrical systems and circuits use RCDs. Today, we will see how you can connect an RCD to the distribution board at your home.

Our home distribution board consists of MCB, MCCB(in some cases), and RCD. So, you should know how to connect an RCD with MCBs.

RCD Wiring Diagram

Here, you can see the wiring diagram of RCD in a distribution board with a single-phase system.

Here, you can see the output from the energy meter is directly connected to a 32A MCB. This MCB is used as the main switch of the whole circuit. When this MCB is turned off, the whole circuit will be disconnected from the power source. After the main MCB, 32A RCD is connected. 

Remember that the current rating of the RCD is equal to the current rating of the MCB. Now, all the single pole MCBs for individual loads are connected to the output of the RCD. So, the main MCB always be connected before the RCD. If connect the RCD before the MCB it will not be protected from the overcurrent and short circuit fault. Because we know that RCD cannot give protection against overcurrent and short circuit faults.

RCD Connection Procedure

• As I told you RCD should be connected after the main MCB.
• Before connecting find out the input and output sides already mentioned written on the RCD.
• Connect the output of the MCB to the input of the RCD and connect the output of the RCD to the load circuit.
• It is better to connect the RCD in exact parallel to the MCB, I mean to say do not interchange the phase and neutral.

RCD Function and Working Principle

The main function of the RCD is to give protection against leakage current faults. You can see here two protective devices are connected in the above wiring diagram - MCB, and RCD. MCB will give protection against short circuit faults and overload faults and the RCD will give protection against leakage faults.

The RCD is a current-controlled device. It has a sensing coil that continuously measures the current flow in both phase and the neutral wire. If there is no leakage in the circuit, the phase and neutral wire will conduct equal current but if there is any leakage in the circuit an unbalanced current will flow through the phase and neutral. In this situation, the sensing coil of the RCD will detect the fault and trip the circuit to disconnect the power supply. If we want to turn On again the RCD without rectifying the fault, the RCD will trip again. Leakage fault causes to flow a very small amount of excessive current in the circuit so the MCB will not trip, the RCD will trip only.

Now, what happens if the short circuit fault happens. During the short circuit fault, a high amount of current flows in the circuit but the current flow in phase and neutral is equal so the RCD cannot sense the fault and it will not trip. But the MCB can sense this high current and it will trip.

So when you see the MCB got tripped, there is an overload or short circuit fault happened. But if you see the RCD got tripped, then there is a leakage current fault happened.

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• List of Electrical Safety/Protective Devices used in Home
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