What is the main function of RCD?

The main function of an RCD (Residual Current Device) is to provide protection against electric shock. Here are the details:
Protection against electric shock:
An RCD continuously monitors the flow of electrical current in a circuit. In a normal electrical installation, the current flowing in the live wire should be equal to the current flowing in the neutral wire. However, if there is a fault such as a person coming into contact with a live wire and a path to ground is created, an imbalance occurs in the current.
When this imbalance, known as residual current, reaches a certain threshold (usually a few milliamperes), the RCD quickly trips and cuts off the power supply. This action helps to prevent serious electric shock injuries or even fatalities.
Prevention of electrical fires caused by leakage current:
In addition to protecting against electric shock, an RCD can also help prevent electrical fires. If there is a leakage of current due to a damaged cable or faulty electrical appliance, the RCD can detect this and disconnect the power before the leakage current can cause overheating and potentially start a fire.