What is RCD?The simple definition as given here is: What current flow goes in must also come out. Other names by which these circuits are known are earth leakage circuit breakers (ELCB) or safety switches.
An RCD is an electrical safety device designed specifically to switch the electricity immediately off when an electricity "leaking" (to earth) is detected at a level harmful to a person who is using an electrical equipment.
An RCD circuit offers a high level of personal protection from electric shock. Fuses or overcurrent circuit breakers do not offer the same level of personal protection against faults involving current flow to earth. Circuit breakers and fuses provide equipment and installation protection and operate only in response to an electrical overload or short circuit.
Short circuit current flow to earth via an installation's earthing system causes the circuit breaker to trip, or fuse to blow, disconnecting the electricity from the faulty circuit.
However, if the electrical resistance in the earth fault current path is too high to allow a circuit breaker to trip (or fuse to blow), electricity can continue to flow to earth for an extended time.
RCDs (with or without an overcurrent device) detect a much lower level of electricity flowing to earth and immediately switch the electricity off.
RCDs are thus designed to prevent electrocution by detecting the leakage current, which can be far smaller (typically 5–30 milliamperes) than the currents needed to operate conventional circuit breakers or fuses (several amperes).
RCDs are intended to operate within 25–40 milliseconds, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock.
How do the RCDs work?
According to the principle "what goes in must come out" (or, more technically, the sum of all the currents meeting in any point in a circuit must be zero, otherwise the circuit will be broken - the Node Law), the residual current device operates by continually comparing the current flow both in the Active (the "IN" part, the Supply) and Neutral (the "OUT" part, the Return) conductors of the electrical circuit.
If the current flow becomes unbalanced (there are some leakages detected), it means that some of the current in the Active conductor is not returning back through the Neutral conductor and is leaking to the earth - this triggers the device, which automatically closes the circuit.
The time frame in which the RCDs are designed to operate is within 10 to 50 milliseconds, and the harmful leakage at which, when detected, they disconnect the circuit is typically 30 mA.
The Residual Current Devices are designed in such a way that it senses, in a very quick time-wise way any earth leakage of power and automatically switches off the circuit, before it can cause injury or damage.
The analyses of the electrical accidents in the past show that the most common and greatest risk of an electric shock comes from the contact between live parts (electrical devices) and earth, via either the human body or conductors (metal parts, etc).
Even though an RCD will not protect against all instances of electric shock, its function is to at least significantly reduce its risk.
If someone touches both the Active and Neutral conductors (and the plugs or electrical appliances are faulty), he will cause the electric current to flow through his body - this contact will not be detected by the RCD unless there is also a current flow to earth.
If a circuit is protected by RCD and a fault causes electricity to flow from the Active conductor to earth through a person's body or through a metal conductor, the RCD will automatically disconnect the electricity supply - avoiding the risk of a fatal shock or short circuit.
Many electrical fires are caused by such electrical power leakages to the earth, due to faulty wiring. Even a leakage current of 1A or a little less can start a fire; normal protection devices will not detect such low current levels. This is why there was the need of the RCDs, who detect low current level leakages to earth, and trip out the circuit, thus greatly reducing the risk of an electrical fire.
Here is a link where you can purchase RCDs and RCBOs at Sparks, made by different electrical manufacturers.