How a Residual Current Device (RCD) Works

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RCD means Residual Current Device. They are so called because they work based on the residual current in a circuit. The RCD is an electrical safety device specifically designed to immediately switch the electrical current flow when current "leaking" to earth is detected at a level harmful to a person using electrical equipment. An RCD offers a high level of personal protection from electric shock.

RCDs also help to reduce the risk of fire by detecting electrical leakage to earth in electrical wiring and accessories. RCDs are designed to operate within 10 to 50 milliseconds and to disconnect the electricity supply when they sense harmful leakage, typically 30mA

RCD Operating Principle
In absence of an earth fault,
the vectorial sum of the currents (I1 + I2) is equal to zero; in case of an earth fault if the (I1 + I2) value exceeds the rated residual operating current IΔn, the circuit at the secondary side of the toroid sends a command signal to a dedicated opening or trip coil causing the tripping of the circuit-breaker.

Types of Residual Current Devices (RCD)
There are three basic types of RCDs:
1. Switchboard mounted
2. Power point type and
3. Plug in (portable).
Switchboard mounted and power point types are referred to as non-portable RCDs. Portable RCDs are plugged into a fixed socket. A non-portable RCD installed at the switchboard is the best option in most wiring situations –residential & industrial - as it protects all the wiring and appliances plugged into the circuit.

Switchboard Mounted RCDs
These are non-portable units installed at the switchboard to provide protection of the complete installation, or protection of a selected circuit.

Power point (Fixed Socket Outlet units) RCDs
These are non-portable units consisting of RCD protection inbuilt into a fixed socket outlet to provide protection to equipment plugged into the outlet.

Portable Units RCDs
Various models are available from simple plug adaptors to units designed for specific equipment such as the portable unit.

Sensitivity of RCDs
The sensitivity and speed of disconnection of RCDs are such that any earth leakage will be detected and automatically switched off before it can cause injury or damage. The sensitivity of an RCD in terms of current is given by:

The choice of sensitivity of the RCD is a function of the resistance RE of the earth electrode for the installation. Typical values of RE and sensitivity, I in amps under a fault voltage of 50V and 25V respectively are given below:
I = 50V or 25V/RE

Fault Current Maximum Resistance of the Earth Electrode (RE)
50V 25V
3A 16Ω 8Ω
1A 50Ω 25Ω
500mA 100Ω 50Ω
300mA 166Ω 83Ω
30mA 1666Ω 833Ω

Limitations of RCDs 
In spite of its versatility and effectiveness, an RCD will not protect against all instances of electric shock. If a person comes into contact with both the active and neutral conductors while handling faulty parts of an electrical installation causing electric current to flow through the person's body, this contact will not be detected by the RCD unless there is also a current flow to earth.

The RCD will only act automatically to disconnect the electricity supply if a fault causes electric current to flow from the active conductor to earth through a person's body.

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