What is a Residual Current Device and How it works?


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Residual Current Device is a sensitive safety device that switches off an electrical device in the event of a fault.

Electrical current flowing through the live and neutral conductors of a circuit generates,around each conductor,an electromagnetic field. The vectorial sum of these electromagnetic field is normally close to zero.

In the event of a fault there is a leakage current flowing to the ground and creating unbalance inside the RTD. The RCD works thanks to this phenomenon.A toroidal transformer interacts with the electromagnetic field around the conductors and when fault unbalances the electromagnetic field its vectorial sum becomes bigger than zero,creating an electrical potential on the coil of the toroidal transformer that acts on the tripping mechanism of the switch.

Residual Current Device _RCD_How it works

Classification of RCD-

Based on Product Standard-

RCCB - Residual Current operated circuit breaker.IEC 61008

It is a device not designed to give protection against overloads and/or short-circuit and must always be be used in conjunction with an overcurrent protective device such as a fuse or circuit-breaker in order to have a complete protection.

RCBO- Residual Current operated Circuit-Breaker with overcurrent protection IEC 61009

It is a device designed to give protection against overloads and/or short-circuits and can be used independently from any other overcurrent protective device within its rated short circuit capacity.

Based on the types of wave form detected-

Type AC - Sensitive to Alternating Current only

Type A - Sensitive to alternating and/or pulsating current with DC component.

Type B - Provide protection in case of alternating residual sinusoidal currents up to 1.000 Hz,pulsating direct currents and smooth direct residual current.

Based on intervention time-

General type - are intended for general purpose use and are also called instantaneous.

Selective type- have a delayed tripping action and are normally used in conjunction with a downstream general type RCD to ensure selectivity.

Voltage Independent RCD-

They use the energy of the earth fault current to trip the mechanism directly. In this type of RCD,the output from the sensing coil operates a specially constructed magnetic relay and so releases the RCD mechanism,independently of the mains voltage.

Voltage Dependent RCD-

They rely on a voltage source,derived from the mains supply or an auxiliary supply,to provide power to the amplifier,that provides an enhanced signal from the sensing coil to operate a trip solenoid or relay.