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What is an RCD?

RCD is a generic term for devices that are designed to provide protection against electrocution or electrical fires by disconnecting the electricity automatically when it senses a ‘leakage’ (residual current) from a circuit.

Different RCD devices

RCCB - Residual Current Operated Circuit-Breaker without Integral Overcurrent protection to BS EN 61008

Provides residual current protection but must be used with an overcurrent protective device such as a fuse or circuit breaker 

RCBO - Residual Current Operated Circuit-Breaker with Integral Overcurrent protection to BS EN 61009

Provides residual current protection as well as protection against overloads and/or short-circuits

CBR - Circuit-Breaker incorporating Residual Current protection to BS EN 60947-2

A circuit-breaker providing overcurrent protection and incorporating residual current protection

SRCD - Socket-Outlet incorporating a Residual Current Device to BS7288

A socket-outlet for fixed installations incorporating an integral residual current protection

FCURCD - Fused Connection Unit incorporating a Residual Current Device to BS7288

A fused connection unit for fixed installations incorporating an integral residual current protection 

Different types of RCDs

Type AC

RCD tripping on alternating sinusoidal residual current, suddenly applied or smoothly increasing.


Type A

RCD tripping on alternating sinusoidal residual current and on residual pulsating direct current, suddenly applied or smoothly increasing

Note:

For RCD Type A tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current up to 6 mA.


Type F

 

RCD for which tripping is achieved as for Type A and in addition:

  • for composite residual currents, whether suddenly applied or slowly rising intended for circuit supplied between phase and neutral or phase and earthed middle conductor.
  • for residual pulsating direct currents superimposed on smooth direct current.

Note:

For RCD Type F tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current up to 10 mA.


Type B

  

RCD for which tripping is achieved as for Type F and in addition:

  • for residual sinusoidal alternating currents up to 1 kHz
  • for residual alternating currents superimposed on a smooth direct current
  • for residual pulsating direct currents superimposed on a smooth direct current
  • for residual pulsating rectified direct current which results from two or more phases
  • for residual smooth direct currents whether suddenly applied or slowly increased independent of polarity.

Note:

For RCD Type B, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current up to 0.4 times the rated residual current or 10mA, whichever is the highest value.


  • Type A RCD can also be used for Type AC applications
  • Type F RCD can also be used for Type A & AC applications
  • Type B RCD can also be used for Type F, A & AC applications

Correct selection of the Type of RCDs must be in accordance to the latest BS7671 IET wiring regulations.