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A Residual Current Device (RCD) is a protective device that is intended to isolate the supply to protected circuits, socket outlets or electrical equipment in the event of a current flow to earth that exceeds a predetermined value, usually 30mA.  It is important to detect this earth current because the current flow path may be through a person, and so the protection provided by an RCD is critical in preventing death by electric shock.


How does an RCD work?
All the active electrical conductors flow through a toroid in the device. These current flows create magnetic fields which should normally cancel each other out in an installation with no leakage, but any imbalance in the current flowing to the load, and the current returning from the load will create a magnetic field, which in turn creates a current in the toroid, which is converted by the secondary into a signal that drives a trip solenoid to open the device.


Different Types of RCD:
The basic RCD mechanism can be incorporated into several different types of protection devices. 
  • Residual Current Circuit Breaker (RCCB) is an RCD with its own poles for isolating the supply.  Sometimes generically called “RCD”, especially in domestic applications.  Can also be called “Safety Switch” in domestic applications. 
  • Residual Current Circuit Breaker with Overcurrent Protection (RCBO) is an RCCB which also incorporates thermal and magnetic protection for the main pole.  Can be called “Combined MCB/RCD” or “Combo” for short in domestic applications. 
  • Add-on RCD is a type of RCD which does not have its own poles for isolating, and so must be attached to a Miniature Circuit Breaker to form an RCBO. 
  • RCD Relay is an RCD which does not have its own poles for isolating but has a relay output which drives a shunt trip to open a separately mounted circuit breaker.
How does the test pushbutton work?
The test pushbutton works by creating a current flow path that bypasses the toroid causing the RCD to trip by its normal protection method.  By doing this it tests every part of the RCD – the toroid, the secondary wiring, the signal processing, the trip solenoid and the main poles.

If you’d like to learn more about RCD technology or standards, please visit our website at https://resources.ipd.com.au/rcd/
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