How does RCD work?


14 October 2021

Residual Current Devices (RCDs)
The basic characteristics of the operation of a RCD are shown below:
  • a magnetic toroid surrounds all the active conductors, including the neutral conductor, if present, supplying an electrical circuit. The magnetic flux generated in the magnetic toroid depends at all times on the vector sum of the currents in the active conductors
  • in the case of a single-phase circuit, the incoming current  (from source to use) is considered to be positive, while the outgoing current   is considered to be negative
  • for a healthy electrical circuit, , there is no magnetic flux: no electromotive force created in the secondary winding
  • in case of earth leakage, a fault current  flows through the magnetic toroid from source to use but returns through the protective conductors (TN method of earthing) or via the earth (TT method of earthing). Consequently, the sum of the incoming and outgoing currents is not zero, i.e. , and this difference in current creates a magnetic flux. The difference in current is called "residual current"
  • the resulting alternating flux in the magnetic toroid consequently induces an electromotive force in the secondary winding so that a current  flows through the tripping control winding of the device.
If the residual current   exceeds the value (the threshold) required to trip the device directly or via an electronic relay, this will cause the associated disconnection device (switch or circuit breaker) to open

With the Voltage Dependent (VD) technology, the summation current transformer measures residual current. An electronic circuit detects the tripping level and then sends an order to tripping unit to open the protected circuit
  • in this case, the power supply of electronic circuit and energy for tripping unit come from the line voltage
  • with this technology, the RCD will be able to detect but not to trip if the line voltage is too low, because the electronic circuit and tripping units needs to be powered.
Reference document - CA908066E