The RCD employs the current balance principle which involves the supply conductors to the load (phase and neutral) wound onto a common transformer core to form the primary windings. Under healthy conditions, the current in the phase conductor is equal to the current in the neutral and the vector sum of the current is zero.

In the event of an earth fault, an amount of current will flow to earth, creating an out of balance situation in the transformer assembly. This out of balance detected by the secondary winding of the transformer will activate the trip mechanism at a pre-determined level. Single phase and neutral or three phases and neutral units (suitable for both 3 wire and 4 wire systems) are available, the latter being suitable for balanced or unbalanced 3 phase loads.

The RCD tripping mechanism will operate at a residual current of between 50%-100% of its rated tripping current. (Sensitivity)


  • 10mA: Suitable for use in special applications where additional protection against contact is essential
  • 30mA: Tripping current to provide additional protection against direct contact shock
  • 100mA: Suitable for use against direct contact shock or where protection is guard against fire hazards etc.
  • 300mA: Suitable for use in large installations where equipment protection are main considerations and high levels of earth leakage are experienced

Semi-conductor devices are extensively integrated in equipments in industries, commerce and in our homes. They can be found in control panels to computers to toys.

As equipments are fed from the mains electrical supply; in the event of an earth fault, the presence of semi-conductors may result in the normal AC waveform being replaced by a non-sinusoidal fault current. In some cases, the waveform may be rectified. These waveforms are said to contain a pulsating DC component which can either partially desensitize a standard type AC RCD.

International standards IEC 1008 (RCCBs) and IEC 1009 (RCBOs) divide RCDs into two performance classes:
Residual Current Circuit Breaker (RCCB)