RCD – Residual Current Device, principle of operation; RCD, RCCB, RCBO, CBR and SRCD

RCD – Residual Current Device

RCD – Residual Current Device – is a generic term for all types of residual current operated device which by definition is: A mechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specified conditions. The more common types are:

  • Residual Current Operated Circuit Breaker (RCCB)
    A mechanical switching device designed to make, carry and break currents under normal service conditions and to cause the opening of the contacts when the residual current attains a given value under specified conditions. Depending on the age of these devices they will comply with either BS EN 61008 or BS 4293.
    The British Standard BS 4293 was withdrawn on 1st July 2000 and its moratorium period ended in July 2005, which means manufacturers continued to produce residual current devices complying with BS 4293 up to 2005 providing the devices are in production before July 2000. In line with the Company’s forward thinking and innovative approach to product development, Hager has already introduced residual current devices complying with the Harmonised Standards.
  • Residual Current Operated Circuit Breaker With Integral Overcurrent Protection (RCBO)
    A residual current operated circuit breaker designed to perform the functions of protection against earth fault currents, overloads and short-circuits. These devices comply with BS EN 61009. ‘Appendix G’ of this Standard also covers RCD add-on blocks for BS EN 60898 circuit breakers.
  • Circuit Breaker Incorporating Residual Current Protection (CBR)
    A circuit breaker providing overcurrent protection and incorporating residual current protection either integrally or in combination with a residual current unit, which may be factory fitted or fitted in the field. These devices comply with BS EN 60947-2: Circuit Breakers – Annex B.
  • Socket Outlet Incorporating A Residual Current Device (SRCD)
    A socket outlet for fixed installations, which incorporates an integral sensing circuit that will cause the switching contacts in the circuit to open automatically at a predetermined value of residual current. These will comply with BS 7288: 1990.
  • Earth Fault Relay
    A device incorporating the means of detection of an earth fault current, comparing its value to the earth fault current operating value and giving a signal to an associated switching device to open the protected circuit when the earth fault current exceeds this value. Relays can be directly connected or fed from a separate torroid. There is currently no specific standard for this type of device. It should therefore comply with IEC 755 general requirements for residual current operated protective devices.

The residual current device – principle of operation

The residual current devices monitor the current flowing in a circuit by means of a torroid, which is a small current transformer specially designed to detect earth fault currents. All live conductors will pass through this coil, the currents flowing in the live conductors of a healthy circuit will balance and therefore no current will be induced in the torroid. Live conductors of a circuit include all phase and neutral conductors. When an earth fault is present on a circuit, current will be flowing to earth through an abnormal or unintended path. This earth fault path could be through a person in contact with live parts as shown in Figure 20 as Ic or where insulation has failed through an exposed conductive part shown as Id.

The Technology behind the RCD’s

There are two types of technology available in residual current devices, electromagnetic & electronic and both offer very reliable performance. Electromagnetic devices use a very sensitive torroid, which operates the trip relay when it detects very small residual currents. These devices generally require no reference earth lead and are unaffected by temporary loss of supply, as the power to trip the device is derived directly from the fault current. Electronic devices do not need such a sensitive torroid as electronic circuits within the device amplify the signal to operate the trip relay. However, these devices often require a safety earth reference lead to ensure that the device will continue to operate in the event of the supply neutral being lost. The power to trip the device is taken from both the fault current and the mains supply, enabling the overall size of the devices to be reduced.
These devices should be disconnected while carrying out insulation resistance tests to prevent damage to the device and to avoid incorrect test results. Hager manufacture a range of devices using both technologies. The RCCB range, CBR CB add-on blocks and two module RCBO range are electromagnetic devices, while the single module RCBO range and Earth Fault Relays use electronic technology. The CBR MCCB add-ons are available in both technologies.

  • Current flowing through torroid in healthy circuit: Ires = I1-I2 = 0
  • Current flowing through torroid in circuit with earth fault: Ires = I1-I2=Ic + Id

This earth fault current, known as ‘Residual Current’ (Ires), is seen by the torroid as an imbalance. When the magnitude of this residual current reaches the sensitivity setting IΔn of the device, it will operate to open the contacts.

The RCD’s are used in many practical applications, like: the consumer units(Hager Consumer Units, the 17th edition consumer units, MK Metal Clads, etc), RCD sockets, RCD modules, MCB’s and RCCB’s, etc.


  1. Hi thanks for the excellent explanation of the RCD. Further, I would like to inquire, do we have standard formula for the computation of the ELCB and RCD? if so, appreciate your help. Another, what is meant by 40A/4P/100mA.


    1. We will soon post about this matter again, but for now this is all the information we have. For more details, visit hager.co.uk, they have a series of technical specs, details, explanations…. – actually, this is the place we get our data from 🙂

  2. hi the electrical report for my new house came back that residual current protection is required for the socket outlet circuits could you help me and explain ! is it a big problem?

    1. This just means that your house wiring doesn't comply with the current 17th edition wiring regulations. This does NOT mean however that it is unsafe. However your mortgage provider may insist that it needs upgrading. Without seeing the installation if your report only mentions that the sockets need RCD protection I would suggest that it isn't a very good report since 17th edition requires all circuits to have RCD protection unless cables are buried over 50mm deep in walls or are protected in metal conduit or similar. Either change the fuseboard (best done by a qualified electrician) or if you can, change the original circuit breakers for RCBO's of the same current ratings (make sure fuseboard main switch is off though)

  3. Pls could you explain how a 3 phase rccd without a neutral works the neutral input has no connection the neutral out is connected to grey phase

  4. the main transformer in my apartment is faulty ,whenever the HG fuse fuses out then there there is a sudden surge in the neutral line which causes 440v and all appliances burst due to this fault.i have installed a rccb 32amps to contain this problem,will it work.

    1. Before anything else, the main transformer must be rectified 1st. Pointless trying to fix any further if you know where the fault lies. Ta

  5. why will ahealthy rcb not trip when test button pressed if put on other side of split load board it does

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