RCDs (short for Residual Current Devices) come in two types, electromechanical and electronic. The latter is usually incorporated in other devices such as Portable RCDs, Combined RCDs and Socket Outlets, or RCBOs.

It is the smaller size which can be achieved which enables the electronic designs to be incorporated in the above units.

RCDs detect fault currents leaking to earth either through metalwork or unfortunately in some cases through a person's or animal's body.

Because of the way in which they work they can detect very small currents which cannot be achieved by any other device and provide the only practical method of protecting against the effects of these small currents.

Effects of Electric Shock

If a person comes into contact with metal which is carrying electric current and at the same time is earthed either through the other hand or the feet, a current will flow through the body dependant on the resistance of the body and the voltage of the supply. At 240 volts this current could vary between 240 thousandths of an amp. shortened to 240 milliamps and 80 milliamps.

The first effect of such a shock is for the hands (if gripping the live metal) to close due to muscular contraction and prevent the hand being removed.

If the live metal is touched by some other part of the hand, the hand will be thrown off violently which may also cause the body to be thrown with consequent physical damage.

If the current is limited by shoes or clothing the first effect on the body may be muscular contraction and restriction of breathing, causing unconsciousness. Providing the person is removed from the electrical supply or the supply is switched off soon enough recovery may be affected by artificial respiration.

For higher currents and/or longer periods of contact the heart may be affected and go into fibrillation which causes it to beat out of sequence. This in turn restricts the circulation of oxygen in the blood round the brain and death can occur within a couple of minutes.

Unfortunately, once fibrillation occurs the heart may not recover even if the supply is removed and only the use of a defibrillator which, of course, would not usually be available except in a hospital, will stop the heart and then allow it to start again in its normal rhythm.

At higher currents, which would normally need higher voltages, burning of the skin could occur serious enough to cause death. This last phenomenon would not occur at voltages up to 415 volts.


Firstly to have a low enough voltage that a dangerous current could not flow though the body. This would be 50 volts or below. Although this is used in special cases it is not practical for normal domestic or commercial premises.

Secondly to prevent people coming into contact with dangerous voltages. This is the object of barriers and enclosures. As we know from experience these are not foolproof.

The third and only practical way on mains voltage is the RCD which will detect that a fault current is leaking to earth and then cut off the supply rapidly enough to prevent dangerous effects.

How an RCD Works

Operation of the RCD

The way the RCD operates is as follows.

The phase and neutral cables from the supply to the load are passed through a magnetic ring called a toroid. On the toroid is wound a detector winding which is taken to a tripping mechanism. It is the make-up of this tripping mechanism which determines whether the RCD is electromechanical or electronic.

Whilst the current flowing down the phase conductor is balanced by that in the neutral the RCD takes no action. However, if part of the current flows down to earth through metalwork or through a person's body an imbalance occurs between phase and neutral.

This causes a magnetic field in the toroid which is picked up by the detector winding, fed to the tripping mechanism and the RCD opens. It is this speed of opening, usually between 30 and 50 milliseconds, which gives the protection.

The mechanism within an electromechanical tripping device consists of a permanent magnet which holds a tripping arm closed thus holding the RCD in the closed position. A spring attempts to pull the arm away from the magnet.

The Electrical Mechanical Device is called the RCCB, Residual Current Circuit Breaker.

A winding on the magnet is connected to the detector winding on the toroid and demagnetises the permanent magnet in the event of a signal on the detector winding due to a fault to earth. The arm is pulled away by the spring and the RCD trips.

In the case of the electronic RCD the signal from the toroid is fed into an electronic circuit which accepts the signal, amplifies it and instructs a relay to open the RCD.

In order to operate the electronic circuit, a mains feed is required into the circuit to feed the amplifier etc and it is a requirement of the British Standard for electronic RCDs that if the supply to the electronic circuit fails and prevents its operation in the event of an earth fault, the RCD must open. This is particularly important in the case of a loss of neutral which could cause the RCD not to work even though the phase supply is still connected to the load.

This problem does not arise in the case of the electromechanical RCCB in which the tripping is powered by the signal induced into the detector winding on the toroid.

A test circuit is connected from neutral to phase across the toroid and when the test button is pressed creates an imbalance of about 2.5 times the normal tripping current across the RCD which trips it. The use of the test button which should be operated at least every six months verifies the operation of the RCD.

Additional Requirements for RCDs

Standard electromechanical RCCBs are designed to operate on normal supply waveforms and cannot be guaranteed to operate where non-standard waveforms are generated by loads. The most common is the half wave rectified waveform sometimes called pulsating dc or generated by speed control devices, semi conductors, computers and even dimmers.

Type A RCCBs are now commonplace and will operate on normal ac and pulsating dc (up to 6mA).

In addition to the RCCBs which are for use on single phase and neutral supplies three phase RCDs are also available and work on the same principle of imbalance across the three phases or three phases and neutral.


You will recall earlier that the level of current which flows through the body in the event of a shock situation is between 80 mA and 240 mA. It is essential that the detection of the RCD is below the minimum anticipated current through the body and in fact the recommended tripping current for shock protection is a maximum of 30 mA and this is the current recommended in the Wiring Regulations.

A tripping level of 100mA will a give degree of shock protection if it is not possible to use a 30 mA device.

A 300mA device should never be recommended for shock protection and is only intended for equipment and fire protection.

The British Standard requirements for RCDs (BS EN 61008) states that the RCD should operate between 50% and 100% of its rated tripping current. That is 15 mA and 30 mA for the 30 mA RCD. Most 30 mA RCDs operate at levels between 18 mA and 23 mA.

The Wiring Regulations recognize two ways to receive an electric shock. Firstly, indirect contact where a shock is received from metalwork made live by a fault and direct contact where a shock is received from metal intended to carry current such as a cable or electrical element.

In the case where an RCD is intended to provide indirect contact protection only, it is required to trip in less than 300 milliseconds at rated tripping current. In addition, for direct contact protection the RCD must trip in less than 40 milliseconds when 5 times rate tripping current is applied.

The only suitable instrument for testing an RCD is an RCD tester. A loop impedance tester cannot give an indication of the correct operation of an RCD and must never be used.

One RCD which does not comply with the requirements just discussed is the time delay RCCB. This device is time delayed or slugged so that it does not commence to operate for a predetermined time for instance 50 milliseconds which will allow downstream instantaneous RCDs to clear the supply without affecting the TD RCCB.

The time delay RCD is used in situations where the fastest tripping times are not required such as fire protection but must never be recommended for shock protection. The time delay RCCB must always be of lower sensitivity than the instantaneous RCD it is feeding in order to guarantee discrimination.


There are a number of queries which arise on RCDs.

Firstly the use of RCDs on reduced voltage such as 110 volts for a 240 volt RCD.

In the case of electronic RCDs they can only be used at their stated voltage because of the requirements of the electronic circuit.

As far as protection in the event of an earth fault is concerned on electromagnetic RCCBs there is no problem since the operation is dependant on residual current only. However the test circuit has to generate sufficient current to trip the RCCB and this is dependant on voltage.

RCDs are designed to operate at specific frequencies; in the case of the standard devices 50/60 Hz. They can be used at higher or lower frequencies but their tripping characteristics will change and they therefore cannot be recommended.

The following is stated within the newly updated wiring regulations (BS7671:2018 AMD2 2022):

Types of RCD

Different types of RCD exist, depending on their behaviour in the presence of DC components and frequencies. The appropriate RCD shall be selected from the following:

  1. RCD Type AC: RCD tripping on alternating sinusoidal residual current, suddenly applied or smoothly increasing

  2. RCD Type A: RCD tripping on alternating sinusoidal residual current and on residual pulsating direct current, suddenly applied or smoothly increasing

    NOTE 1: For RCD Type A, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current up to 6 mA.

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

    1. for composite residual currents, whether suddenly applied or slowly rising, intended for circuit supplied between line and neutral or line and earthed middle conductor

    2. for residual pulsating direct currents superimposed on smooth direct current.

    NOTE 2: For RCD Type F, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current up to 10 mA.

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

    1. for residual sinusoidal alternating currents up to 1 kHz

    2. for residual alternating currents superimposed on a smooth direct current

    3. for residual pulsating direct currents superimposed on a smooth direct current

    4. for residual pulsating rectified direct current which results from two or more phases

    5. for residual smooth direct currents, whether suddenly applied or slowly increased, independent of polarity.

    NOTE 3: 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 (IΔn) or 10 mA, whichever is the highest value

    RCD Type AC shall only be used to serve fixed equipment, where it is known that the load current contains no DC components.