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What is an RCD?

An RCD (Residual Current Device) is a device designed to provide protection against electrocution or electrical fires, by cutting off the flow of electricity when it detects a fault.

The device detects “leakage” of electric current from a circuit or appliance which can occur when there is a fault within the electrical installation.

The line and neutral conductors of a circuit pass through a transformer within the RCD. In a healthy circuit, the line and neutral currents are equal and opposite.

RCD diagram

If there is a fault, the line and neutral currents are not equal. The imbalance will cause the RCD to trip and break the circuit.

Why do I need RCD protection?

It only requires a very small continuous electric current (40mA or more), flowing through the human body to cause irreversible damage to the normal cardiac cycle (ventricular fibrillation). When somebody comes into direct contact with mains voltage and earth, the current flowing through the body is of the order of 230mA.


Whilst an RCD will not always prevent an electric shock, RCDs rated at 30mA are designed to disconnect the supply before damage or electrocution is likely to occur.


A fuse or circuit breaker alone will not provide the same level of protection.

Where do I need RCD protection?

The current edition of BS7671 IET Wiring Regulations now requires the majority of circuits to have RCD protection within a domestic property. These include:

  • Socket outlets

  • Mobile equipment for use outdoors.

  • Luminaires

  • Locations containing a bath or shower.

Previous editions of the regulations did not require the same level of RCD protection and therefore your installation may not comply with current regulations. but this does not necessarily mean that it is unsafe for continued use. However, the electrical safety would be enhanced by the provision of RCD protected circuits. 

For further information regarding identifying and testing RCDs, please click here.

Typical Risks

Please click on image to enlarge.

  • Faulty appliances or equipment.

  • Accidental mechanical damage to cables - including penetration of cable insulation in walls and beneath floorboards during DIY, or cutting the supply lead/extension lead for an electric lawn mower, hedge trimmer or other garden equipment.

  • Ingress of water - due to a water leak or deteriorated/poorly fitted outside equipment.

  • Vermin – mice, rats and other vermin chewing through cables.

  • Fire risk - when the insulation breaks down due to deterioration or external damage, which can result in ignition of flammable material. 

  • Bad wiring practice – inadequate earthing or bonding, wires trapped during installation, insulation damage during or after installation, or bad system design. 

Types of RCD

The majority of older installations have Type AC RCDs, which were designed for resistive loads. However, with increasing technological advances, more and more equipment relies on electronic components.


Modern appliances, such as LED lighting, washing machines, dishwashers, induction hobs, tumble dryers and electric vehicle charging equipment are becoming increasingly more sophisticated, and have an element of DC residual fault current as part of their normal operation.


This DC current saturates the magnetic core of a Type AC RCD which can either prevent the RCD from operating or reduce the sensitivity, also known as “blinding”. In fact, an incorrectly selected RCD is potentially as dangerous as no RCD at all. 


For this reason, the appropriate type of RCD must be selected according to the connected equipment.

Identifying RCD type

Type AC

Type A

Type B

Type F

RCD characteristics symbols

In the majority of cases when installing RCD protection, we fit Type A RCDs/RCBOs to overcome this problem. However, some manufacturers of appliances or equipment may specify in their instructions that a Type B or Type F is installed. We would always recommend consulting manufacturer’s instructions for confirmation regarding the appropriate selection of RCD device, prior to installation or use of new equipment. 

Please feel free to contact us to find out more about how RCDs can protect you and your home.

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