What is an RCD? A Guide to Safety Devices

Residual current devices are classified as different types of RCD according to where they are fitted in an electrical circuit. RCDs installed in consumer units are also classified according to the curve type they are designed for. As well as this, RCDs are also classified according to whether or not they feature overcurrent protection and their current rating.  

RCD fitting locations

RCDs can be fitted as:

• Fixed RCDs in the consumer unit

• Socket RCDs within an electrical socket

• Portable RCDs that plug into a socket

Fixed RCDs

Fixed RCDs are installed in the consumer unit. They provide a high level of protection, covering faults in any circuits they cover as well as to any appliances plugged into those circuits. Since 2008, it has been mandatory in the UK to install an RCD in virtually all new or rewired domestic circuits.

Fixed RCDs are generally installed in a new consumer unit during a house rewire or other electrical work.

RCD sockets

RCD sockets are plug sockets with built-in RCDs. Any device that is plugged into the socket will be protected by the socket outlet RCD. Socket outlet RCDs do not provide the same level of coverage as fixed RCDs. RCD sockets do, however, make a good choice of socket where appliances need to be protected at a particular location.

Portable RCDs

RCD adapters are RCD devices that plug into a standard electrical socket. When an appliance is then plugged into the portable RCD, it is protected by it. Portable RCDs do not provide the same level of coverage as fixed RCDs. They are useful, however, where a fixed or socket outlet RCD is not available. Portability is also useful in certain situations.

Fixed RCD curve types

RCDs are also classified according to the currents that activate them. It is important that the type of RCD selected for a circuit matches the equipment used on the circuit. Type AC RCDs, for example, are incompatible with modern electrical equipment used in domestic circuits. 

Here are examples of the types of equipment and loads each type is used with: 

1. Type AC – Resistive, capacitive, inductive loads generally without any electronic components.

2. Type A – Single phase circuits with electronic components.

3. Type F – Frequency controlled equipment and appliances.

4. Type B – Three phase electronic equipment.

5. 5. Type S - Timed RCD used in industrial settings to protect against fires, these do not offer protection against electric shocks.

It is very important that RCDs are matched to circuits by a qualified professional.

Overcurrent protection: RCCBs and RCBOs

RCCB and RCBO are both acronyms that stand for different types of RCD. 

RCDs can be grouped into two major types according to whether or not the device features an integrated overcurrent protection capacity. An RCD can be referred to as a residual current circuit breaker (RCCB) or a residual current breaker with overcurrent protection (RCBO).

• RCCB – An RCCB is an RCD with no integrated overcurrent protection

• RCBO – An RCBO is an RCD with integrated overcurrent protection

In many cases, the term RCD is used on its own to refer to RCCBs. RCCBs and RCBOs are, however, both different types of RCD. RCD simply means residual current device (RCD)

RCDs current rating

RCDs are all designed to activate and disconnect a circuit at a set current level.

The level of injury that a person will experience when they suffer an electric shock is highly dependent on the amount of current they are exposed to. RCDs are designed to activate at a low enough current to minimise the risk of injury as much as possible. Current ratings, however, also need to be set high enough to avoid nuisance tripping.

In the UK, domestic RCDs operate at 30 milliamps (mA) because the risk of death from an electrical shock significantly increases above this level.

RCDs are very effective safety devices that significantly reduce the risk of injury or death caused by electric shocks. 

Changes to regulations in recent years also mean that RCDs are now required on virtually all circuits. Previously, some circuits in a configuration, such as lighting circuits were often left without RCD protection. Modern practices, however, have shifted so that virtually all circuits are protected by an RCD.

Modern circuit configurations are often designed with multiple RCDs in a high-integrity layout. This reduces the problem caused by nuisance tripping where single RCDs cover groups of circuits.

An RCD is an essential part of modern consumer units.

RCDs and electric vehicle charging

A common, new feature in both domestic and commercial properties is electric vehicle charging capacity. The regulations regarding electric vehicle charging and the use of RCDs are constantly evolving.

In many cases, electric vehicle charging circuits are required to have their own RCD. 

Is an RCD the same as a circuit breaker?

Residual current devices (RCCB type) offer a much better level of protection against the risk of electrocution or electrical fires than circuit breakers do.

Circuit breakers are designed to offer protection against excess current in an electrical circuit. Circuit breakers prevent overheating and damage to electrical circuits caused by overloading or a short circuit. They are not designed to offer protection against electrical shocks. 

Circuit breakers are still an essential part of every electrical circuit. Miniature circuit breakers ensure the power is cut off to prevent damage to a circuit, typically triggered by an overload or short.

RCDs and overcurrent protection

RCDs can be designed with in-built additional overcurrent protection on top of the normal function of an RCD.

• RCDs with overcurrent protection are referred to as residual current breakers with over-current protection (RCBOs).

• RCDs with no overcurrent protection are referred to as residual current circuit breakers (RCCBs).

What causes an RCD to trip?

RCDs trip when a fault in an electrical circuit, such as when someone touches a live wire, causes an imbalance in the electrical circuit. 

Internally, how RCDs work is through electromagnetism.

Within the mechanism of an RCD is an iron toroid. A live copper wire from the circuit is wrapped around one half of the toroid. A neutral copper wire is wrapped around the other half of the toroid. Another copper wire then feeds from the toroid to a circuit disconnector switch.

Under normal conditions, the live and neutral copper wires are equal in current, and no there is no net electromagnetic in the toroid. However, if, there is an imbalance between the two wires, a magnetic field is created in the toroid. This creates a current that flows down the third wire to the circuit disconnector switch, and the circuit is disconnected.