RCD and RCBO Protection for London Homeowners: What It Is and Why It Matters

What an RCD Does and Why It Is Important

A residual current device, or RCD, is an electrical safety device that monitors the balance of current flowing through the live and neutral conductors of a circuit and disconnects the circuit automatically within thirty milliseconds if it detects a difference between the two, which indicates that some current is flowing to earth through an unintended path. This unintended earth path may be through a person who has received an electric shock, through damaged insulation in a cable or appliance, or through moisture that has created a leakage path to earth. At thirty milliamps of earth leakage current, the RCD trips and disconnects the circuit before the current can cause cardiac fibrillation in a person, which is the principal cause of electrocution fatalities. An RCD provides a level of protection against electric shock that overcurrent devices such as fuses and miniature circuit breakers cannot provide, because overcurrent devices only operate when fault current is high enough to exceed their rated current, while an RCD operates at thirty milliamps regardless of the total circuit current.

In addition to protection against electric shock, RCDs provide protection against electrical fires caused by earth leakage. When insulation degrades in a cable or appliance and current leaks to earth through building materials such as timber joists or plasterwork, the leakage current is typically too low to operate a fuse or MCB but sufficient over time to cause ignition of surrounding materials. An RCD detects this leakage and disconnects the circuit before ignition can occur, which is why RCD protection is now required under BS 7671 for virtually all circuits in a domestic installation including socket outlet circuits, lighting circuits, and heating circuits.

RCDs Versus RCBOs: Understanding the Difference

A standard RCD is typically installed in a consumer unit to protect groups of circuits rather than individual circuits. A thirty milliamp RCD protecting six socket outlet circuits will trip and disconnect all six circuits if any one of them develops an earth fault, removing power from all sockets in the affected group. This is the traditional arrangement in UK domestic consumer units fitted with RCD protection from the late 1990s through to the mid-2000s, where two banks of circuits were each protected by one thirty milliamp RCD. The disadvantage of this arrangement is that a nuisance trip on one circuit, caused for example by a faulty appliance, removes power from all circuits on that bank simultaneously.

An RCBO, which stands for residual current circuit breaker with overcurrent protection, combines the functions of a miniature circuit breaker and an RCD in a single device that protects and monitors one circuit individually. A consumer unit fitted with individual RCBOs for each circuit provides earth fault protection equivalent to a standard RCD arrangement, but a fault on one circuit only trips that circuit, leaving all others unaffected. The BS 7671 eighteenth edition published in 2018 effectively requires individual circuit protection for new installations, making RCBO-equipped consumer units the current standard for new and replacement consumer units in London domestic properties. Prestige Engineers carry out consumer unit upgrades installing modern RCBO-equipped units, and carry out EICR inspections that identify whether existing RCD protection in a London property meets current standards and whether upgrade is required.