RCD, RCCB, and RCBO Protection: Types, Testing, and Selection for 18th Edition Compliance

Residual current devices (RCDs) are the front line of shock protection in UK electrical installations. The 18th Edition of BS 7671 — along with Amendment 2 (2022) — has significantly expanded the requirement for RCD protection, making it essential for electricians to understand the full range of device types, their applications, and how to test them correctly on site.

This guide covers RCCBs, RCBOs, SRCD sockets, combined arc fault detection, and how to specify the right device for each circuit type under current UK regulations.

What Is a Residual Current Device?

An RCD monitors the current flowing out through the live conductor and returning through the neutral. Under normal conditions, these currents are equal. If current leaks to earth — through a person receiving a shock, a wiring fault, or a deteriorated appliance — there is a measurable imbalance. When that imbalance exceeds the device's rated sensitivity (typically 30mA for general protection), the RCD trips within milliseconds.

At 30mA the trip time must not exceed 40ms — well within the threshold at which ventricular fibrillation becomes likely. This is why 30mA protection is accepted as the standard for shock risk reduction, while 100mA or 300mA devices are used for fire protection only and must not be relied upon to protect persons.

Types of RCD Device

RCCB — Residual Current Circuit Breaker

An RCCB (sometimes referred to simply as an RCD) provides residual current protection only — it does not incorporate an overcurrent protective function. RCCBs are used in conjunction with MCBs, which provide overload and short-circuit protection upstream or downstream.

In a typical split-load consumer unit, an RCCB covers one group of MCBs — for example, all ring final circuits or all lighting circuits. A single 63A or 80A RCCB feeding multiple MCBs is cost-effective but has a drawback: if any circuit on the group has a fault, the entire RCCB trips, taking out all circuits on that side of the board.

RCCBs are available in 2-pole (single-phase, live and neutral switched) and 4-pole (three-phase, all four conductors switched) configurations. On domestic split-load boards, 2-pole RCCBs are standard. In commercial and industrial three-phase installations, 4-pole RCCBs protect groups of circuits from a 400V three-phase supply.

Current ratings for RCCBs in domestic installations are typically 40A, 63A, or 80A. The rating must be at least equal to the maximum demand of all circuits it protects simultaneously.

RCBO — Residual Current Circuit Breaker with Overcurrent Protection

An RCBO combines the functions of an MCB and an RCD in a single 17.5mm or 18mm module. Each circuit has its own independent RCD protection: if one circuit trips on a residual fault, no other circuit is affected.

This selectivity makes RCBOs the preferred choice for modern consumer unit designs. Under the 18th Edition and Amendment 2, many electricians now fit all-RCBO boards, providing both circuit-selective tripping and full RCD protection on every final circuit — including those that previously relied on the main switch or upstream RCCB.

RCBOs are available in Type B, C, and D trip characteristics to match inductive loads, motor starting currents, and high-inrush equipment. The common ratings are 6A, 10A, 16A, 20A, 32A, and 40A, with 30mA sensitivity for general circuits. Dual-function RCBOs (Type B+D, or combined AFDD+RCBO in a single module) are also available for high-specification installations.

RCD Socket Outlets (SRCD)

Socket outlets with integral RCD protection — commonly known as SRCD or RCD sockets — provide point-of-use protection at the outlet itself. They are commonly used to retrofit protection where an upstream RCD or RCBO cannot easily be added, and are mandatory in certain locations under BS 7671.

SRCD outlets are available as single or double gang, in white, brushed chrome, and other finishes. Always check that the socket is RoHS compliant and carries a BS 1363 approval mark. SRCD sockets have a test button and a reset button on the face and should be tested periodically using the integral test function.

PRCD — Portable Residual Current Device

PRCDs are plug-in RCDs used with portable equipment — particularly power tools on construction sites, agricultural settings, and outdoor locations. They are not a substitute for fixed RCD protection in the consumer unit, but provide additional protection where portable equipment may be used beyond the protection zone of the main installation.

Site PRCDs must be rated at 30mA and tested before use each working day. They should carry the CE mark and be rated to the appropriate current for the load connected.

RCD Types by Waveform Sensitivity

A critically important distinction — and one that is often overlooked — is the type of residual current waveform that an RCD can detect. BS EN 61008 and BS EN 61009 classify devices into types:

• Type AC: Detects sinusoidal AC residual currents only. Provides no protection against pulsating DC residual currents produced by variable speed drives, EV chargers, switch-mode power supplies, and inverters. Type AC is being phased out for new installations under the 18th Edition.
• Type A: Detects sinusoidal AC and pulsating DC residual currents. Suitable for most domestic and commercial circuits including those feeding washing machines, dishwashers, tumble dryers, and standard EV charge points. Type A is the minimum recommended type for most new installations.
• Type F: Detects AC, pulsating DC, and high-frequency residual currents associated with variable speed drives and some inverter-driven equipment. Required for circuits feeding frequency converter loads where pulsating DC may be produced.
• Type B: Detects AC, pulsating DC, smooth DC, and high-frequency residual currents. Required for three-phase EV chargers, PV inverters connected to three-phase supplies, and medical equipment. Type B devices are significantly more expensive than Type A and should only be specified where the load genuinely requires them.

The 18th Edition requires that the type of RCD is selected to be appropriate for the waveform of residual current that may arise from the load. Fitting a Type AC RCBO to a circuit feeding a modern EV wallbox, for example, would fail to provide protection against the smooth DC residual current that the charger can produce. This is a common compliance failure that electricians should audit for during periodic inspection work.

18th Edition RCD Requirements — Amendment 2 (2022)

Amendment 2 to the 18th Edition (effective from January 2022 for new designs) significantly expanded mandatory RCD protection. Key changes affecting domestic and commercial installations include:

• All socket outlets rated up to 32A in residential and similar premises must be protected by a 30mA RCD, regardless of location. Previously, some older installations relied on EEBAD (earthed equipotential bonding and automatic disconnection) without RCD protection on general sockets.
• Cable-in-wall circuits not protected by an earthed metallic covering or installed at a depth of at least 50mm from all surfaces must be RCD-protected at 30mA. In practice, this means virtually all concealed wiring in partition walls and timber-framed structures requires 30mA RCD protection.
• Mobile or transportable units (site offices, exhibition units, food vehicles) must have RCD protection at the incoming supply as well as per-circuit protection.
• AFDD requirements: Regulation 421.1.7 now requires Arc Fault Detection Devices in certain high-risk locations — specifically in accommodation rented under a tenancy agreement, and in locations where risk of fire from arc faults is high. This is an expanding requirement and increasingly standard in new social housing and rental property work.

Surge Protection Devices (SPDs) and RCDs

Where surge protection devices are fitted (now mandatory in many new and replacement consumer unit installations — see our SPD and AFDD guide), the interaction with RCDs must be considered. SPDs can introduce a measured leakage current under normal operating conditions, particularly Type 2 devices with varistor-based clamping. This can cause nuisance tripping on sensitive 10mA RCDs. For most Type 2 SPDs, 30mA RCDs are unaffected, but check the manufacturer's data sheet for maximum leakage current before specifying.

RCD Testing on Site

Operational Push-Button Test

The test button on an RCD simulates a residual current condition and should cause the device to trip immediately. This tests the mechanical operation of the device but not its calibrated trip time or trip current. Every RCD should be tested in this way quarterly or after any event that may have subjected the installation to abnormal conditions.

When testing on a split-load board, note that the test button trips the RCCB (or RCBO) and disconnects all circuits on that group. Always warn occupants and isolate sensitive equipment before testing.

Instrument-Based Testing

During periodic inspection and testing (EICR), or as part of initial verification after installation, RCDs must be tested with a calibrated RCD tester. This confirms both the trip time and, for some testers, the trip current.

For a 30mA general-purpose RCD:

• Trip current test (I∆n × ½): At half the rated sensitivity (15mA), the device must NOT trip. This confirms the device will not nuisance-trip under small leakage conditions.
• Trip time test (I∆n): At rated sensitivity (30mA), the device must trip within 300ms.
• High-current trip time (5 × I∆n): At 5 times rated sensitivity (150mA), the device must trip within 40ms. This is the standard test used in socket outlet testing to confirm protection within the 40ms requirement.

Record all RCD test results on the schedule of test results or periodic inspection report, including the measured trip time at each test level. See our article on electrical installation testing for the full range of initial verification measurements required under BS 7671.

Nuisance Tripping: Causes and Remedies

Nuisance RCD tripping — where the device trips without an apparent fault — is one of the most common complaints from occupants of recently rewired properties. Understanding the causes allows the electrician to diagnose and resolve the issue systematically rather than simply resetting the device.

Common causes include:

• Accumulated earth leakage: Every appliance, length of cable, and fluorescent tube has a small inherent leakage to earth. If multiple appliances are connected to one RCCB and their combined leakage approaches or exceeds 15–30mA, the device may trip intermittently. The solution is to divide the load across multiple RCBOs so each circuit carries less total leakage.
• Damp or contamination in sockets, FCUs, or wiring: Water ingress dramatically increases leakage current. Inspect accessories in bathrooms, kitchens, and utility rooms for signs of moisture.
• New appliances with switch-mode power supplies: Modern computers, televisions, and LED drivers have capacitive filters to ground that can sum up to several milliamps per appliance. On an RCCB feeding many such circuits, the aggregate leakage may be significant.
• Incorrect neutral grouping: If neutral conductors from two separate RCD groups are mixed at a terminal block, residual currents from one group can return via the other group's neutral, causing false imbalance readings and spurious tripping.
• Faulty or ageing appliance: Identify by process of elimination — disconnect appliances one at a time and measure leakage current with an insulation resistance tester at 250V DC (appropriate for capacitive loads). See our testing guide for methodology.

RCD Protection for Outdoor and Special Locations

Certain locations require enhanced RCD protection beyond the standard 30mA general circuits:

• Swimming pools and fountains (Zone 0/1/2): Supplementary equipotential bonding and additional RCD requirements apply. All circuits serving these zones must be RCD-protected. See BS 7671 Chapter 70 (Special Installations).
• Agricultural and horticultural premises: 30mA RCD protection is mandatory for all socket outlets and for circuits to mobile or portable equipment due to the high risk of contact with earth.
• Construction sites: As above — all temporary supplies must be RCD-protected at the source and at the point of use.
• Caravan and camping parks: Each pitch supply point must incorporate a 30mA RCCB covering all socket outlets and luminaires on that supply.
• Locations with floor heating: Regulation 753 requires 30mA RCD protection for all electric floor heating systems including electric underfloor heating mats and cables. See our electric underfloor heating guide.

Consumer Unit Design Considerations

When designing a new consumer unit or replacing an existing board, selecting the RCD protection strategy is one of the most important decisions:

Option 1 — Split-load board with RCCBs: Lower device cost but poor selectivity. Any residual fault on the protected group trips all circuits on that RCCB. Best suited to straightforward domestic rewires where cost is the primary constraint and the number of circuits is modest.

Option 2 — All-RCBO board: Each circuit independently protected. A fault on the kitchen ring does not affect lighting or heating circuits. Higher device cost but significantly better selectivity and end-user experience. Now the standard for quality domestic installations and all commercial fit-outs.

Option 3 — Dual RCD with selective protection: Two RCCBs with time-delayed (S-type) discrimination upstream and instant-trip downstream. Less common in domestic work but used in medical and commercial locations where continuity of supply to critical circuits is essential.

For consumer unit selection, sizing, and MCB/RCBO specification, see our detailed guide to consumer units, MCBs, and RCBOs. For wiring methods and cable selection that inform the overcurrent protection rating needed, see our cable selection guide.

Part P, NAPIT, and NICEIC Requirements

All consumer unit replacements in England are notifiable under Part P of the Building Regulations, regardless of whether the work is otherwise straightforward. Where the work is carried out by a registered competent person (NAPIT, NICEIC, ELECSA, or STROMA), it can be self-certified. Where carried out by an unregistered electrician, a Building Control notification must be submitted before work commences.

Evidence of RCD protection — including the schedule of test results showing RCD trip times — must be provided in the Electrical Installation Certificate (EIC) issued upon completion. Without this, the installation cannot be said to comply with BS 7671 or Part P. See our Part P guide for full details of notification requirements and the self-certification process.

Product Recommendations

APM Electricals stocks a range of RCBOs, RCCBs, and consumer unit accessories from leading brands. Our range includes Type B and Type C RCBOs in 30mA sensitivity for all standard domestic and commercial circuit types, compatible with major consumer unit ranges. Browse our RCBO range for current stock and pricing.

Visit us at APM Electricals, 24 Western Avenue, Acton, London, W3 7TZ or call 020 8896 0800 for trade and retail electrical and plumbing supplies.