WIRING GUIDE

RCD Types UK Guide: RCCB, RCBO, SRCD and RCDM Explained

Everything electricians and property owners need to know about RCD types in UK installations — RCCB, RCBO, SRCD, and RCDM explained with BS 7671 18th Edition requirements, sensitivity ratings, and practical selection guidance.

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12 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

  • 1RCDs (Residual Current Devices) detect earth leakage currents and disconnect the circuit within milliseconds, protecting people from electric shock and reducing fire risk.
  • 2BS 7671 18th Edition requires additional protection by 30mA RCDs for socket-outlet circuits rated up to 20A in most installations — this is one of the most commonly cited requirements during EICRs.
  • 3RCCBs protect multiple circuits but provide no overcurrent protection — they must be used with MCBs upstream or downstream.
  • 4RCBOs combine MCB overcurrent protection and RCD earth-fault protection in a single device, eliminating nuisance tripping between circuits.
  • 5SRCDs (socket-outlet RCDs) provide protection at the point of use and are suitable for retrofit situations where a full board upgrade is not practical.
  • 6Fire-protection RCDs rated at 100mA or 300mA are used where 30mA would cause nuisance tripping but earth-fault protection is still required.
01 · Wiring Guide

What Is an RCD?

A Residual Current Device (RCD) is a protective device that monitors the balance of current flowing in the live and neutral conductors of a circuit. Under normal conditions, the current flowing out through the live conductor equals the current returning through the neutral conductor. If a fault develops — for example, because someone touches a live conductor and current flows through their body to earth — the balance is disrupted and a residual current flows.

When the RCD detects a residual current above its rated threshold, it disconnects the circuit within milliseconds. This rapid disconnection is what makes RCDs effective at preventing electrocution: a standard 30mA RCD must trip within 40 milliseconds at rated current, well within the time needed to cause cardiac arrest.

  • Personal protection — 30mA RCDs protect against electric shock from direct or indirect contact. They are the primary means of additional protection required by BS 7671 18th Edition for socket-outlet circuits.
  • Fire protection — 100mA and 300mA RCDs detect sustained earth faults that generate sufficient heat to start a fire, even where 30mA protection would cause nuisance tripping.
  • Not overcurrent protection — RCDs detect earth leakage only. They do not provide protection against overload or short-circuit currents. Separate MCBs or fuses must be provided for overcurrent protection (unless an RCBO is used).

There are four main types of RCD used in UK domestic and commercial installations: RCCB, RCBO, SRCD, and RCDM. Each has different applications, advantages, and limitations.

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02 · Wiring Guide

RCCB — Residual Current Circuit Breaker

The RCCB (Residual Current Circuit Breaker) is the most familiar type of RCD in UK domestic installations. It is a stand-alone device installed in the consumer unit that provides earth-leakage protection for a group of circuits. RCCBs come in 2-pole versions (for single-phase circuits) and 4-pole versions (for three-phase circuits).

  • No overcurrent protection — an RCCB provides only earth-fault protection. Individual MCBs must be provided for each circuit downstream (or upstream, depending on the board design). In a typical split-load consumer unit, the RCCB covers one half of the board while individual MCBs protect each circuit.
  • Nuisance tripping risk — because one RCCB protects multiple circuits, an earth fault on any one circuit trips protection for all circuits on that RCCB. This is the main practical disadvantage compared to individual RCBOs.
  • Cost-effective for new installations — RCCBs are significantly cheaper than RCBOs. In a new consumer unit, using an RCCB to protect a group of circuits with individual MCBs is a cost-effective approach, particularly where circuits are unlikely to cause individual nuisance tripping.
  • Types — Type AC (responds to sinusoidal AC residual currents), Type A (responds to AC and pulsating DC residual currents), and Type B (responds to AC, pulsating DC, and smooth DC). Type A is now standard for most domestic installations. Type B is required for some EV chargers and inverter-connected equipment.
03 · Wiring Guide

RCBO — Combined MCB and RCD Protection

An RCBO (Residual Current Breaker with Overcurrent protection) combines the functions of a Miniature Circuit Breaker (MCB) and a Residual Current Device (RCD) in a single device. Each RCBO provides overcurrent protection and 30mA earth-fault protection for one individual circuit, fitting in a single-module consumer unit slot.

  • No nuisance tripping between circuits — because each circuit has its own RCBO, an earth fault on one circuit only trips that circuit. Other circuits remain live. This is the principal advantage of the all-RCBO consumer unit, particularly in domestic properties where a tripped RCCB at 2am can be extremely disruptive.
  • Higher cost — RCBOs cost three to five times more per unit than a comparable MCB. An all-RCBO consumer unit is typically £150 to £300 more expensive than an equivalent split-load RCCB/MCB board, though this gap has narrowed as RCBOs have become more common.
  • Preferred for new consumer unit installations — many electricians now specify all-RCBO boards as standard for new domestic consumer unit replacements, as the additional cost is modest relative to the labour and the customer experience benefit is significant.
  • RCBO types — like RCCBs, RCBOs come in Type AC, Type A, and Type B variants. BS 7671 18th Edition (as amended) requires Type A or Type B RCDs for circuits supplying EV charge points and equipment with electronic power conversion.
04 · Wiring Guide

SRCD — Socket-Outlet Residual Current Device

An SRCD (Socket-outlet RCD) is a socket outlet with a built-in 30mA RCD. It fits in a standard single-gang or double-gang back box and provides additional protection at the point of use. SRCDs are identifiable by their test and reset buttons on the socket face.

  • Retrofit solution — SRCDs are particularly useful for retrofitting 30mA additional protection on individual sockets without replacing the consumer unit. They are commonly used to provide RCD protection on a single socket where a specific risk exists, such as a garage or outdoor socket on an older installation.
  • Point-of-use protection only — an SRCD protects only the equipment plugged into that socket. It does not protect the cable feeding the socket, other sockets on the same circuit, or provide any overcurrent protection.
  • Depth consideration — SRCDs are thicker than standard sockets due to the built-in electronics. A 35mm back box is usually required (standard sockets often use 25mm boxes). This must be checked before ordering.
  • Monthly testing — like all RCDs, SRCDs should be tested monthly using the integral test button to verify correct operation. The test button must be pressed and the socket must disconnect. This is particularly important in rental properties where the equipment is used by tenants unfamiliar with electrical safety.
05 · Wiring Guide

RCDM — RCD Module

An RCDM (RCD Module) is a modular device that plugs into a spare way in a consumer unit or distribution board to add RCD protection for a specific circuit or group of circuits. RCDMs are used in retrofit situations where individual circuit protection needs to be added to an existing board without full replacement.

  • Board compatibility — RCDMs are manufacturer-specific and must be compatible with the consumer unit they are installed in. They cannot be mixed between brands in most cases. Always check the manufacturer's compatibility list before specifying an RCDM for a retrofit installation.
  • Requires a spare way — an RCDM occupies one or more ways in the consumer unit. If the board is already full, an RCDM cannot be added without first freeing up a way or replacing the board with a larger unit.
  • Practical use case — RCDMs are particularly useful in commercial and industrial distribution boards where individual circuits need RCD protection added without replacing the entire board. They are also used in older domestic consumer units where a full replacement is not justified but one or two circuits require additional protection.

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06 · Wiring Guide

30mA, 100mA and 300mA — Understanding RCD Sensitivity

RCDs are available in different sensitivity ratings, expressed as the rated residual operating current (IΔn). The sensitivity rating determines both the level of protection provided and the susceptibility to nuisance tripping from normal circuit leakage.

  • 30mA — Personal protection — the standard sensitivity for additional protection in domestic and commercial installations. A 30mA RCD must disconnect within 40ms at rated current. At 150mA (5 × IΔn), it must disconnect within 40ms. The 30mA threshold was established as the level below which ventricular fibrillation is extremely unlikely in healthy adults, assuming rapid disconnection.
  • 100mA — Fire protection (medium sensitivity) — used where 30mA would cause nuisance tripping but earth-fault protection against fire risk is required. Common in commercial and industrial installations with large numbers of connected appliances that have inherent leakage capacitance.
  • 300mA — Fire protection (low sensitivity) — used for the main incoming protection on large installations or where significant normal leakage makes 100mA impractical. A 300mA RCD will not provide personal protection but will disconnect on sustained earth faults that could start a fire. Often used as the upstream RCD in a selective discrimination scheme with downstream 30mA devices.

Selective coordination (discrimination) between RCDs is achieved by ensuring that the upstream device has a higher sensitivity rating and/or a time delay. For example, a 300mA time-delayed RCD upstream of multiple 30mA RCDs ensures that a single-circuit fault only trips the 30mA device, not the entire installation.

07 · Wiring Guide

BS 7671 Requirements for RCD Protection

BS 7671 18th Edition (as amended) sets out where RCD additional protection is required. Understanding these requirements is essential for EICR inspections and new installation design alike.

  • Socket-outlet circuits ≤20A — BS 7671 18th Edition requires additional protection by a 30mA RCD for all socket-outlet circuits with a rated current not exceeding 20A. This is one of the most commonly cited requirements during EICR inspections of older properties, where sockets on ring final circuits may have no RCD protection at all.
  • Mobile equipment circuits ≤32A outdoors — circuits supplying mobile equipment for use outdoors require 30mA additional protection. This includes garden sockets, garage sockets, and outdoor supplies for garden equipment, power tools, and similar.
  • Locations with increased risk — bathrooms, shower rooms, swimming pools, agricultural premises, construction sites, and caravans all require enhanced RCD protection under their specific BS 7671 chapters. Bathroom circuits require 30mA RCD protection for all circuits within the room.
  • Cables buried in walls — cables buried in walls at a depth less than 50mm that are not protected by earthed metallic covering, mechanical protection, or a 30mA RCD must be installed in a zone above or beside an accessory. This requirement ensures that anyone drilling into a wall is protected if they strike a cable.

The absence of RCD protection on socket-outlet circuits is a C2 observation (potentially dangerous) on an EICR for most domestic and commercial properties, making the report Unsatisfactory. Landlords and property owners must arrange for RCD protection to be installed as a remedial action.

08 · Wiring Guide

Choosing the Right RCD Type for Your Installation

Selecting the correct RCD type depends on the installation type, the number of circuits, the budget, and the sensitivity to nuisance tripping. The following guidance covers the most common UK domestic and light commercial scenarios.

  • New domestic consumer unit — specify an all-RCBO board with Type A RCBOs as standard. The additional cost over a split-load RCCB/MCB board is modest and eliminates nuisance tripping completely. Where budget is constrained, a split-load board with two RCCBs (one for lighting, one for power) provides acceptable protection.
  • EV charging circuit — EV charger circuits require a Type A or Type B RCD (not Type AC). If the EV charger has built-in Type B protection, a Type A upstream RCD is acceptable. Check the charger manufacturer's specifications carefully, as requirements vary between products.
  • Retrofit on existing board — where the existing board is in good condition, individual RCBOs can replace MCBs for circuits requiring additional protection. Alternatively, an RCDM can be added for grouped circuit protection, or SRCDs can provide point-of-use protection for individual sockets.
  • Commercial installation with high leakage — consider 100mA or 300mA RCDs as the upstream device with 30mA RCDs downstream. Design the system for selective coordination so that downstream faults trip only the local 30mA device.
09 · Wiring Guide

For Electricians: RCD Work and Certification

Consumer unit replacements to install RCD or RCBO protection are notifiable work in England and Wales under Part P of the Building Regulations. The work must be carried out by a competent person registered with an approved scheme (NICEIC, NAPIT, ELECSA, or similar) or notified to the local building control authority.

Certificate and Test Results

Every consumer unit replacement or new circuit installation requires an Electrical Installation Certificate (EIC) with a Schedule of Test Results. Use the Elec-Mate EIC app to complete certification on site and export a compliant PDF instantly.

EICR RCD Testing

During an EICR, RCDs must be tested using a calibrated RCD tester. The test verifies trip time at IΔn (rated current) and at 5 × IΔn. The integral test button test alone is not sufficient for an EICR. Record all RCD test results on the Schedule of Test Results.

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