RCD Protection — What Trade Electricians Need to Know (2026)

What an RCD Actually Does

An RCD (Residual Current Device) continuously compares the current flowing in the line conductor against the current returning in the neutral. In a healthy circuit these are equal. If some current leaks to earth — for example through a person touching a live part, or through damaged insulation — the line and neutral currents no longer balance. The RCD detects this residual imbalance and disconnects the supply.

The standard rating for additional protection against electric shock is 30mA. At that sensitivity an RCD will disconnect fast enough to dramatically reduce the risk of a fatal shock if someone contacts a live conductor. RCDs also reduce fire risk by cutting off persistent earth leakage faults that would otherwise heat up over time. It is important to understand what an RCD does not do: a 30mA RCD is not a substitute for correct overcurrent protection, and it does not protect against a line-to-neutral shock where current still balances.

The Regulations: BS 7671

RCD requirements in the UK are governed by BS 7671 — the IET Wiring Regulations — currently the 18th Edition incorporating Amendment 2 (2022). BS 7671 is not statutory law in itself, but it is the recognised standard for demonstrating compliance with the Electricity at Work Regulations 1989 and, in dwellings, Part P of the Building Regulations. If your installation does not meet BS 7671, it is very hard to defend the work as safe, and a non-compliant certificate exposes you to real liability.

Amendment 2 in particular expanded the scope of additional protection and introduced surge protection and arc fault considerations. The practical effect for everyday domestic work is that almost every new final circuit now needs 30mA RCD protection — the days of leaving a few circuits unprotected are largely gone.

Where 30mA Additional Protection Is Required

This is the area inspectors look at first. Under BS 7671, 30mA RCD additional protection is required in the following situations. Treat this as a working checklist rather than an exhaustive legal text — always refer to the current regulation numbers on the job.

• Socket-outlets rated up to 32A for use by ordinary persons and intended for general use. In domestic premises this effectively means all general-purpose sockets.
• Circuits in locations containing a bath or shower — bathrooms and shower rooms require 30mA protection across the circuits serving the location.
• Cables concealed in walls or partitions less than 50mm deep without earthed mechanical protection, where the cable does not run in a prescribed safe zone with metallic covering or conduit.
• Mobile equipment used outdoors with a rating not exceeding 32A — for example sockets feeding lawnmowers, pressure washers or power tools used in the garden.
• Luminaires (lighting circuits) in domestic premises — added under Amendment 2, so new lighting final circuits in dwellings now need 30mA protection too.

The common misconception is that RCD protection is only about sockets and bathrooms. Since Amendment 2, the practical default in a new domestic consumer unit is that every final circuit gets 30mA protection. If you are designing a board and leaving a circuit unprotected, you need a specific, documented reason.

RCD Devices: RCCB, RCBO, SRCD and PRCD

"RCD" is a category, not a single product. Choosing the right device is as important as choosing the right rating.

RCCB (Residual Current Circuit Breaker)

An RCCB provides residual current protection only — it has no overcurrent (overload or short-circuit) protection built in, so it must be used together with MCBs. A single RCCB is often used to protect a group of final circuits on a split-load board. Its weakness is exactly that grouping: a single earth fault on one circuit trips the RCCB and takes out every circuit downstream of it.

RCBO (RCD + MCB Combined)

An RCBO combines residual current protection and overcurrent protection in a single module that protects one circuit. RCBOs have become the increasingly standard default for individual circuit protection precisely because they avoid the "whole house off" problem — a fault on one circuit trips only that circuit, leaving the rest of the installation live. For most new domestic and commercial boards in 2026, fully RCBO-populated consumer units are the sensible specification despite the higher component cost. They make fault-finding easier and customer disruption far lower.

SRCD and PRCD

An SRCD (socket-outlet RCD) is an RCD built into a socket-outlet — useful for retrofitting protection at a specific point. A PRCD (portable RCD) is a plug-in or in-line device that provides protection to equipment connected through it, commonly used to protect mobile tools where the fixed installation does not provide 30mA protection. Both have their place, but neither replaces correctly designed protection at the board for new work.

RCD Types: AC, A, F and B

Beyond the device format, RCDs are classified by the kind of residual current they can reliably detect. This is where a lot of older installations now fall short, because modern equipment with electronic components produces leakage that a basic Type AC device cannot see.

• Type AC: detects only sinusoidal AC residual currents. Once the default, it is now widely considered inadequate for circuits feeding modern electronic loads. BS 7671 guidance steers installers away from Type AC where electronic equipment is connected.
• Type A: detects sinusoidal AC plus pulsating DC residual currents. This is the practical baseline for most general-purpose circuits today, because almost everything from LED drivers to washing machines and chargers introduces some DC component.
• Type F: as Type A but additionally suited to circuits supplying equipment with frequency-controlled drives, such as some modern appliances and variable-speed motor loads.
• Type B: detects AC, pulsating DC and smooth DC residual currents. Required where smooth DC leakage can occur — for example certain three-phase equipment, some PV/inverter arrangements and particular EV charging installations.

EV chargers are the headline case. DC fault current from a charger can "blind" a standard Type A RCD. The installation must therefore provide protection against DC fault current — either by using a Type B RCD, or by relying on a charge point with integrated 6mA DC fault protection combined with a Type A device, depending on the equipment manufacturer's instructions and the relevant section of BS 7671. Always follow the charge point installation instructions and specify the protection accordingly; getting this wrong is a common and serious certification failure.

Quick Reference: RCD Type vs Application

Testing RCDs

Specifying the right device is only half the job — it has to be proven to operate. RCD testing happens at two levels.

The User Test Button

Every RCD carries a test button marked "T". Pressing it creates an internal imbalance and should trip the device immediately, confirming the mechanism operates. BS 7671 and manufacturers recommend the user presses this button quarterly (roughly every three months). It is worth labelling the board and telling the customer to do this — the test button is a mechanical check only and does not verify trip time, but a button that fails to trip is an obvious fault.

Instrument Testing

The electrician's test is the meaningful one. Using a calibrated RCD tester you verify operation at the rated residual current (IΔn) and at five times the rated current (5×IΔn), checking the disconnection times against the limits in BS 7671:

• At 1× rated current a 30mA general-type RCD must trip within 300ms.
• At 5× rated current it must trip within 40ms, which proves fast disconnection under a heavier fault.
• The test is typically performed at both 0° and 180° phase angles, and the worst-case (longest) trip time is the one recorded.

Results are recorded on the Electrical Installation Certificate (EIC) for new work, the Minor Works Certificate for small additions, or the Electrical Installation Condition Report (EICR) for periodic inspection. A missing or out-of-limit RCD trip time is a classic reason for a circuit to be coded on an EICR. Record the actual measured times, not just a tick.

Selectivity, Discrimination and Circuit Division

Even with correct devices and passing tests, board design determines how much disruption a single fault causes. Selectivity (also called discrimination) is about making sure that when a fault occurs, only the protective device closest to the fault operates — not a device further upstream that would take out a larger part of the installation.

The most common failure of this principle is the cheap split-load board with two RCCBs, each protecting a bank of circuits. A single earth fault — a failing appliance, water ingress, a damaged flex — trips one RCCB and kills every circuit on that side. In a real home that can mean losing all the upstairs lighting and the boiler at the same time. Spreading circuits across multiple RCDs helps, but the cleanest solution is individual RCBOs per circuit, so a fault isolates only the affected circuit and the rest of the installation stays live.

Where time-graded selectivity is genuinely needed — for instance an upstream RCD protecting a sub-main feeding downstream 30mA RCDs — use a time-delayed (Type S) RCD upstream so the downstream device trips first. Designing this properly matters in commercial and larger domestic installations and is the kind of detail that distinguishes a considered design from a box of breakers thrown together.

Practical Specification Checklist

Before you order parts or sign a certificate, run through the essentials:

• Confirm where 30mA protection is mandatory for the circuits in scope — sockets up to 32A, bathrooms, shallow concealed cables, outdoor mobile equipment and domestic lighting.
• Default to RCBOs for new boards to avoid losing multiple circuits on a single trip, unless cost or space genuinely forces an RCCB grouping.
• Pick the correct RCD type — Type A as the general baseline, Type F or B where electronic, frequency-controlled or EV/PV loads are present. Follow EV charge point instructions for DC fault protection.
• Test and record trip times at 1× and 5× rated current, and document them on the EIC or EICR. Tell the customer to press the test button quarterly.
• Design for selectivity so a single fault isolates the smallest possible part of the installation.

Why This Matters for Your Trade Business

RCD protection is not just a technical detail — it is one of the most visible markers of whether an installation was done properly. Inspectors check it, insurers rely on it, and a coded EICR can stall a house sale or a landlord's compliance. Getting RCD specification, testing and documentation right protects your customers, protects you from liability, and gives you a clear story to tell when a competitor has cut corners with a Type AC board and grouped RCCBs. Accurate certificates and clean records are part of running a credible electrical business — and they are exactly the kind of detail that earns repeat work and referrals.