An RCD could save your life. It detects earth leakage current and disconnects the supply in milliseconds — before the current through your body can cause a fatal electric shock. This guide explains how RCDs work, why 30mA matters, and when BS 7671 requires them.
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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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An RCD (Residual Current Device) is a life-saving protective device fitted in the consumer unit or distribution board. Its job is to detect earth leakage current — current that is flowing to earth through an unintended path, such as through a person's body or through damaged cable insulation — and disconnect the supply before the leakage can cause a fatal electric shock or a fire.
Every modern domestic installation in the UK has RCD protection. You will see RCDs as wide devices in the consumer unit, typically rated at 63A or 80A with a 30mA sensitivity, each protecting a group of circuits. Alternatively, the installation may use RCBOs — individual devices that combine RCD and MCB protection for each circuit.
The RCD is arguably the most important protective device in a domestic installation. While an MCB protects the cable from overcurrent, the RCD protects people from electric shock. The two devices serve different purposes and are both essential.
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The operating principle of an RCD is elegantly simple. Inside the device is a toroidal (ring-shaped) transformer core with two windings — the line conductor and the neutral conductor both pass through the core.
This is why the RCD is sometimes called a "core balance" device or a "current balance" device — it works by detecting the imbalance between line and neutral currents.
The 30mA (0.03A) trip threshold is not arbitrary. It is based on research into the physiological effects of electric current on the human body.
The 30mA RCD trips before the current through the body reaches a level that would cause fibrillation, provided the exposure time is short. This is the fundamental reason why BS 7671 requires 30mA RCD protection on circuits that present the highest risk of direct or indirect contact — socket outlets, bathrooms, outdoor circuits, and concealed cables.
Elec-Mate records RCD trip times, ramp test results, and test button confirmations directly into the EICR schedule of test results.
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Not all RCDs are the same. Different types are designed to detect different waveforms of leakage current. See our detailed RCD types guide for the full breakdown.
In a traditional split-load consumer unit, two RCDs each protect a group of circuits. If any circuit in a group develops an earth fault, the RCD trips and disconnects all circuits in that group. This means a faulty toaster can knock out the lights, the fridge, and the heating — everything on that RCD.
Two RCDs, each protecting a group of circuits (typically 5-8 circuits each). Cost effective. The downside is that a fault on any circuit trips the entire group. Common in older consumer units and budget installations.
Every circuit has its own RCBO. A fault on one circuit trips only that circuit — everything else stays on. Better selectivity, better for the occupant. Higher initial cost, but increasingly the standard for new installations and consumer unit upgrades.
For landlords and commercial properties, an RCBO board is particularly valuable. A tripped RCD that takes out the freezer, the fire alarm, or the server room can cause significant damage and liability. With individual RCBOs, the impact of any single fault is contained to the affected circuit.
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Start free trialBS 7671:2018+A4:2026 requires 30mA RCD protection for the following:
In practice, this means almost every circuit in a modern domestic installation requires 30mA RCD protection. The few exceptions (such as smoke alarm circuits) are typically protected anyway because it is simpler to RCD-protect everything than to create an unprotected group.
An RCD trips because current is leaking to earth. Finding the source is a systematic process. See our detailed RCD keeps tripping guide for the full fault-finding procedure.
The most common cause. A washing machine with a failing heater element, a kettle with internal corrosion, or any appliance with damaged insulation can leak current to earth. Unplug appliances one at a time and reset the RCD to identify the culprit.
Water in an outdoor socket, a leaking roof dripping onto a junction box, or condensation in a light fitting. Moisture provides a path for current to leak to earth. Often worse in wet weather or after rain.
In older properties, the PVC insulation on cables can degrade over time — especially in hot locations (near boilers, in lofts during summer) or where cables have been damaged during building work. The insulation resistance test during an EICR identifies this.
Every appliance has a small, normal standing leakage current (typically 0.5-3.5mA each). When many appliances are connected to circuits protected by the same RCD, the cumulative leakage can approach or exceed 30mA — causing nuisance tripping even though no single appliance is faulty. This is a design issue best solved by splitting circuits across multiple RCDs or using RCBOs.
During an EICR, the electrician tests each RCD to verify it trips correctly. The tests include:
All test results are recorded on the schedule of test results section of the EICR. The trip times must be compared against the maximum permitted values to determine compliance.
Elec-Mate walks you through every test, records RCD trip times by voice entry, cross-references against BS 7671 limits…
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