What Is an RCD? Meaning, Types and How They Work (UK)

An MCB will save your house from a fire. An RCD will save you from electrocution. They protect against completely different dangers — and every modern electrical installation needs both.

Yet the RCD is the less understood of the two. Most people know their circuit breaker trips when they overload a socket. Far fewer understand why that small button labelled TEST on their consumer unit is one of the most important safety features in their home.

This guide explains what an RCD is, what the letters stand for, how it works, how the types and ratings differ, what UK wiring regulations require, and how you can safely simulate RCD behaviour using ElectraSim — a free, browser-based electrical circuit simulator.

Try it now: ElectraSim includes a fully functional RCD component. Build a protected circuit, simulate an earth fault, and watch the RCD trip — all in your browser, no sign-up required. Open ElectraSim →

What Does RCD Stand For?

RCD stands for Residual Current Device. The name describes exactly what the device measures.

An RCD continuously compares the current going out along the Live conductor with the current coming back along the Neutral. In a healthy circuit those two are equal, so the difference is zero. Whatever difference is left over — the residual current — is current that has found another path back to the supply, usually through earth.

When that residual current rises above the device rating, the RCD opens the circuit and disconnects the supply. A 30 mA RCD is the standard domestic rating because 30 mA of current through a person can be lethal, and the device is designed to disconnect well before that current becomes dangerous.

Other Names for the Same Idea

The RCD has accumulated a lot of names. They are not always interchangeable, but they all describe a device that trips on earth leakage rather than overload:

In the UK the plain term RCD is the one used by BS 7671, manufacturers, and electricians. If a label on a consumer unit says RCCB or RCD, it is an earth leakage device and it will not trip on overload. If it says RCBO, it does both jobs.

What Is an RCD and What Does It Do? (Quick Answers)

If you only want the headline answers, start here:

An RCD is a sensitive, life-saving safety switch. It cuts off power in a fraction of a second when it detects earth leakage, by comparing the current flowing out through the Live conductor with the current returning through the Neutral. If the two do not balance, current is leaking to earth — often through a person or damaged wiring — and the device trips to prevent a fatal shock or an electrical fire.

Why an MCB Cannot Protect Against Electrocution

This is the most important distinction in electrical protection — and the one most people misunderstand.

An MCB trips on overcurrent: it only activates when far more current flows than the circuit is designed for (typically 3–20× the rated current, depending on trip curve). A potentially lethal shock through a human body draws around 30–100 milliamps — a fraction of what even the smallest 6 A MCB needs to trip. The MCB will not even notice.

An RCD trips on imbalance: it monitors the difference between Live and Neutral current with extreme sensitivity — typically 30 mA for personal protection. A 30 mA RCD is designed to disconnect quickly enough to reduce the risk of fatal electric shock; the exact trip time depends on the test current and device type.

The brutal truth: without an RCD, you could receive a potentially fatal shock and the MCB would never trip. The circuit would remain live the entire time.

If you want the overcurrent side explained separately, see what an MCB is and how it works and why an MCB keeps tripping.

How an RCD Works: The Core Principle

Inside every RCD is a toroidal transformer — a ring-shaped core with two windings:

1. The Live wire passes through the ring in one direction
2. The Neutral wire passes through the ring in the opposite direction

In a healthy circuit, the current in Live equals the current in Neutral. Their magnetic fields cancel each other out exactly, and the transformer core sees zero net flux.

When a fault occurs — say, 30 mA leaks to earth through a person — the Live current is now 30 mA higher than the Neutral current returning. This imbalance creates a small net magnetic flux in the toroidal core. This induces a voltage in a third sensing winding, which triggers the trip solenoid, snapping the switch contacts open.

The entire process — from fault to contacts opening — is very fast, but the permitted trip time depends on the residual current. A standard 30 mA RCD is commonly checked at rated residual current and at five times rated residual current; the 5× test is the one associated with the familiar 40 ms maximum.

Note that the Earth conductor does not pass through the toroid. That is deliberate: earth is the path the fault current escapes along, so the imbalance is detected precisely because the earth wire bypasses the sensing core.

RCD Ratings: 10 mA, 30 mA, 100 mA and Above

RCD sensitivity is quoted as the rated residual operating current, written as IΔn on the device. The IEC divides devices into three broad groups:

For home wiring, 30 mA is the standard for any circuit where a person could come into contact with a live conductor. Lower ratings such as 10 mA exist for special locations such as medical areas, where even 30 mA is considered too high. Higher ratings such as 100 mA or 300 mA are not personal protection: they are too insensitive and too slow to prevent electrocution, and are chosen for fire protection or for a main incoming device that must not trip every time a 30 mA device downstream operates.

How Fast Does an RCD Trip?

Speed is the whole point. A 30 mA shock is survivable or fatal depending on how long it lasts, so the product standards (BS EN 61008 for RCCBs and BS EN 61009 for RCBOs) set maximum disconnection times:

Two things follow from that table. First, the device reacts faster to a larger leakage current — exactly the behaviour you want when someone has touched a live conductor. Second, a 30 mA RCD that takes a fraction of a second to operate is still fast enough to be the difference between a shock and a fatality, which is why the regulation is written around 30 mA rather than a larger, less nuisance-prone value.

Time-Delayed (S-Type) RCDs

Some RCDs are deliberately slower. An S-type (selective, or time-delayed) RCD introduces a short intentional delay and typically has a higher trip threshold. This is not a fault — it exists so that a device fitted at the origin of an installation does not operate before the 30 mA device on the faulty final circuit. That coordination is called discrimination, and it is the reason a TT-earthed property can have a 100 mA time-delayed device upstream of 30 mA protection on each circuit. The layout choices are covered in detail in RCD protection zones and consumer unit design.

RCD Types Explained: AC, A, F and B

Not all RCDs are the same. They differ by trip current sensitivity and application. The type letter describes which shapes of residual current the device can detect:

This matters more than it used to. Virtually every modern appliance contains switched-mode power electronics, which can produce pulsating DC earth fault currents that a Type AC device is not designed to detect. That is why BS 7671 Amendment 2 (2022) requires Type A — or better — RCD protection for socket outlets up to 32 A and for final circuits in domestic premises. An old Type AC RCD is not automatically dangerous, but it is no longer the right device for new domestic work. The selection trade-offs are set out in the RCD protection zones guide.

Poles: 2-Pole and 4-Pole RCDs

A single-phase device is normally 2-pole: it switches both Live and Neutral. Two-pole operation is important because a fault can be cleared only if the device breaks the Neutral as well as the Live — and some RCDs use a “solid neutral” arrangement, which is worth checking before you assume what a trip actually isolates.

Three-phase installations use 4-pole devices, which switch all three line conductors plus the neutral. In a three-phase circuit the toroid sums the currents in all four conductors: a healthy balanced or unbalanced circuit still adds to zero, so any leftover is again genuine leakage.

What RCDs Cannot Protect Against

RCDs are specifically designed for earth fault and electrocution protection. They do not protect against:

• Overload — too much current on one circuit (that’s the MCB’s job)
• Short circuit between Live and Neutral (the MCB handles this)
• Live-to-Neutral shock — if you touch both Live and Neutral simultaneously, current flows between them, not to earth. The RCD sees no imbalance and does not trip. This is why safe working practice still matters even with RCDs installed.

An RCD also cannot fix a circuit whose earthing is inadequate. If there is no effective fault path, the leakage current may be too small or too unpredictable to trip the device quickly. Earthing systems are explained in TN-S, TN-C-S and TT earthing, and the conductor roles in live, neutral and earth wires.

Where RCD Protection Is Required (UK)

BS 7671 (the IET Wiring Regulations) does not require an RCD on every circuit in every installation, but it requires one wherever the risk of earth fault shock is significant. The main domestic triggers are:

• Regulation 411.3.3 — socket outlets: most socket outlets rated up to 32 A in domestic premises must have 30 mA RCD protection
• Regulation 411.3.4 — domestic lighting: AC final circuits supplying luminaires in domestic premises must also have 30 mA RCD protection
• Regulation 522.6.7 — cables in walls: any cable buried less than 50 mm deep without earthed metallic containment needs 30 mA RCD protection
• Section 701 — bathrooms: all circuits in a bathroom location must be RCD protected
• TT earthing systems: where the loop impedance cannot be met by overcurrent protection alone, an RCD is the means of achieving disconnection
• Outdoor, garage and shed circuits: sockets and equipment usable outdoors must be RCD protected, which in practice means 30 mA

An RCD that has been correctly installed is also checked as part of an EICR — the inspection and testing report a landlord or homeowner commissions periodically. If you are buying or renting a property, the RCD arrangement is one of the things that report describes; see when to get an EICR.

RCD vs MCB vs RCBO

The answer to “which do I need?” is both — they protect against entirely different hazards and are not interchangeable.

For any new installation or rewire, RCBOs on every circuit is the current best practice and what most modern wiring standards mandate. The full comparison — including how to read the labels on the devices in your own board — is in RCD vs MCB vs RCBO.

Consumer Unit Layouts: Single RCD, Split-Load, and RCBO Boards

How RCDs are arranged determines what happens to the rest of the house when one circuit faults.

Single RCD (Older Installations)

One 30 mA RCD covers half or all of the consumer unit. Simple and cheap, but one fault on any circuit trips everything on that RCD — including the fridge and any medical equipment.

Split-Load Consumer Unit

The consumer unit is divided: one half is protected by an RCD, the other is not. In the past, critical circuits (fridge, freezer, alarm) were put on the unprotected side — but an ordinary socket circuit on the unprotected side is no longer compliant for new work, because Regulation 411.3.3 requires 30 mA protection for sockets in domestic premises.

Individual RCBOs (Modern Best Practice)

Every circuit has its own RCBO — a combined RCD + MCB. A fault on the kitchen sockets trips only that RCBO, and every other circuit stays live. This is the current best practice for new domestic installations and the layout most electricians now fit by default.

Testing Your RCD: The TEST Button

Every RCD and RCBO has a TEST button. Pressing it creates an internal fault current that bypasses the toroidal transformer, simulating a real earth leakage. The RCD should trip immediately — the button press should feel firm and the device should click off.

How often to test: At least every 6 months for domestic installations. Monthly for high-risk environments (workshops, outdoor circuits used regularly).

What to do if it doesn’t trip: The RCD is faulty and must be replaced immediately. An RCD that doesn’t trip on test provides no protection.

Important: A TEST button failure means the RCD is defective — not that the circuit is fine. Replace it before using any circuits it protects.

The TEST button confirms the trip mechanism works, but it is not a calibration check. Proving that the device still operates at its rated residual current, within the permitted time, needs an instrumented test — that is part of periodic inspection and testing, not something a homeowner can do with the button.

Why Does My RCD Keep Tripping?

An RCD that trips is usually doing its job. The most common causes are a faulty appliance, moisture in an outdoor socket or light fitting, a neutral-to-earth fault, cumulative leakage from several devices, or damaged cable insulation.

The useful diagnostic is which circuits lose power and when the trip happens. A trip that only occurs when it rains points somewhere completely different from a trip that happens as soon as the washing machine heater energises.

Because the safe method is a process of elimination rather than guesswork, it is covered step by step in why your RCD keeps tripping.

Common RCD Mistakes

• Assuming the RCD replaces the MCB. A plain RCD provides no overload protection at all — the circuit still needs overcurrent protection, or a combined RCBO.
• Fitting Type AC devices in a modern installation. Type A is now required for domestic socket and final circuits because of pulsating DC leakage from modern electronics.
• Repeatedly resetting a tripping RCD. If it trips again immediately, something real is leaking. Reset once, then stop and investigate.
• Treating “RCD protected” as “safe to touch”. An RCD limits shock duration; it does not prevent you from touching live parts, and it does not detect a Live-to-Neutral shock.
• Only ever pressing the TEST button. The button checks the mechanism, not the trip threshold — instrumented testing is needed to prove the device still meets its rating.
• Ignoring the supply side. If the RCD trips, the connected circuit is isolated, but cables and terminals upstream of the device may still be live.

How to Simulate RCD Protection in ElectraSim

ElectraSim includes a full RCD component so you can understand and verify RCD behaviour before working on real circuits. Here’s how to build and test an RCD-protected socket circuit:

Building the circuit:

1. Open ElectraSim
2. Place a Live (L) terminal and a Neutral (N) terminal
3. Place an MCB — wire Live → MCB
4. Place an RCD — wire MCB output → RCD L-in, wire Neutral → RCD N-in
5. Place a 3-Pin Socket — wire RCD L-out → Socket L, RCD N-out → Socket N
6. Wire an Earth (E) terminal directly to Socket E
7. Press Run — the socket is live and fully protected

What this demonstrates:

• The circuit has two layers of protection: the MCB handles overloads and short circuits; the RCD handles earth faults
• The Live and Neutral both pass through the RCD, so any imbalance is detected
• The Earth wire bypasses the RCD entirely — it provides a low-resistance fault path back to the source, which is exactly what triggers the RCD to trip

This is the fundamental wiring pattern behind every protected socket outlet, bathroom circuit, and outdoor power point in a modern home.

The RCD component guide covers the terminals and internals of the device itself, and Challenge Mode includes a mission to build an RCBO-protected socket circuit once you are comfortable with the basics.

Frequently Asked Questions

What does RCD stand for?

RCD stands for Residual Current Device. The “residual” is the leftover current that remains when you subtract the current returning on the neutral from the current leaving on the live conductor. In a healthy circuit that value is zero; RCDs trip when it is not.

Is an RCD the same as a circuit breaker?

Not quite. In everyday speech people say “circuit breaker” for anything in the consumer unit, but an MCB trips on overload and short circuit, while an RCD trips on earth leakage. They protect against different hazards. An RCBO is both devices in one unit, which is why modern boards often contain nothing else.

What is the difference between a 30 mA and a 100 mA RCD?

30 mA is a personal protection rating — sensitive enough to reduce the risk of a fatal shock. 100 mA is a fire protection or upstream rating. It is too insensitive to protect a person, and is normally used as a time-delayed device at the origin of an installation, ahead of 30 mA devices downstream.

Does an RCD protect me from electric shock?

It limits the duration of a shock. If you touch a live conductor and current flows to earth, the resulting imbalance trips the RCD in a fraction of a second — reducing the risk of a fatal outcome. It does not stop you receiving a shock, and it will not respond at all if the current flows between live and neutral rather than to earth.

Can I replace an MCB with an RCBO?

An RCBO combines overcurrent and earth leakage protection in one device, so in principle it replaces both the MCB and the separate RCD. Whether one physically fits depends on the make, model and busbar arrangement of your consumer unit, and adding RCD protection to an existing circuit is a change that needs to be designed and certified correctly. This is electrician work, not a like-for-like swap.

Why does my RCD trip when it rains?

Rain is a strong hint that the leakage path is outside — water bridging the terminals of an outdoor socket, a garden light fitting full of water, or damaged cable insulation in a wet location. If your board is split, note which circuits lose power; isolating the outdoor circuit is usually the quickest way to confirm the cause.

How often should I test my RCD?

Press the TEST button at least every six months, and more often if the device protects outdoor or workshop circuits. The test is quick, requires no tools, and is the only way to know the trip mechanism still works.

Will an RCD protect my lighting circuit?

In modern UK domestic installations, yes — BS 7671 requires 30 mA RCD protection for final circuits supplying luminaires in domestic premises. In older consumer units with a single shared RCD, the lighting may sit on the unprotected side, which is one reason older boards are recommended for replacement.

Key Takeaways

• RCD stands for Residual Current Device — it measures the imbalance between Live and Neutral current and trips when that difference means current is leaking to earth
• A 30 mA RCD trips on a current too small for any MCB to notice — this is by design, because 30 mA can kill
• Type A is the modern domestic requirement; Type AC is common in older installations but is not the right choice for new work
• MCBs and RCDs protect against different things — every circuit needs both, ideally as a combined RCBO
• Press the TEST button every 6 months, and remember it checks the mechanism, not the trip threshold
• RCDs do not prevent overloads, Live-to-Neutral shocks, or poor earthing — they limit the duration of an earth fault shock
• You can safely simulate and explore RCD-protected circuits for free in ElectraSim — no installation, no sign-up

Ready to build your first RCD-protected circuit? Open ElectraSim now → and wire up a protected socket outlet in under two minutes.