What Is an RCD? How It Works, Types, and When BS 7671 Requires One

The Residual Current Device — the RCD — is one of the most important protective devices in modern electrical installations. It provides additional protection against electric shock by detecting current leakage to earth and disconnecting the supply in milliseconds — far faster than an MCB or fuse could react.

 

Understanding how RCDs work, the different types available, and where BS 7671 requires them is essential for the IET exam. This guide covers all of it.

 

How an RCD Works

An RCD monitors the current flowing in the Line and Neutral conductors of a circuit. Under normal conditions, the current flowing out on the Line is exactly equal to the current returning on the Neutral — the two cancel each other out.

 

If a fault occurs — say, current leaks through a person touching a live part to earth — some current takes a different path and doesn’t return via the Neutral. The RCD detects this imbalance and trips.

 

 

The Core Transformer

Inside every RCD is a toroidal (ring-shaped) current transformer. Both the Line and Neutral conductors pass through this core. Under normal conditions:

• The magnetic field created by the Line current is exactly opposed by the Neutral current
• The net magnetic flux in the core is zero
• No voltage is induced in the sensing coil — the RCD stays on

 

When there’s a leakage to earth:

• Less current returns via the Neutral than went out on the Line
• The magnetic fields don’t cancel — there’s a net flux in the core
• A voltage is induced in the sensing coil
• This triggers the trip mechanism and disconnects the supply

 

A standard 30 mA RCD will trip when the leakage current reaches 30 milliamps — well below the level that would cause a fatal electric shock (typically 50 mA and above for a sustained period).

 

Types of RCD

 

RCCB (Residual Current Circuit Breaker)

An RCCB provides earth fault protection only — it does not protect against overload or short circuit. It’s typically installed in a consumer unit to protect a group of circuits, with individual MCBs providing overcurrent protection for each circuit.

 

• Typical ratings: 63A or 80A, 30 mA sensitivity
• Disadvantage: A fault on any circuit downstream trips the RCCB, cutting power to all circuits it protects
• Often found in split-load consumer units (one RCCB protecting half the circuits)

 

RCBO (Residual Current Breaker with Overcurrent)

An RCBO combines an RCD and an MCB in a single device. It provides earth fault, overload, AND short circuit protection for an individual circuit.

 

• Typical ratings: 6A to 40A, 30 mA, Type B
• Advantage: A fault on one circuit only trips that circuit’s RCBO — all other circuits remain live
• The preferred solution in modern consumer units for full discrimination

 

RCD Socket Outlet

A socket with a built-in RCD. Provides 30 mA protection to whatever is plugged into it.

 

• Useful for outdoor sockets, garages, and retrofitting older installations
• Self-contained — doesn’t require RCD protection at the consumer unit

 

Where BS 7671 Requires RCD Protection

Regulation 411.3.3 is the key regulation. A 30 mA RCD is required for:

 

 

Key Requirements

• Socket outlets ≤ 32A — must have 30 mA RCD protection (Reg. 411.3.3)
• TT systems — all circuits require RCD protection for fault protection, since the high Ze means overcurrent devices can’t clear faults fast enough (Reg. 411.5.2)
• Cables in walls — if cables are installed at a depth less than 50 mm and are not otherwise protected, 30 mA RCD protection is required (Reg. 522.6.202)
• Bathrooms — all circuits in locations containing a bath or shower require 30 mA RCD protection (Reg. 701.411.3.3)
• Outdoor equipment — circuits supplying outdoor lighting and equipment require 30 mA RCD protection (Reg. 714.411.3.3)

 

Exceptions

Not every socket needs RCD protection. Exceptions include:

• Socket outlets for specific items of equipment that are labelled and maintained by a skilled or instructed person
• Socket outlets that are not expected to supply portable equipment used outdoors
• Specific industrial applications where risk assessment justifies an alternative approach

 

RCD Testing

RCDs must be tested both by the user (via the test button) and by an electrician (using a calibrated RCD tester).

 

Test Button

Every RCD has a test button that simulates a small leakage. BS 7671 recommends pressing this button quarterly (every 3 months) to check mechanical operation. The test button only checks the mechanism — it does not verify the trip time or trip current.

 

Instrument Testing (Regulation 612.13)

During initial verification and periodic inspection, an electrician tests the RCD with a calibrated instrument that injects a known test current:

Test	Current Applied	Maximum Trip Time
1× rated residual current (IΔn)	30 mA (for a 30 mA RCD)	300 ms
5× rated residual current	150 mA	40 ms
½× rated residual current	15 mA	Must NOT trip
Ramp test (optional)	Gradually increasing	Records actual trip current

 

The half-rated test (½× IΔn) is important — it confirms the RCD doesn’t trip too easily, which would cause nuisance tripping.

 

RCD vs MCB — What’s the Difference?

Feature	MCB	RCD (RCCB)	RCBO
Overload protection	✓	✗	✓
Short circuit protection	✓	✗	✓
Earth fault protection	✗	✓	✓
Additional protection	✗	✓	✓
Detects	Overcurrent	Current imbalance	Both
Trip speed (earth fault)	Up to 5 seconds	Under 300 ms	Under 300 ms

 

Important: An RCD is not a substitute for an MCB — they protect against different things. An RCCB needs MCBs downstream. Only an RCBO provides all three types of protection in one device.

 

Common Exam Questions

Question	Answer
”What does an RCD detect?”	An imbalance between Line and Neutral current
”What trip current for additional protection?“	30 mA (Reg. 415.1.1)
“Maximum trip time at IΔn?“	300 ms
”Maximum trip time at 5× IΔn?“	40 ms
”Why is RCD essential on TT systems?”	High Ze means MCBs can’t clear earth faults fast enough
”How often should the test button be pressed?”	Quarterly (every 3 months)
“Does an RCCB provide overload protection?”	No — MCBs are needed downstream

 

Key Regulations

• Reg. 411.3.3 — Additional protection by RCDs (≤ 30 mA) for socket outlets ≤ 32A
• Reg. 411.5.2 — RCD required for fault protection on TT systems
• Reg. 415.1.1 — Additional protection: RCD with IΔn ≤ 30 mA
• Reg. 522.6.202–203 — RCD protection for cables in walls
• Reg. 531.2 — Requirements for RCDs
• Reg. 612.13 — RCD testing during verification
• Reg. 701.411.3.3 — RCD requirements for bathrooms

 

Practice and Further Study

RCD protection is covered under Part 4: Protection for Safety of BS 7671. Test your knowledge:

• Part 4 — Protection for Safety quiz

 

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