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Educational use: ElectraCore supports learning and preliminary checks. It does not replace a competent electrician or engineer. Verify results, equipment data, and current local regulations before installation or live work.
Courses/Domestic Wiring/Consumer Unit Design

lesson · 9min · Lesson 3 of 39

RCDs vs RCBOs: when to use which

Course syllabusCourse overview
01Consumer Unit Design
  1. ReadConsumer unit layout and components
  2. ReadMCBs: types, ratings, and selection
  3. ReadRCDs vs RCBOs: when to use which
  4. ReadSplit load vs dual RCD boards
  5. quizModule quiz
02Ring Final Circuits
  1. ReadHow ring circuits work: the topology
  2. ReadCable selection for ring finals
  3. ReadSpur outlets: fused and unfused
  4. ReadRadial circuits: when to use them
  5. exerciseWiring practice problems
03Lighting Circuits
  1. ReadJunction box vs loop-in wiring methods
  2. ReadOne-way switching: full diagram
  3. ReadTwo-way switching: staircase wiring
  4. ReadIntermediate switching for long runs
  5. ReadDimmer circuits and LED compatibility
  6. quizLighting circuit quiz
04Earthing & Bonding
  1. ReadTypes of earthing: TN-S, TN-C-S, TT
  2. ReadMain protective bonding
  3. ReadSupplementary bonding requirements
  4. ReadEarth electrode installation for TT
  5. quizBonding quiz
05Special Locations
  1. ReadBathroom zones: Zone 0, 1, 2
  2. ReadKitchen wiring requirements
  3. ReadGarage and outbuilding circuits
  4. ReadGarden power: outdoor sockets and lighting
06Cables & Containment
  1. ReadTwin-and-earth cable construction
  2. ReadInstalling in walls: notching and chasing
  3. ReadTrunking, conduit and dado systems
  4. ReadCable in ceilings and floor voids
  5. quizDepths and zones quiz
07Fault Finding
  1. ReadSystematic fault finding process
  2. ReadOpen circuits: symptoms and finding them
  3. ReadShort circuits and cross-connections
  4. ReadRCD nuisance tripping
  5. exerciseFault finding case studies
08Wiring Regulations
  1. ReadBS 7671 structure and using the regs
  2. ReadNotification, certification and sign-off
  3. ReadPart P and building control
  4. quizFinal assessment
Lesson · 9min
RCD · CORE BALANCEL → IN ← ITRIPhealthy: I(L)=I(N)fault: ΔI ≥ IΔntime limit depends on device type and test currentverify current rules and manufacturer instructions
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In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
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ElectraCore lesson handout

RCDs vs RCBOs: when to use which

Domestic Wiring · Lesson 3 · Beginner

BeginnerReview: professional review pending

Purpose

Distinguish RCCB and RCBO functions and explain how circuit separation changes the consequences of residual-current operation.

Before you beginConsumer-unit layout · Earth-leakage concept

Learning objectives

  • Distinguish RCCB from RCBO
  • Explain residual-current sensing
  • Compare shared and individual protection
  • Recognise RCD type and selectivity considerations

Residual-current devices compare current in live conductors and operate when the imbalance exceeds their characteristic. The product arrangement determines whether overcurrent protection is also present.

CONSUMER UNIT PROTECTION PATHSCONSUMER UNIT PROTECTION PATHSCUisolate · protect · identifydevice / pointtrace every conductor path

Core theory

An RCCB provides residual-current protection to one or more downstream circuits but needs coordinated overcurrent protection. An RCBO adds overcurrent protection for one outgoing circuit.

Individual RCBOs reduce the number of circuits lost after one residual-current fault and reduce cumulative leakage on a shared device.

Selection also considers rated current, residual operating current, RCD type/waveform sensitivity, conditional short-circuit protection, neutral switching, selectivity, and manufacturer instructions.

Terms, symbols, and units
TermMeaningSymbolUnit
RCCBResidual-current circuit-breaker without overcurrent protectionNot applicableNot applicable
RCBOResidual-current breaker with overcurrent protectionNot applicableNot applicable
Residual currentVector imbalance of currents in live conductorsIΔA
Worked exampleCompare one RCCB serving five circuits with five individual RCBOs when one appliance develops earth leakage.

Assumptions: The fault operates the relevant 30 mA protection; No upstream device operates.

  1. Shared RCCB: All five downstream circuits lose supply.
  2. RCBO arrangement: Normally only the affected final circuit loses supply.
  3. Design implication: Division can reduce inconvenience and hazards caused by loss of unrelated circuits.

Individual RCBOs provide more granular interruption, though final design still requires full coordination.

Reasonableness check: Each RCBO measures one circuit instead of the aggregate leakage of several circuits.

Common mistakes
  • Saying an RCCB protects against overload
  • Treating 30 mA as guaranteed personal safety
  • Ignoring DC components and RCD type

Where this appears in practice

RCD architecture affects shock/fire protection, continuity of service, fault finding, and cumulative leakage.

SafetyRCD protection is additional protection, not permission for live work and not a substitute for earthing, bonding, overcurrent protection, isolation, or competent design.
Local code checkThis lesson uses UK terminology and BS 7671 concepts. The current standard, amendments, Building Regulations, distribution-network requirements, manufacturer instructions, and local notification rules must be checked by a competent designer or installer.

Knowledge check

What protection does an RCBO add to the RCCB function?

Overcurrent protection. It combines residual-current and overload/short-circuit functions for a circuit.

Answer: Overcurrent protection. It combines residual-current and overload/short-circuit functions for a circuit.

Practical exercise

Given a dwelling circuit list, propose two RCD-division strategies and explain the consequences of one device tripping.

Summary

  • RCCBs need overcurrent coordination
  • RCBOs protect individual circuits
  • RCD type and circuit division are design decisions

Sources and review

  • Consumer Units and Protective Devices FAQs: IET; Current online guidance; United Kingdom
  • Best Practice Guide 1: Replacing a consumer unit: Electrical Safety First; Issue 5.1; United Kingdom
  • Electrical standards and approved codes of practice: HSE; Current web guidance; Great Britain

Editorial review date: 2026-08-21. Professional electrical review is pending.

Educational material for learning and preliminary checks. Verify current local requirements and exact equipment instructions. This lesson does not replace competent professional work.

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