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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 · 8min · Lesson 4 of 39

Split load vs dual RCD boards

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 · 8min
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

Split load vs dual RCD boards

Domestic Wiring · Lesson 4 · Beginner

BeginnerReview: professional review pending

Purpose

Compare split-load, dual-RCD, and individual-RCBO arrangements through continuity, cumulative leakage, selectivity, and maintainability.

Before you beginRCCB and RCBO functions · Domestic circuit grouping

Learning objectives

  • Describe common arrangements
  • Assess unwanted-tripping consequences
  • Distribute circuits deliberately
  • Avoid treating legacy layouts as default best practice

Consumer-unit architecture has evolved. A drawing must be assessed by the circuits and protective functions it actually provides, not by an informal product label.

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

Core theory

A dual-RCD arrangement shares each RCCB across a group of MCB-protected circuits. Circuit allocation should minimise danger and inconvenience if one group is lost.

A traditional split-load board may leave some ways outside an RCD group; whether that is acceptable depends on the actual circuits and current requirements.

All-RCBO designs give individual residual protection, but upstream devices, surge protection, isolation, neutral arrangements, and total assembly ratings still need coordination.

Terms, symbols, and units
TermMeaningSymbolUnit
Split loadConsumer unit divided into differently protected sectionsNot applicableNot applicable
Dual RCDTwo RCCBs each serving a circuit groupNot applicableNot applicable
Unwanted trippingOperation without the intended fault conditionNot applicableNot applicable
Worked exampleAllocate upstairs lights, downstairs lights, sockets, boiler, and freezer across two shared RCD groups to reduce consequences of one trip.

Assumptions: All circuits otherwise suitable for the proposed protection.

  1. Avoid concentration: Do not place all lighting on one RCD.
  2. Separate dependencies: Avoid grouping a freezer and all nearby lighting/sockets so one event removes every aid to response.
  3. Document: Record the allocation and residual-current assumptions for verification.

Interleave lighting and power functions across groups; exact allocation depends on the installation risk assessment.

Reasonableness check: One device operation should not unnecessarily remove every related safety or essential function.

Common mistakes
  • Calling every two-row board dual RCD
  • Grouping all lights together
  • Assuming RCBOs eliminate all upstream selectivity issues

Where this appears in practice

Architecture decisions affect safety, availability, testing, diagnosis, and future alterations.

SafetyChanging consumer-unit architecture can expose defects in existing circuits and requires pre-work assessment, isolation, inspection, testing, certification, and competent fault resolution.
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

Why should final circuits be divided between shared RCDs?

To minimise danger and inconvenience if one device operates. One trip should not unnecessarily remove all lighting or related essential functions.

Answer: To minimise danger and inconvenience if one device operates. One trip should not unnecessarily remove all lighting or related essential functions.

Practical exercise

Compare a dual-RCD and all-RCBO schedule for the same fictional dwelling, listing single-fault consequences.

Summary

  • Architecture controls outage scope
  • Shared RCD groups require deliberate allocation
  • Existing circuits must be assessed before replacement

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.

Personal study record

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