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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/Earthing & Bonding

lesson · 10min · Lesson 17 of 39

Types of earthing: TN-S, TN-C-S, TT

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 · 10min
EARTHING · TN-C-S (PME) vs TTTN-C-SsourceLPENMET (split N/PE)TTLNinstallation electrode; verify ADS and RCD requirements
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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

Types of earthing: TN-S, TN-C-S, TT

Domestic Wiring · Lesson 17 · Beginner

BeginnerReview: professional review pending

Purpose

Distinguish TN-S, TN-C-S, and TT earthing arrangements by the source and path of the installation protective connection.

Before you beginFault current · Protective conductors

Learning objectives

  • Decode the earthing letters
  • Trace the fault return path
  • Distinguish PE from PEN
  • Avoid identifying a supply by appearance alone

Earthing-system labels describe how the source and exposed conductive parts connect to earth. They affect fault paths, protective measures, testing, and special risks.

EARTHING AND BONDING PATHSEARTHING AND BONDING PATHSMETisolate · protect · identifydevice / pointtrace every conductor path

Core theory

TN-S provides separate neutral and protective functions throughout the supply path; TN-C-S combines them as PEN upstream and separates them for the installation.

TT uses a consumer earth electrode rather than a distributor protective conductor as the installation earth connection; RCD-based automatic disconnection is normally central because the electrode path may not operate an overcurrent device quickly enough.

The earthing arrangement must be established by enquiry and inspection/testing, not guessed from cable appearance; PNB and multiple-source installations add further distinctions under current requirements.

Terms, symbols, and units
TermMeaningSymbolUnit
PEProtective conductor functionNot applicableNot applicable
PENCombined protective-earth and neutral conductorNot applicableNot applicable
TTSource and installation exposed parts use separate earth connectionsNot applicableNot applicable
Worked exampleA property has a consumer electrode and no distributor earth terminal. Which broad arrangement is indicated, and what remains to verify?

Assumptions: The observation is accurate.

  1. Indication: A consumer electrode with no distributor earth indicates a TT arrangement.
  2. Protection: Confirm electrode resistance and RCD-based disconnection design.
  3. Evidence: Verify supply characteristics, conductors, bonding, device coordination, and current certification requirements.

It indicates TT, but the complete earthing and protective arrangement still requires verification.

Reasonableness check: Topology identifies the arrangement; measured performance establishes whether protection is adequate.

Common mistakes
  • Calling every cable-sheath earth TN-S
  • Confusing PEN with an installation neutral
  • Using typical Ze values as measured results

Where this appears in practice

Correct identification underpins circuit design, ADS checks, EV/PV decisions, inspection, and fault diagnosis.

SafetyA broken or diverted PEN can create dangerous touch voltages. Do not alter distributor equipment; contact the DNO and apply the current risk controls.
Local code checkThis lesson uses UK earthing and bonding terminology. Confirm the actual supply arrangement with the DNO where required and verify current BS 7671 Amendment 4 requirements, BS 7430 guidance, manufacturer data, and local Building Regulations with a competent designer.

Knowledge check

Why should the earthing arrangement be confirmed rather than assumed?

Its topology and measured characteristics determine protective design. Appearance and typical values are not authoritative site evidence.

Answer: Its topology and measured characteristics determine protective design. Appearance and typical values are not authoritative site evidence.

Practical exercise

Compare paper diagrams of TN-S, TN-C-S, and TT, marking normal neutral current and fault-current return paths.

Summary

  • TN-S keeps PE and N separate
  • TN-C-S separates an upstream PEN
  • TT relies on a consumer earth connection

Sources and review

  • Earthing and Bonding FAQs: IET; Current online guidance; United Kingdom
  • Earthing and bonding: Electrical Safety First; Current safety guidance; United Kingdom
  • BS 7671 Amendment 4 pocket guide: Electrical Safety First; 2026; United Kingdom

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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