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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/Protection & Fault Analysis/Fault Loop Impedance

lesson · 10min · Lesson 6 of 29

What is earth fault loop impedance?

Course syllabusCourse overview
01Overcurrent Protection Devices
  1. ReadRewirable fuses vs cartridge fuses
  2. ReadMCB operating characteristics: B, C, D curves
  3. ReadRCBO: combined RCD + MCB operation
  4. ReadAFDD: arc fault detection
  5. quizDevice selection quiz
02Fault Loop Impedance
  1. ReadWhat is earth fault loop impedance?
  2. ReadZe: external impedance measurement
  3. ReadZs: total loop impedance calculations
  4. ReadDisconnection time requirements: Table 41.1
  5. exerciseZs calculation exercises
03RCDs and Residual Current
  1. ReadHow RCDs work: the core balance principle
  2. ReadRCD types: Type AC, A, F, B
  3. ReadRCD ratings: 10, 30, 100, 300mA
  4. ReadRCD testing and nuisance trips
  5. quizRCD selection quiz
04Prospective Fault Current
  1. ReadPFC: what it is and why it matters
  2. ReadCalculating PFC at the origin
  3. ReadPFC at distribution boards downstream
  4. ReadFault current rating of devices
  5. exercisePFC worked problems
05Discrimination & Coordination
  1. ReadWhat is discrimination and why it matters
  2. ReadCurrent discrimination
  3. ReadTime discrimination
  4. ReadEnergy discrimination for MCBs
  5. exerciseDiscrimination case study
06Testing for Protection
  1. ReadTesting Zs with loop testers
  2. ReadMeasuring PFC at the board
  3. ReadRCD trip time testing
  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

What is earth fault loop impedance?

Protection Fault Analysis · Lesson 6 · Intermediate

IntermediateReview: professional review pending

Purpose

Model the complete earth-fault loop and connect its impedance to fault current, touch voltage, and automatic disconnection.

Before you beginOhm's law · Earthing systems · Protective devices

Learning objectives

  • Trace a TN fault loop
  • Define Ze and R1+R2
  • Relate Zs to fault current
  • Recognise TT/RCD differences

Earth fault loop impedance is the opposition around the complete fault-current path from source line through the fault and protective return to the source.

EARTH-FAULT LOOP AND ADSEARTH-FAULT LOOP AND ADSSOURCETRIPfault / loadZs to fault current to operating time

Core theory

For a TN circuit, the loop includes source impedance, line conductors to the fault, protective conductors back to the source, connections, and the fault itself; parallel metallic paths can influence a measurement.

A lower loop impedance generally permits greater fault current, helping an overcurrent device disconnect faster, but the exact time comes from its curve and the applicable minimum-voltage/temperature basis.

In TT systems the electrode path often cannot operate an overcurrent device within the required time, so RCD fault protection and the applicable RA relationship are central; a high result may still reveal a defect.

Terms, symbols, and units
TermMeaningSymbolUnit
ZsEarth fault loop impedance of a circuitNot applicableΩ
ZeExternal earth fault loop impedanceNot applicableΩ
R1+R2Circuit line plus protective-conductor resistanceNot applicableΩ
Conceptually: Zs ≈ Ze + (R1 + R2); fault current ≈ U / Zs

Use the current standard's voltage factors, temperature basis, device data, and measured or calculated method, not this conceptual screening equation alone.

Ω and A
Worked exampleA simplified 230 V loop has Zs=1.0 Ω. What idealised fault current follows before required correction and curve checks?

Assumptions: Purely resistive screening; Nominal voltage only.

  1. Equation: I=U/Z.
  2. Substitute: I=230/1.0.
  3. Screen: Idealised current is 230 A; now use the actual design basis and device curve.

230 A idealised screening current, not a compliance conclusion.

Reasonableness check: One ohm at 230 volts gives hundreds, not tens, of amperes.

Common mistakes
  • Treating Zs=Ze+R1+R2 as exact under all conditions
  • Ignoring conductor temperature
  • Calling low Zs proof of every protective measure

Where this appears in practice

Loop analysis supports ADS design, maximum circuit length, device selection, and diagnostic testing.

SafetyLoop testing can energise exposed conductive parts during a defect and may operate RCDs. Only competent persons should use suitable instruments and controls.
Local code checkUse the current BS 7671 tables/equations and device/manufacturer data for the exact protective device and required disconnection time. Account for conductor temperature, supply minimum voltage factors, measurement uncertainty, parallel paths, RCD method, DNO limits, and safe live-testing controls.

Knowledge check

Does a calculated fault current alone prove required disconnection?

No. Compare the applicable current/time with the exact protective-device characteristic and full requirements.

Answer: No. Compare the applicable current/time with the exact protective-device characteristic and full requirements.

Practical exercise

Draw TN-S, TN-C-S, and TT fault loops and mark which impedance components dominate each conceptual path.

Summary

  • Zs covers the complete fault path
  • Impedance influences fault current
  • Device/time and earthing method decide compliance

Sources and review

  • Determining maximum earth fault loop impedance: IET Wiring Matters; 2023 technical article; United Kingdom
  • How to verify automatic disconnection of supply for RCD-protected circuits: IET Wiring Matters; Issue 105, May 2025; United Kingdom
  • Electricity at Work Regulations 1989: Guidance on Regulations: HSE; HSR25; Great Britain
  • Inspection and Testing FAQs: IET; Current online guidance; 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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