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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/Inspection & Testing/Earth Fault Loop Impedance

lesson · 9min · Lesson 16 of 36

External impedance Ze measurement

Course syllabusCourse overview
01Test Sequence & Preparation
  1. ReadWhy test sequence matters: safety
  2. ReadDead tests before live tests
  3. ReadTest instrument calibration and leads
  4. ReadBS 7671 Appendix 6 test schedule
  5. quizPreparation quiz
02Continuity Testing
  1. ReadRing final continuity: three tests explained
  2. ReadProtective conductor continuity
  3. ReadMain and supplementary bonding continuity
  4. ReadContinuity test results: limits and recording
  5. exerciseContinuity exercise
03Insulation Resistance
  1. ReadIR testing principles: 500V, 1000V
  2. ReadTesting between live conductors and earth
  3. ReadLow IR readings: causes and diagnosis
  4. ReadElectronic equipment and IR testing
  5. quizIR testing quiz
04Earth Fault Loop Impedance
  1. ReadExternal impedance Ze measurement
  2. ReadTotal loop impedance Zs measurement
  3. ReadComparing Zs to Table 41.1 limits
  4. ReadCorrecting excessive Zs readings
  5. exerciseZs measurement exercise
05RCD Testing
  1. ReadRCD tripping current tests: half-rated and rated
  2. ReadRCD trip time testing at 1× and 5× rated current
  3. ReadMaximum trip time limits for different RCD types
  4. ReadRecording RCD test results
  5. quizRCD testing quiz
06Polarity & PFC
  1. ReadPolarity testing: dead and live methods
  2. ReadPFC measurement at origin and boards
  3. ReadVerifying PFC against device ratings
  4. quizPolarity and PFC quiz
07Certification & EICRs
  1. ReadEIC: Electrical Installation Certificate
  2. ReadMinor Works Certificate: when to use
  3. ReadEICR: periodic inspection
  4. ReadObservation codes C1, C2, C3, FI
  5. exerciseEICR coding exercise
08Final Assessment
  1. exerciseFull mock test schedule: complete it
  2. quizFinal written assessment
Lesson · 9min
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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

External impedance Ze measurement

Inspection Testing · Lesson 16 · Advanced

AdvancedReview: professional review pending

Purpose

Establish external earth-fault loop impedance Ze without creating greater danger than the information gained.

Before you beginSafe isolation and test preparation · Continuity and protective-device fundamentals

Learning objectives

  • Select a safe valid method
  • Protect connected equipment
  • Interpret measured versus design values
  • Diagnose without bypassing protective evidence

Ze is the impedance of the fault loop external to the installation. It may be obtained from reliable distributor data or measured at the origin by a competent controlled method.

EARTH-FAULT LOOP AND DISCONNECTIONEARTH-FAULT LOOP AND DISCONNECTIONORIGINCIRCUITtest pointinspect · isolate · prove · measure · interpret · restore · record

Core theory

A direct Ze measurement typically requires installation circuits isolated and parallel installation earth/bond paths managed so the external path is measured. This can expose shock and diverted-neutral current hazards.

Before disturbing a TN-C-S/PME earthing or bonding connection, assess diverted neutral currents and shared extraneous paths under current GN3 guidance. Do not disconnect a live protective path casually.

Record supply voltage, earthing arrangement, method/source and result. Ze is not a generic DNO maximum assumption and must not include R1+R2 when used in Zs=Ze+(R1+R2).

Terms, symbols, and units
TermMeaningSymbolUnit
ZeExternal earth-fault loop impedanceNot applicableNot applicable
METMain earthing terminalNot applicableNot applicable
Diverted neutral currentNeutral current flowing through shared earth/bond pathsNot applicableNot applicable
Zs ≈ Ze + (R1+R2)

Useful design/verification relationship when values share compatible conditions and boundaries.

ohms
Worked exampleZe=0.24 Ω and corrected circuit R1+R2=0.51 Ω.

Assumptions: Supplied values and device limits are fictional design evidence; Instrument uncertainty and current edition data remain to be checked.

  1. Add: Zs≈0.24+0.51=0.75 Ω.
  2. Boundary: Ze excludes internal circuit path; R1+R2 supplies it.
  3. Verify: Compare using applicable temperature/method and device data.

Calculated illustrative Zs is 0.75 Ω.

Reasonableness check: The conclusion retains units, test boundary and protective-device context; one favorable reading cannot excuse an unsafe method.

Common mistakes
  • Testing connected electronics at an unsuitable DC voltage
  • Treating parallel paths as the designed CPC
  • Comparing a measured Zs with the wrong device curve or an unqualified tabulated value

Where this appears in practice

IR and loop verification provide complementary evidence: insulation limits unintended current paths while loop impedance supports automatic disconnection under faults.

SafetyIR testing charges capacitance and can damage equipment; isolate, disconnect/protect devices, control access and discharge after test. Loop testing is live and Ze procedures may disturb earthing and bonding. Use declared data or a controlled competent method, check diverted current and restore every connection.
Local code checkUse the BS 7671 edition applicable to the work, current Guidance Note 3, exact protective-device data, supply operator information and instrument/manufacturer instructions. During the 2026 transition, record the actual edition used. Ze work can remove a protective/bonding path and expose diverted-neutral current hazards; direct measurement is not automatically preferable to reliable declared supply data.

Knowledge check

Should the main earthing conductor be disconnected live whenever a direct Ze reading is desired?

No. Use a justified controlled method or reliable declared data; assess diverted-current danger.

Answer: No. Use a justified controlled method or reliable declared data; assess diverted-current danger.

Practical exercise

Write a Ze method statement choosing between DNO data and direct measurement for TN-S, TN-C-S and TT examples.

Summary

  • Choose voltage and connections from actual equipment
  • Loop boundaries must be explicit
  • Compare results with exact current design evidence

Sources and review

  • BS 7671 and Guidance Note 3: Inspection & Testing: IET; BS 7671:2018+A4:2026 / GN3 10th edition; transition rules apply; United Kingdom
  • Guidance Note 3 errata and diverted-neutral safety note: IET; GN3 9th-edition errata; use current GN3 procedure; United Kingdom
  • GS38: Electrical test equipment for low-voltage systems: HSE; Fourth edition; Great Britain

Editorial review date: 2026-08-22. 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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