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Courses/Inspection & Testing/Earth Fault Loop Impedance

lesson · 9min · Lesson 18 of 36

Comparing Zs to Table 41.1 limits

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

Comparing Zs to Table 41.1 limits

Inspection Testing · Lesson 18 · Advanced

AdvancedReview: professional review pending

Purpose

Compare Zs with the exact protective-device disconnection requirement and applicable maximum measured value.

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

Zs limits depend on system voltage, earthing/protective measure, device type/rating/curve, required disconnection time and the standard edition. Table 41.1 is not a universal device lookup table.

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

Core theory

Determine required disconnection time and the current needed to operate the exact fuse, circuit-breaker or protective function. Use applicable BS 7671 table/curve or manufacturer data.

Relate Zs to U0/Ia and account for conductor temperature and the chosen measured-versus-tabulated method. Current guidance can publish maximum permissible measured EFLI values; do not improvise a blanket percentage.

Include instrument uncertainty and worst credible conditions. Record the source/edition. RCD additional/fault protection may change the ADS assessment but does not excuse an absent CPC or other requirements.

Terms, symbols, and units
TermMeaningSymbolUnit
IaCurrent causing operation within required disconnection timeNot applicableNot applicable
U0Nominal line-to-earth voltageNot applicableNot applicable
Maximum measured ZsField comparison value derived for declared conditions/methodNot applicableNot applicable
Zs,max = U0/Ia

Core ADS relationship; exact standard factors, voltage and device data apply.

ohms
Worked exampleU0=230 V and supplied device data requires Ia=400 A for the required time.

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

  1. Divide: Zs,max=230/400=0.575 Ω.
  2. Compare: A measured 0.62 Ω exceeds this supplied design maximum.
  3. Act: Do not round into compliance; investigate and redesign/repair using exact current rules.

Supplied maximum is 0.575 Ω; 0.62 Ω fails that stated criterion.

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

Is Table 41.1 the correct source for every fuse, breaker and RCD-protected circuit?

No. Use the applicable device table/curve, protective measure and current edition/method.

Answer: No. Use the applicable device table/curve, protective measure and current edition/method.

Practical exercise

Build a comparison sheet for several device curves, disconnection times and measured/corrected Zs values.

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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