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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/Polarity & PFC

lesson · 8min · Lesson 27 of 36

PFC measurement at origin and boards

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 · 8min
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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

PFC measurement at origin and boards

Inspection Testing · Lesson 27 · Advanced

AdvancedReview: professional review pending

Purpose

Determine prospective fault current at origins and distribution points using safe measurement, calculation or reliable enquiry.

Before you beginTest sequence and safe live testing · ADS, RCD types and fault current

Learning objectives

  • Use the current required method
  • Separate field verification from type testing
  • Prove polarity through the complete path
  • Compare fault current with every affected rating

Prospective short-circuit current (line-neutral/line-line) and prospective earth-fault current can differ. The governing prospective fault current at a point is the greater credible value for equipment rating checks.

POLARITY AND PROSPECTIVE FAULT CURRENTPOLARITY AND PROSPECTIVE FAULT CURRENTORIGINCIRCUITtest pointinspect · isolate · prove · measure · interpret · restore · record

Core theory

At the origin determine supply configuration and sources. Use a suitable instrument/method, calculation from loop impedance/voltage, or reliable DNO/manufacturer data; declare the method.

For three-phase systems evaluate line-line and line-earth conditions as applicable. Downstream fault current generally reduces with added impedance but local transformers, generators, motors or storage can contribute.

Measurement may expose high fault energy and instrument limitations. Do not open covers or probe busbars when a safer valid method exists; verify leads, CAT rating, access barriers and PPE/risk controls.

Terms, symbols, and units
TermMeaningSymbolUnit
PSCCProspective short-circuit currentNot applicableNot applicable
PEFCProspective earth-fault currentNot applicableNot applicable
PFCGoverning prospective fault current at a pointNot applicableNot applicable
Ipf ≈ U/Zloop

Screening relationship; use the correct voltage and loop for the fault type and source model.

amperes
Worked exampleAt one point PSCC=5.6 kA and PEFC=3.9 kA.

Assumptions: Supplied readings and criteria are fictional evidence; Current edition and exact equipment data remain governing.

  1. Compare: 5.6 kA exceeds 3.9 kA.
  2. Record: Governing PFC at that point is 5.6 kA.
  3. Continue: Compare every affected device/assembly rating and backup combination.

Governing supplied PFC is 5.6 kA.

Reasonableness check: The decision distinguishes the measured quantity, protective function and equipment rating instead of treating one tester result as a complete certificate.

Common mistakes
  • Applying obsolete half-rated/5× rules as universal requirements
  • Selecting an RCD type from trip time alone
  • Checking PFC only against the final-circuit breaker

Where this appears in practice

RCD, polarity and PFC verification confirms correct conductor/device operation and that installed equipment can safely interrupt credible faults.

SafetyThese are often live tests. Complete dead-test prerequisites, restore covers/protective conductors, restrict access, use GS38-suitable leads, prefer the least exposed point, verify tester settings and avoid repeated RCD operation where connected equipment or continuity of service creates risk.
Local code checkUse the applicable BS 7671 edition and current Guidance Note 3, exact RCD/product/manufacturer instructions, supply characteristics, device breaking-capacity data and instrument specifications. Current BS 7671 field compliance testing no longer universally requires ½× or 5× tests; record the actual required/diagnostic method and never infer device suitability from trip time alone.

Knowledge check

Is earth-fault current always the highest prospective fault current?

No. Compare credible short-circuit and earth-fault values.

Answer: No. Compare credible short-circuit and earth-fault values.

Practical exercise

Calculate/compare PFC through an origin, sub-board and local generator contribution using supplied source impedances.

Summary

  • Current field RCD testing is intentionally simplified
  • Polarity requires both topology and final confirmation
  • PFC is a worst-case rating and coordination check

Sources and review

  • Changes to RCD testing in BS 7671:2018+A2:2022: IET Wiring Matters; 2022; confirm A4:2026/GN3 status; United Kingdom
  • BS 7671 and Guidance Note 3: Inspection & Testing: IET; BS 7671:2018+A4:2026 / GN3 10th edition; transition rules apply; United Kingdom
  • Minimizing unnecessary live testing for initial verification: IET Wiring Matters; 2025; 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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