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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/Testing for Protection

lesson · 8min · Lesson 27 of 29

Measuring PFC at the board

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 · 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

Measuring PFC at the board

Protection Fault Analysis · Lesson 27 · Advanced

AdvancedReview: professional review pending

Purpose

Measure or otherwise establish prospective fault current at a board with correct fault-loop selection, instrument capability, and risk controls.

Before you beginPEFC and PSCC · Loop testing

Learning objectives

  • Choose the relevant fault loops
  • Validate instrument capability
  • Interpret low impedance
  • Compare PFC with equipment ratings

PFC at a board is the greatest relevant prospective current for the supply configuration and fault type. A live instrument derives it from a measured loop impedance, so small impedance errors can create large current errors.

PROTECTION TEST EVIDENCEPROTECTION TEST EVIDENCESOURCETRIPfault / loadmethod to reading to evidence to decision

Core theory

For single-phase systems assess prospective line-neutral short-circuit current and line-earth fault current as applicable; record PFC using the governing result. Multi-phase and multiple-source systems require every credible combination.

At low impedance, lead resistance, contact quality, supply fluctuation, test method, and instrument range/resolution materially affect the derived current. DNO/source data or calculation may provide safer or stronger evidence.

Compare the established value with device breaking or conditional ratings, assembly short-circuit capability, protective-conductor withstand, busbar capability, and manufacturer backup data, not only the number printed on one breaker.

Terms, symbols, and units
TermMeaningSymbolUnit
PSCCProspective short-circuit current between live conductorsNot applicableA or kA
PEFCProspective earth-fault current through the earth-fault loopNot applicableA or kA
CAT ratingInstrument transient-overvoltage category for its intended measurement environmentNot applicableNot applicable
Worked exampleA single-phase board has 0.052 Ω line-neutral and 0.061 Ω line-earth measured loop values at 230 V. Which simple derived value governs?

Assumptions: Readings and 230 V basis are valid; No other source or phase combination gives more.

  1. PSCC: 230 V / 0.052 Ω ≈ 4.42 kA.
  2. PEFC: 230 V / 0.061 Ω ≈ 3.77 kA.
  3. PFC: Use the higher relevant value, about 4.42 kA, then address uncertainty and ratings.

The simple governing result is approximately 4.42 kA PSCC.

Reasonableness check: The lower impedance produces the higher prospective current.

Common mistakes
  • Measuring only line-earth
  • Trusting excess displayed digits
  • Ignoring generators, PV, or motors

Where this appears in practice

PFC evidence is required when selecting switchgear and assessing distribution boards or supply changes.

SafetyOrigin testing is at a point of high fault energy. Prefer authoritative enquiry/calculation where adequate; any live measurement needs appropriately rated leads, probes, instrument, PPE, barriers, and procedure.
Local code checkConfirm the current BS 7671 edition, exact device and assembly data, manufacturer selectivity/backup tables, required disconnection times, test-instrument ratings, safe system of work, and certificate schedule. A generic rating ratio or one field reading is not proof of coordination or compliance.

Knowledge check

Why is PFC normally the greater relevant prospective value?

Equipment must withstand or interrupt the worst credible fault duty. State fault type, point, source configuration, and uncertainty.

Answer: Equipment must withstand or interrupt the worst credible fault duty. State fault type, point, source configuration, and uncertainty.

Practical exercise

Calculate PSCC and PEFC from supplied fictional impedances, then list capability checks before acceptance.

Summary

  • Test every relevant fault basis
  • Low impedance magnifies uncertainty
  • Compare against system capability

Sources and review

  • Guidance Note 6: Protection Against Overcurrent: IET; 10th edition, BS 7671:2018+A4:2026; United Kingdom
  • Inspection and Testing FAQs: IET; Current online guidance; United Kingdom
  • Electricity at Work Regulations 1989: HSR25: Health and Safety Executive; Third edition; Great Britain

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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