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Courses/Protection & Fault Analysis/Prospective Fault Current

lesson · 9min · Lesson 17 of 29

Calculating PFC at the origin

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 · 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

Calculating PFC at the origin

Protection Fault Analysis · Lesson 17 · Intermediate

IntermediateReview: professional review pending

Purpose

Calculate an origin PFC estimate from verified impedance data while accounting for test uncertainty, declared DNO data, and unnecessary-live-testing guidance.

Before you beginPFC definition · Ohm's law

Learning objectives

  • Choose the correct loop
  • Calculate screening current
  • Assess instrument resolution
  • Use DNO enquiry appropriately

At an installation origin, PFC can be established by appropriate measurement, calculation from verified loop results, or permitted DNO enquiry depending on installation and current guidance.

PROSPECTIVE FAULT CURRENTPROSPECTIVE FAULT CURRENTSOURCETRIPfault / loadsource Z to PFC to device capability

Core theory

For single-phase screening, PSCC may be estimated from line-neutral loop impedance and PEFC from line-earth loop impedance using the relevant voltage basis; three-phase calculation needs the correct line-line/source model.

Small impedance errors produce large current differences because current is inversely proportional to impedance. Low displayed impedance can exceed an instrument range's useful accuracy/resolution.

Current guidance permits Ze and PFC by DNO enquiry in specified verification contexts, reducing hazardous live work; records must identify whether data are measured, calculated, or declared.

Terms, symbols, and units
TermMeaningSymbolUnit
ResolutionSmallest display incrementNot applicableNot applicable
AccuracyDeclared closeness to the true value under conditionsNot applicableNot applicable
Declared PFCNetwork information supplied for design/verificationNot applicableNot applicable
Single-phase screening: I = U / Z

State whether Z is line-neutral or line-earth and use the applicable voltage and uncertainty method.

V / Ω = A
Worked exampleA verified line-neutral loop impedance is 0.05 Ω at 230 V nominal. Find the raw screening PSCC and flag the measurement issue.

Assumptions: Resistive screening only.

  1. Calculate: 230/0.05=4600 A=4.6 kA.
  2. Sensitivity: A 0.01 Ω change would materially change the result.
  3. Evidence: Check instrument range/accuracy or use a suitable alternative/DNO method.

Raw screening PSCC is 4.6 kA, subject to strong low-impedance uncertainty sensitivity.

Reasonableness check: A very low impedance produces a kiloampere-scale result.

Common mistakes
  • Reporting more precision than the impedance supports
  • Using Ze for line-neutral PSCC
  • Measuring live when authoritative enquiry suffices

Where this appears in practice

Origin PFC supports service equipment, consumer-unit, tails, and main-device capability checks.

SafetyOrigin testing can expose very high fault energy. Use enquiry or safer methods when permitted; live measurement requires competent planning and CAT-rated equipment.
Local code checkVerify the current BS 7671 edition, exact device/product standard and manufacturer instructions, waveform and delay characteristics, supply data, instrument category/accuracy, safe live-testing method, and recorded circuit conditions. Nominal labels or a single displayed result do not establish protection.

Knowledge check

Why is a PFC calculated from 0.05 Ω highly sensitive to a 0.01 Ω error?

Current is the reciprocal of impedance. Small absolute impedance errors become large relative current errors.

Answer: Current is the reciprocal of impedance. Small absolute impedance errors become large relative current errors.

Practical exercise

Calculate a PFC interval using stated impedance tolerance rather than reporting a single false-precision result.

Summary

  • Choose the correct loop
  • Low impedance magnifies uncertainty
  • Record the evidence method

Sources and review

  • Minimizing unnecessary live testing for initial verification: IET Wiring Matters; Issue 105, May 2025; United Kingdom
  • Best Practice Guide 7: Test instrument accuracy and consistency: Electrical Safety First; Issue 2.1; United Kingdom
  • BS 7671:2018+A4:2026 model EICR schedule: IET; Amendment 4:2026; 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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