Skip to main content
ElectraCore
LearnCalculatorsCircuit DesignerReferences
Saved
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/Prospective Fault Current

lesson · 9min · Lesson 16 of 29

PFC: what it is and why it matters

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
Lesson contents 0%

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
0% read
ElectraCore lesson handout

PFC: what it is and why it matters

Protection Fault Analysis · Lesson 16 · Intermediate

IntermediateReview: professional review pending

Purpose

Define prospective fault current as the greatest credible current at a point for the fault types and supply configuration assessed.

Before you beginFault loops · Short circuits

Learning objectives

  • Distinguish PEFC and PSCC
  • Explain why PFC varies by location
  • Relate PFC to device capability
  • Include multiple-source scenarios

Prospective fault current is the current expected if a negligible-impedance fault occurs at the point considered. Both line-earth and live-conductor short-circuit paths may need assessment.

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

Core theory

Prospective earth fault current follows the earth-fault loop; prospective short-circuit current follows the live-conductor loop. The recorded PFC basis must cover the greater applicable result.

Source impedance, transformer/network strength, conductor impedance, parallel supplies, motors, generators, PV/storage converters, and operating configuration affect magnitude and X/R characteristics.

PFC establishes required device and assembly breaking/short-circuit withstand, conditional backup arrangements, conductor thermal stress, arc energy context, and test safety.

Terms, symbols, and units
TermMeaningSymbolUnit
PEFCProspective earth fault currentNot applicableA or kA
PSCCProspective short-circuit currentNot applicableA or kA
PFCProspective fault current basis recorded at a pointNot applicableA or kA
Screening only: If ≈ U / Zloop

Use the correct fault voltage, loop, source contributions, impedance angle, and current-edition method.

A
Worked exampleAt one board PEFC is 2.8 kA and PSCC is 4.1 kA. What PFC screening value is recorded for device capability?

Assumptions: Both results valid for the same configuration.

  1. Compare: 4.1 kA exceeds 2.8 kA.
  2. Record: Use 4.1 kA as the greater prospective current basis.
  3. Verify: Check all device/assembly ratings and source configurations.

Use 4.1 kA as the greater PFC result for this screening context.

Reasonableness check: Capability must cover the more severe credible fault current.

Common mistakes
  • Recording only earth-fault current
  • Ignoring alternate sources
  • Calling the breaker rating the measured PFC

Where this appears in practice

PFC is required at origins and relevant distribution points for switchgear and protective-device selection.

SafetyHigh PFC increases arc and blast energy. Measurement at live origins requires competent risk controls and correctly rated instruments/leads/PPE.
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 compare PEFC and PSCC?

The equipment must withstand or interrupt the greater applicable prospective current. Their loop impedances can differ.

Answer: The equipment must withstand or interrupt the greater applicable prospective current. Their loop impedances can differ.

Practical exercise

For three source configurations, list every contribution and the point at which PEFC/PSCC must be established.

Summary

  • PFC depends on fault type and point
  • Use the greater applicable value
  • All credible sources matter

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.

Personal study record

Lesson notes

Ready

Notes are stored on this device unless you export them.