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Courses/Protection & Fault Analysis/Prospective Fault Current

lesson · 8min · Lesson 18 of 29

PFC at distribution boards downstream

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

PFC at distribution boards downstream

Protection Fault Analysis · Lesson 18 · Intermediate

IntermediateReview: professional review pending

Purpose

Estimate downstream PFC from upstream source impedance plus distribution-circuit impedance and explain why fault current usually falls with distance.

Before you beginOrigin PFC · Cable impedance

Learning objectives

  • Convert current to equivalent source impedance
  • Add downstream impedance
  • Calculate downstream current
  • Account for local source contributions

A downstream board sees the supply source through additional conductor impedance, so its grid-derived fault current is normally lower than at the origin.

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

Core theory

Convert the upstream Thevenin fault level to equivalent loop impedance only when voltage basis and fault type match; add distribution-circuit line/neutral or line/protective impedance for the downstream fault considered.

Conductor resistance changes with temperature; reactance becomes more important for larger conductors/three-phase systems, and parallel conductors need correct division.

Local generators, motors, inverters, transformers, or interconnections may add fault current downstream, so a simple monotonic reduction is not universal.

Terms, symbols, and units
TermMeaningSymbolUnit
Thevenin impedanceEquivalent source impedance seen from a pointNot applicableNot applicable
Downstream boardDistribution point supplied through an upstream circuitNot applicableNot applicable
Source contributionFault current supplied by one source or rotating machineNot applicableNot applicable
Zsource = U / If,origin; If,down ≈ U / (Zsource + Zdistribution)

Use consistent phase/fault bases and include relevant resistance, reactance, and sources.

Ω and A
Worked exampleAt 230 V, origin PSCC is 4.6 kA and downstream line-neutral impedance adds 0.10 Ω. Estimate downstream PSCC.

Assumptions: Single source; Resistive screening; Same voltage basis.

  1. Source Z: 230/4600=0.050 Ω.
  2. Total: 0.050+0.100=0.150 Ω.
  3. Current: 230/0.150≈1533 A=1.53 kA.

Estimated downstream PSCC is about 1.53 kA.

Reasonableness check: Added impedance reduces the grid contribution below 4.6 kA.

Common mistakes
  • Adding current values instead of compatible impedances
  • Mixing line-earth and line-neutral loops
  • Ignoring a downstream generator or motor

Where this appears in practice

Downstream PFC informs distribution-board, device, busbar, and enclosure capability.

SafetyA lower calculated current does not mean low hazard; 1.5 kA can still cause severe arc energy and equipment rupture.
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 does grid-supplied PFC usually decrease downstream?

Additional circuit impedance is inserted between source and fault. Check for local sources that may reverse the assumption.

Answer: Additional circuit impedance is inserted between source and fault. Check for local sources that may reverse the assumption.

Practical exercise

Estimate downstream PFC for three feeder impedances and state when a full complex/multi-source study is required.

Summary

  • Convert consistent fault levels to impedance
  • Add feeder impedance
  • Include every credible source

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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