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Courses/Protection & Fault Analysis/Fault Loop Impedance

lesson · 10min · Lesson 8 of 29

Zs: total loop impedance calculations

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 · 10min
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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

Zs: total loop impedance calculations

Protection Fault Analysis · Lesson 8 · Intermediate

IntermediateReview: professional review pending

Purpose

Calculate and assess Zs from external and circuit components while keeping temperature, voltage, parallel paths, uncertainty, and device data explicit.

Before you beginZe · R1+R2 · Device curves

Learning objectives

  • Combine loop components
  • Correct conductor resistance conceptually
  • Compare with exact limits
  • Reconcile calculated and measured evidence

A design value of Zs combines source/external impedance with the circuit line and protective return path at the point under consideration.

EARTH-FAULT LOOP AND ADSEARTH-FAULT LOOP AND ADSSOURCETRIPfault / loadZs to fault current to operating time

Core theory

At a conceptual level Zs=Ze+(R1+R2), but design resistance must reflect conductor material, length, cross-section, operating temperature, joints, and the current standard's factors.

The maximum permitted Zs comes from the exact protective-device/time basis or applicable equation/manufacturer data; tabulated maximum and maximum measured values may use different temperature assumptions.

A measured result includes actual supply and parallel paths at test time plus instrument uncertainty. A large discrepancy requires investigation, not selection of whichever value passes.

Terms, symbols, and units
TermMeaningSymbolUnit
Design ZsCalculated loop impedance for worst applicable design conditionsNot applicableNot applicable
Measured ZsLoop-test result under recorded site conditionsNot applicableNot applicable
Maximum measured valueLimit adjusted to the stated measurement temperature basisNot applicableNot applicable
Zs,design = Ze + (R1 + R2) at the applicable design temperature

Use current factors and exact device limits; do not mix design-temperature and measured-temperature values.

Ω
Worked exampleZe=0.30 Ω and corrected R1+R2=0.72 Ω. Find the conceptual design Zs.

Assumptions: Values use compatible bases; No additional reactance term required for this exercise.

  1. Sum: 0.30+0.72=1.02 Ω.
  2. Limit: Compare 1.02 Ω with the exact device and required-time limit.
  3. Record: Document data sources, temperature factors, and uncertainty.

Conceptual design Zs is 1.02 Ω; compliance is not decided until the exact limit is checked.

Reasonableness check: The total must exceed either positive component.

Common mistakes
  • Applying a temperature factor to Ze
  • Comparing measured cold values with an incompatible hot-design table
  • Ignoring a surprising measured/calculated mismatch

Where this appears in practice

Zs calculation supports device choice, conductor sizing, maximum length, and pre-work design verification.

SafetyA passing calculation does not replace continuity, polarity, inspection, and required tests; an open CPC can make the real fault path absent.
Local code checkUse the current BS 7671 tables/equations and device/manufacturer data for the exact protective device and required disconnection time. Account for conductor temperature, supply minimum voltage factors, measurement uncertainty, parallel paths, RCD method, DNO limits, and safe live-testing controls.

Knowledge check

Why must calculated and measured Zs use clearly stated temperature bases?

Conductor resistance changes with temperature. Mixing bases can create a false pass or false fail.

Answer: Conductor resistance changes with temperature. Mixing bases can create a false pass or false fail.

Practical exercise

Calculate three conceptual Zs values with supplied corrected R1+R2 data, then list the exact evidence needed for final comparison.

Summary

  • Combine compatible impedance components
  • Temperature basis matters
  • Investigate conflicting evidence

Sources and review

  • Determining maximum earth fault loop impedance: IET Wiring Matters; 2023 technical article; United Kingdom
  • How to verify automatic disconnection of supply for RCD-protected circuits: IET Wiring Matters; Issue 105, May 2025; United Kingdom
  • Electricity at Work Regulations 1989: Guidance on Regulations: HSE; HSR25; Great Britain
  • Inspection and Testing FAQs: IET; Current online guidance; 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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