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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/Fault Loop Impedance

lesson · 9min · Lesson 7 of 29

Ze: external impedance measurement

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

Ze: external impedance measurement

Protection Fault Analysis · Lesson 7 · Intermediate

IntermediateReview: professional review pending

Purpose

Explain what Ze represents and plan its confirmation without presenting a hazardous live-origin measurement as a routine learner task.

Before you beginEarth fault loop · Safe isolation · Supply earthing

Learning objectives

  • Define the external loop boundary
  • Distinguish enquiry and measurement
  • Recognise parallel-path effects
  • Escalate abnormal results to the DNO

Ze describes the source and distributor portion of the earth-fault loop at the installation origin, excluding the installation circuit conductors.

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

Core theory

The boundary and method depend on the earthing arrangement. DNO declared data and competent measurement/confirmation serve different evidential purposes.

A direct measurement at the origin can require disconnection of installation earthing/bonding and energised testing, creating severe shock and loss-of-protection risks; it needs a specific safe system of work.

Parallel paths can make an in-situ loop result lower than the true external path. An unexpectedly high value or ineffective distributor earth must be investigated and referred to the DNO, not hidden by adding RCDs.

Terms, symbols, and units
TermMeaningSymbolUnit
Declared ZeExternal impedance information supplied by the network operatorNot applicableNot applicable
Measured ZeResult obtained by an appropriate origin test under controlled conditionsNot applicableNot applicable
Parallel pathAdditional conductive return path affecting a loop resultNot applicableNot applicable
Worked exampleA TN installation's origin result is far above the DNO's expected range. Should the designer simply rely on 30 mA RCBOs?

Assumptions: Instrument and method checked.

  1. Validate: Confirm test method, connections, supply arrangement, and instrument.
  2. Escalate: Treat a possible distributor earth defect as a DNO matter.
  3. Protect: Do not proceed on an unverified means of earthing merely because RCDs are present.

No; stop and have the external earthing condition investigated by the DNO.

Reasonableness check: Additional protection does not repair an inadequate external earth connection.

Common mistakes
  • Disconnecting bonding live without controls
  • Using a typical value as a measurement
  • Assuming the smallest displayed loop value is true Ze

Where this appears in practice

Ze evidence establishes the supply-side input to ADS calculations and installation verification.

SafetyThis lesson does not authorise live origin testing. It requires competent planning, secure control, suitable PPE/instruments, and restoration checks.
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 can connected bonding make an in-situ loop reading lower?

Parallel conductive paths can share the return current. Interpret the result using the actual connection state and approved method.

Answer: Parallel conductive paths can share the return current. Interpret the result using the actual connection state and approved method.

Practical exercise

Write a desk-based evidence checklist distinguishing DNO enquiry, supply inspection, safe-test planning, result validation, and escalation.

Summary

  • Ze is external to circuit wiring
  • Measurement can be high risk
  • Abnormal supply evidence goes to the DNO

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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