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Courses/Protection & Fault Analysis/Fault Loop Impedance

lesson · 9min · Lesson 9 of 29

Disconnection time requirements: Table 41.1

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

Disconnection time requirements: Table 41.1

Protection Fault Analysis · Lesson 9 · Intermediate

IntermediateReview: professional review pending

Purpose

Determine the required disconnection time from the earthing system, circuit type, voltage, and protective measure before selecting a Zs limit.

Before you beginAutomatic disconnection of supply · Protective device curves

Learning objectives

  • Use Table 41.1 context
  • Distinguish final and distribution circuits
  • Relate time to device current
  • Avoid memorising 0.4 s as universal

Disconnection-time requirements limit how long dangerous touch voltage may persist. The applicable time is a classification result, not a universal domestic constant.

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

Core theory

Identify TN or TT arrangement, nominal line-to-earth voltage, final/distribution circuit, socket/current limits and other conditions, then use the current Table 41.1 and related regulations.

Once the required time is known, determine the current Ia that makes the exact device operate within that time and derive/check the permitted loop impedance using the current voltage factor or table/manufacturer data.

RCD fault protection uses its own operating characteristics and RA/Zs provisions; overcurrent protection, additional protection, continuity, bonding, and special-location rules remain coordinated.

Terms, symbols, and units
TermMeaningSymbolUnit
Disconnection timeMaximum permitted duration before ADS removes the faultNot applicableNot applicable
IaCurrent causing operation within the specified timeNot applicableA
U0Nominal AC voltage to EarthNot applicableV
Zs × Ia ≤ U0 × applicable minimum-voltage factor

Apply the equation and factors from the exact current edition; tables may already embody stated assumptions.

Ω × A ≤ V
Worked exampleA learner remembers 0.4 s for every circuit. What classification questions must be answered first?

Assumptions: No circuit details supplied.

  1. System: Establish TN/TT and U0.
  2. Circuit: Classify final versus distribution and the applicable rating/use conditions.
  3. Rules: Check current Table 41.1, related exceptions, protective measure, and device data.

No time can be assigned until the system and circuit are classified under the current rules.

Reasonableness check: Different exposure and circuit contexts have different permitted durations.

Common mistakes
  • Applying 0.4 s universally
  • Reading a table without its notes
  • Using an instantaneous multiplier where a time curve is required

Where this appears in practice

Time classification precedes Zs limits, maximum length, and protection verification.

SafetyA marginal calculated value requires competent resolution; do not energise a circuit merely to see whether a protective device trips during a real fault.
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

What comes before choosing a maximum Zs from a table?

Classifying the system, circuit, required time, and exact protective device. The table value only applies within its stated scope.

Answer: Classifying the system, circuit, required time, and exact protective device. The table value only applies within its stated scope.

Practical exercise

For four fictional circuit descriptions, identify missing classification data before looking up any disconnection time.

Summary

  • Required time is context-dependent
  • Time determines device operating current
  • Read table scope and notes

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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