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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/Overcurrent Protection Devices

lesson · 9min · Lesson 4 of 29

AFDD: arc fault detection

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

AFDD: arc fault detection

Protection Fault Analysis · Lesson 4 · Intermediate

IntermediateReview: professional review pending

Purpose

Describe AFDD detection, scope, limitations, coordination, and test/indicator handling without presenting it as universal fire protection.

Before you beginSeries and parallel arc concepts · Protective devices

Learning objectives

  • Distinguish arc signatures
  • Explain AFDD role
  • Check current requirement and product scope
  • Coordinate other protective measures

An AFDD analyses current characteristics associated with certain arc faults and disconnects when its algorithm and product criteria identify a hazardous signature.

PROTECTIVE DEVICE OPERATING PATHSPROTECTIVE DEVICE OPERATING PATHSSOURCETRIPfault / loadcurrent / time / energy

Core theory

Series arcs may not produce enough current for ordinary overcurrent operation; parallel arcs can involve greater fault current. AFDD signal processing seeks characteristic high-frequency/time patterns while rejecting normal switching noise.

An AFDD does not replace overload, short-circuit, residual-current, earthing, bonding, cable selection, sound connections, arc-resistant construction, inspection, or maintenance.

Current BS 7671 application depends on premises/circuit classification and risk provisions. Confirm Amendment 4 wording, product standard, poles, rated/breaking values, combined functions, indicator state, test procedure, and manufacturer instructions.

Terms, symbols, and units
TermMeaningSymbolUnit
Series arcArcing discontinuity in series with load currentNot applicableNot applicable
Parallel arcArc between conductors at different potentialsNot applicableNot applicable
AFDDArc fault detection deviceNot applicableNot applicable
Worked exampleA loose series connection draws less than the MCB rating but chars an accessory. Why might overcurrent protection not operate?

Assumptions: Arc current remains below overcurrent thresholds.

  1. Current: The load/arc path limits current below the breaker operating requirement.
  2. Heating: High local resistance and arcing concentrate energy at the defect.
  3. Protection: Connection quality and inspection remain primary; an applicable AFDD adds detection for some signatures.

Low total current can coexist with dangerous local arc heating.

Reasonableness check: Overcurrent devices respond to circuit current, not directly to connection temperature.

Common mistakes
  • Saying AFDDs detect every loose connection
  • Using one instead of correct termination
  • Quoting an obsolete application rule

Where this appears in practice

AFDD assessment is relevant to specified AC final circuits and fire-risk design under the current standard.

SafetyScorching, crackling, or heat requires immediate isolation and competent inspection. Never energise a suspected arc to demonstrate it.
Local code checkVerify the current BS 7671 edition and transition status, the exact product standard, time-current/energy data, manufacturer coordination tables, prospective fault current, conductor data, and installation conditions. Device family names alone do not approve a design.

Knowledge check

Does an AFDD replace an MCB or RCD function unless explicitly combined and rated for it?

No. Confirm every integrated function on the exact product.

Answer: No. Confirm every integrated function on the exact product.

Practical exercise

Create a protection-layer table comparing overload, short circuit, residual current, series arc, and parallel arc responses.

Summary

  • Arcs can occur below overcurrent thresholds
  • AFDD scope is defined and limited
  • Sound installation remains essential

Sources and review

  • Guidance Note 6: Protection Against Overcurrent: IET; 10th edition, BS 7671:2018+A4:2026; United Kingdom
  • Consumer Units & Protective Devices FAQs: IET; Current online guidance; United Kingdom
  • Wiring Regulations Help: Electrical Safety First; Current technical 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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