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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/Testing for Protection

lesson · 9min · Lesson 28 of 29

RCD trip time testing

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
RCD · CORE BALANCEL → IN ← ITRIPhealthy: I(L)=I(N)fault: ΔI ≥ IΔntime limit depends on device type and test currentverify current rules and manufacturer instructions
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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

RCD trip time testing

Protection Fault Analysis · Lesson 28 · Advanced

AdvancedReview: professional review pending

Purpose

Verify RCD operation using the current required sequence, correct instrument settings, and documented interpretation.

Before you beginRCD types and ratings · Safe live testing

Learning objectives

  • Identify the required test
  • Select waveform/type settings
  • Control leakage and loads
  • Interpret trip time with device data

RCD testing verifies a device within an installation under stated conditions. The tester waveform, selected IΔn, device type, supply state, standing leakage, and test point all affect the result.

PROTECTION TEST EVIDENCEPROTECTION TEST EVIDENCESOURCETRIPfault / loadmethod to reading to evidence to decision

Core theory

Current BS 7671 guidance for the common verification case uses an AC test at rated residual current IΔn at 0° or 180°, together with operation of the integral test button. Older automatic sequences can add diagnostic information but should not be misrepresented as universally required current tests.

Set the tester for the exact RCD type and rating; identify time-delayed devices, disconnect or control sensitive loads as the procedure permits, and account for standing leakage that adds vectorially to test current.

Record device identification, type, IΔn, location, test current/phase, result, instrument, and any limitations. A trip-time result does not prove earthing, overcurrent protection, isolation, or the suitability of the selected RCD type.

Terms, symbols, and units
TermMeaningSymbolUnit
IΔnRated residual operating currentNot applicableA or mA
Trip timeTime from applied residual test current to openingNot applicablems
Test buttonIntegral functional test using the device's internal test circuitNot applicableNot applicable
Worked exampleA 30 mA Type A RCD is tested with an instrument left on Type AC. Can the result verify the declared device type?

Assumptions: Instrument has a suitable Type A setting.

  1. Mismatch: The applied waveform does not represent the selected Type A test function.
  2. Correct: Select the manufacturer/instrument-approved Type A setting and IΔn.
  3. Repeat: Repeat under controlled conditions and record the valid configuration.

No; repeat using a compatible Type A test configuration.

Reasonableness check: A result is only meaningful when the stimulus matches the function being verified.

Common mistakes
  • Mandating every legacy half-times/five-times test
  • Selecting the wrong RCD waveform
  • Ignoring standing leakage

Where this appears in practice

RCD trip-time evidence forms part of initial verification, periodic assessment, fault diagnosis, and post-replacement testing.

SafetyRCD testing is live and may disconnect critical loads. Obtain permission, control consequences, verify instrument ratings, maintain safe access, and restore every circuit and setting afterward.
Local code checkConfirm the current BS 7671 edition, exact device and assembly data, manufacturer selectivity/backup tables, required disconnection times, test-instrument ratings, safe system of work, and certificate schedule. A generic rating ratio or one field reading is not proof of coordination or compliance.

Knowledge check

Does pressing the RCD test button prove the circuit's earth-fault loop?

No. It is a functional device test, not complete circuit verification.

Answer: No. It is a functional device test, not complete circuit verification.

Practical exercise

Prepare a fictional RCD test record including device type, IΔn, waveform, phase, time, instrument, and restoration checks.

Summary

  • Match test to device type
  • Use the current required sequence
  • Record conditions and restore service

Sources and review

  • Guidance Note 6: Protection Against Overcurrent: IET; 10th edition, BS 7671:2018+A4:2026; United Kingdom
  • Inspection and Testing FAQs: IET; Current online guidance; United Kingdom
  • Electricity at Work Regulations 1989: HSR25: Health and Safety Executive; Third edition; Great Britain

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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