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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/RCDs and Residual Current

lesson · 9min · Lesson 12 of 29

RCD types: Type AC, A, F, B

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 types: Type AC, A, F, B

Protection Fault Analysis · Lesson 12 · Intermediate

IntermediateReview: professional review pending

Purpose

Select Type AC, A, F, or B RCD response from credible residual-current waveforms and equipment instructions.

Before you beginCore-balance operation · Rectifiers and drives

Learning objectives

  • Distinguish waveform capabilities
  • Avoid B-curve/Type-B confusion
  • Use equipment data
  • Assess DC blinding risk

The RCD type letter describes residual-current waveform capability, not its overcurrent curve. Electronic loads can create pulsating DC, mixed-frequency, or smooth DC residual components.

RESIDUAL-CURRENT SENSINGRESIDUAL-CURRENT SENSINGSOURCETRIPfault / loadcurrent balance to residual trip

Core theory

Type AC is for sinusoidal residual current and under current UK rules is restricted to fixed equipment known not to contain DC components.

Type A adds pulsating DC capability; Type F addresses certain mixed-frequency single-phase inverter loads; Type B extends to smooth DC and broader waveform conditions within its product scope.

Selection follows the load/PCE manufacturer, possible DC magnitude, frequency content, upstream/downstream interaction, EV/PV/storage provisions, device standard, and all required integrated detection.

Terms, symbols, and units
TermMeaningSymbolUnit
DC blindingResidual DC impairing an RCD not designed for itNot applicableNot applicable
Pulsating DCUnidirectional residual-current pulsesNot applicableNot applicable
Smooth DCResidual DC with low rippleNot applicableNot applicable
Worked exampleAn appliance manufacturer specifies Type F protection. May a designer substitute Type AC because both devices are 30 mA?

Assumptions: No equivalent documented solution.

  1. Rating: 30 mA describes residual operating rating, not waveform capability.
  2. Waveform: Type AC lacks the declared Type F mixed-frequency capability.
  3. Decision: Follow manufacturer and current standard or obtain a competent documented equivalent.

No; equal IΔn does not make the waveform types interchangeable.

Reasonableness check: Sensing response depends on waveform as well as magnitude.

Common mistakes
  • Confusing Type B RCD with B-curve MCB
  • Using Type AC for unspecified socket loads
  • Assuming a higher-cost type is universally compatible

Where this appears in practice

Type selection matters for LED drivers, appliances, heat pumps, variable-speed drives, EV charging, PV, and storage.

SafetyWrong type selection can delay or prevent operation. Do not trial fault conditions to identify waveform response.
Local code checkVerify the current BS 7671 edition, exact device/product standard and manufacturer instructions, waveform and delay characteristics, supply data, instrument category/accuracy, safe live-testing method, and recorded circuit conditions. Nominal labels or a single displayed result do not establish protection.

Knowledge check

What does Type B mean on an RCD?

A residual-current waveform capability, including specified smooth DC response. It is different from an MCB B operating curve.

Answer: A residual-current waveform capability, including specified smooth DC response. It is different from an MCB B operating curve.

Practical exercise

Match four published equipment residual-current requirements to candidate types and identify missing coordination data.

Summary

  • RCD types are waveform-specific
  • Equipment data drives selection
  • IΔn alone is insufficient

Sources and review

  • Changes to RCD testing in BS 7671:2018+A2:2022: IET Wiring Matters; 2022 technical article; United Kingdom
  • Wiring Regulations Help: Electrical Safety First; Current technical guidance; United Kingdom
  • Which RCD Type?: IET Wiring Matters; 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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