Skip to main content
ElectraCore
LearnCalculatorsCircuit DesignerReferences
Saved
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/Discrimination & Coordination

lesson · 8min · Lesson 24 of 29

Energy discrimination for MCBs

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 · 8min
MCB OPERATING REGIONS · SCHEMATIC ONLYtime ↓multiple of In →thermal (overload)B 3–5×C 5–10×D 10–20×use the manufacturer time-current curve for coordination
Lesson contents 0%

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
0% read
ElectraCore lesson handout

Energy discrimination for MCBs

Protection Fault Analysis · Lesson 24 · Advanced

AdvancedReview: professional review pending

Purpose

Use let-through energy and manufacturer coordination data to assess short-circuit selectivity for current-limiting devices.

Before you beginI²t · MCB operation

Learning objectives

  • Interpret let-through energy
  • Distinguish pre-arcing and total I²t
  • Check peak current
  • Avoid generic MCB assumptions

At high fault currents, both devices may enter their rapid operating regions before ordinary time-current plots show useful separation. Current-limiting behavior and tested device-pair data then become decisive.

SELECTIVITY AND COORDINATIONSELECTIVITY AND COORDINATIONSOURCETRIPfault / loadcurves / time / I-squared-t / paired data

Core theory

I²t represents current-squared energy over time. A downstream current-limiting device can reduce the energy and peak current seen upstream, but its published values depend on fault current, voltage, device, and test standard.

Fuse coordination may compare downstream total clearing I²t with upstream pre-arcing I²t under specified conditions. Circuit-breaker combinations normally require manufacturer-tested selectivity tables or software; a generic ratio is insufficient.

Energy selectivity is checked alongside peak withstand, breaking/conditional rating, conductor adiabatic withstand, enclosure/assembly rating, polarity, and every credible source contribution.

Terms, symbols, and units
TermMeaningSymbolUnit
Pre-arcing I²tEnergy integral before a fuse element begins arcingNot applicableA²s
Total clearing I²tEnergy integral through complete interruptionNot applicableA²s
Peak let-throughHighest limited instantaneous current passed by a deviceNot applicableA
Worked exampleA downstream device has total clearing I²t of 18,000 A²s and an upstream fuse pre-arcing I²t of 30,000 A²s at the stated condition. What does the comparison establish?

Assumptions: Values are comparable and manufacturer criteria permit this method.

  1. Compare: 18,000 A²s is below 30,000 A²s.
  2. Interpret: The upstream element may remain below its pre-arcing energy threshold.
  3. Complete: Confirm voltage, fault range, tolerances, peak current, and exact manufacturer coordination declaration.

The energy comparison supports selectivity for the stated condition but does not alone prove the complete design.

Reasonableness check: The downstream device clears before the stated upstream melting-energy threshold is reached.

Common mistakes
  • Treating one I²t value as universal
  • Applying fuse rules automatically to MCB pairs
  • Ignoring peak current and assembly withstand

Where this appears in practice

Energy coordination is used where high prospective current makes ordinary curve separation inadequate.

SafetyFault-energy studies are calculation and data tasks. Never perform uncontrolled short-circuit trials in an installation.
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

What is normally needed for MCB-to-MCB energy selectivity?

Exact manufacturer-tested coordination data. Generic curve letters and rating ratios do not establish the pair's high-current behavior.

Answer: Exact manufacturer-tested coordination data. Generic curve letters and rating ratios do not establish the pair's high-current behavior.

Practical exercise

Compare a sample selectivity table with its voltage, fault-current, and device-reference boundaries.

Summary

  • High-current behavior needs energy data
  • MCB pairs are manufacturer-specific
  • Check both I²t and peak withstand

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.

Personal study record

Lesson notes

Ready

Notes are stored on this device unless you export them.