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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/Discrimination & Coordination

lesson · 7min · Lesson 22 of 29

Current discrimination

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 · 7min
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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

Current discrimination

Protection Fault Analysis · Lesson 22 · Advanced

AdvancedReview: professional review pending

Purpose

Assess current-based selectivity using pickup thresholds and time-current envelopes rather than rated-current ratios alone.

Before you beginSelectivity · Time-current curves

Learning objectives

  • Locate pickup regions
  • Compare curve tolerances
  • Check overload and fault ranges
  • State a selectivity limit

Current discrimination separates operating thresholds so the downstream device responds at currents for which the upstream device remains stable. Real device tolerances and overlapping instantaneous bands restrict that separation.

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

Core theory

For overloads, compare the downstream maximum operating curve with the upstream minimum operating or non-operating boundary at the same current and stated temperature.

For short circuits, instantaneous pickup bands may overlap. A downstream device can therefore fail to clear alone even when its nominal rating is much smaller.

A defensible result identifies device references, settings, curve edition, current scale, fault type, available minimum/maximum fault current, and the highest current to which separation is demonstrated.

Terms, symbols, and units
TermMeaningSymbolUnit
PickupThreshold region at which a protective function begins to operateNot applicableNot applicable
Curve tolerancePermitted band between minimum and maximum operating behaviorNot applicableNot applicable
Selectivity limitHighest fault current for which the downstream device alone is declared to operateNot applicableNot applicable
Worked exampleTwo published curves separate through the overload region but their instantaneous bands overlap above 600 A. What can be claimed?

Assumptions: Same voltage and stated test conditions; No stronger manufacturer table.

  1. Overload: Record selectivity where the envelopes do not cross.
  2. Fault: Treat the overlap as loss of curve-based proof above 600 A.
  3. Site: Compare 600 A with the prospective current at the downstream board.

Only partial selectivity up to the evidenced boundary can be claimed.

Reasonableness check: An overlapping upstream band permits both devices to operate.

Common mistakes
  • Using only nominal In
  • Reading the centre of a tolerance band
  • Ignoring temperature or adjustable settings

Where this appears in practice

Current separation is used for fuses, circuit-breakers, and some staged residual-current arrangements.

SafetyDo not change settings or device ratings to create separation without repeating conductor, disconnection, breaking-capacity, and assembly checks.
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

Which curve boundaries should be compared conservatively?

Downstream maximum operating behavior against upstream minimum operating behavior. The tolerance bands, not their centre lines, determine assured separation.

Answer: Downstream maximum operating behavior against upstream minimum operating behavior. The tolerance bands, not their centre lines, determine assured separation.

Practical exercise

Mark overlap and a provisional selectivity limit on anonymised manufacturer curves.

Summary

  • Compare tolerance envelopes
  • Instantaneous overlap limits claims
  • State the proven current range

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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