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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/Industrial Control & PLCs/Safety Systems

lesson · 9min · Lesson 17 of 30

Safety relays and safety PLCs

Course syllabusCourse overview
01Motor Starters
  1. ReadDOL starter: main and control circuit
  2. ReadStar-delta starter: wiring and timer
  3. ReadSoft starters: operation and parameters
  4. ReadVariable speed drives: inverter drives
  5. quizMotor starter quiz
02Contactors & Overloads
  1. ReadContactor construction and ratings
  2. ReadAuxiliary contacts and interlocking
  3. ReadThermal overload relays: setting the dial
  4. ReadElectronic overloads and motor protection relays
  5. exerciseContactor circuit problems
03Control Circuit Diagrams
  1. ReadIEC 60617 symbols: reading control diagrams
  2. ReadLadder diagrams: European and American styles
  3. ReadDrawing a full DOL control circuit
  4. ReadForward-reverse motor control
  5. exerciseDiagram reading exercises
04Safety Systems
  1. ReadEmergency stop requirements: IEC 60204
  2. ReadSafety relays and safety PLCs
  3. ReadLight curtains and safety interlocks
  4. ReadSafe isolation: EAWR and multi-energy control
  5. quizSafety systems quiz
05PLC Fundamentals
  1. ReadWhat is a PLC and where is it used?
  2. ReadPLC architecture: CPU, I/O modules, power
  3. ReadLadder logic basics: contacts and coils
  4. ReadTimers and counters in ladder logic
  5. exerciseWriting a simple motor control program
06Panel Layout & Installation
  1. ReadPanel design: layout and component spacing
  2. ReadCable management inside panels
  3. ReadLabelling, ferrules, and documentation
  4. ReadPanel testing and commissioning checklist
  5. 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

Safety relays and safety PLCs

Industrial Control · Lesson 17 · Advanced

AdvancedReview: professional review pending

Purpose

Select and validate safety relays or safety PLCs as logic subsystems within complete safety functions.

Before you beginMotor starter circuits · Machine hazard and protection fundamentals

Learning objectives

  • Read state and cross-references
  • Separate standard control from safety functions
  • Analyse faults and reset behavior
  • Validate the complete input-logic-output path

A safety relay or safety PLC cannot create safety by itself. Required PLr/SIL comes from risk assessment and must be met by the complete sensor, logic, output and machine response.

MACHINE SAFETY FUNCTION PATHMACHINE SAFETY FUNCTION PATHSUPPLYSTARTERmotor · loadshort circuit · overload · command · interlock · isolation

Core theory

Define each safety function, safe state, demand rate, response time and required performance before choosing architecture.

Assess categories/architecture, MTTFd or PFHd data, diagnostic coverage, common-cause failures, software/parameter controls, output-device monitoring and systematic capability under the chosen standard.

Validate wiring faults, channel discrepancy, contactor welding, reset, muting/mode selection, power loss/recovery and configuration changes. Record calculations, versions, proof tests and results.

Terms, symbols, and units
TermMeaningSymbolUnit
PLrRequired performance level under ISO 13849-1Not applicableNot applicable
SILSafety integrity level under the applicable functional-safety frameworkNot applicableNot applicable
EDMExternal device monitoring of final switching elementsNot applicableNot applicable
Worked exampleA dual-channel guard feeds a safety relay but both channels share one vulnerable cable route.

Assumptions: Symbols and terminal designations are defined by the supplied drawing set; Required safety performance comes from the machine risk assessment.

  1. Architecture: Two inputs are present.
  2. Common cause: One cable damage event may defeat both channels.
  3. Resolve: Assess routing, fault exclusion/diagnostics and achieved PL/SIL rather than claiming redundancy.

Dual terminals alone do not prove the required performance; common-cause and diagnostic evidence is missing.

Reasonableness check: The conclusion is checked across normal, demand, fault, reset and restart states rather than inferred from one component label.

Common mistakes
  • Reading a contact in the energized instead of defined normal state
  • Using an ordinary PLC bit as an unvalidated safety function
  • Assuming emergency stop isolates every energy source

Where this appears in practice

Clear diagrams and validated safety functions let competent people build, test, diagnose and modify machinery without losing the intended protective behavior.

SafetyDo not bypass guards, light curtains, interlocks or safety outputs for production or diagnosis. Control hazardous electrical, pneumatic, hydraulic, gravitational and stored energy; isolate, lock off and prove dead using the site procedure before access.
Local code checkUse the current machine risk assessment, applicable type-C standard, IEC/BS EN 60204-1, ISO/BS EN ISO 13849-1 or IEC/BS EN 62061 design route, IEC 61496 where applicable, Electricity at Work Regulations, site lockout procedure and manufacturer validation instructions. BS 7671 Section 514 identification/notices does not by itself define or authorize the complete safe-isolation procedure.

Knowledge check

Does a safety-rated PLC automatically make ordinary field devices safety-rated?

No. The entire safety function must achieve and validate the required performance.

Answer: No. The entire safety function must achieve and validate the required performance.

Practical exercise

Build a safety-requirements specification and validation matrix for guard, E-stop and final contactors.

Summary

  • A diagram is a state model plus physical cross-reference
  • Safety performance belongs to the complete function
  • Stop, emergency stop and isolation are distinct

Sources and review

  • IEC 60204-1:2016+AMD1:2021: Electrical equipment of machines: IEC; 6.1; International
  • ISO 13849-1:2023: Safety-related parts of control systems: ISO; 2023; International
  • IEC 62061:2021 with current amendments: Functional safety of machinery control systems: IEC; 2021; confirm current consolidated version; International
  • IEC 61496-1:2020: Electro-sensitive protective equipment: IEC; 2020; International
  • Electricity at Work Regulations 1989: UK Legislation; Current official text; Great Britain

Editorial review date: 2026-08-22. 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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