Functional safety ensures that automated systems respond correctly to potentially dangerous conditions, bringing machinery to a safe state when hazards are detected. In the UK, compliance with functional safety standards is not optional; it is a legal requirement under the Machinery Directive and the Supply of Machinery (Safety) Regulations.
Key Safety Standards
IEC 62061 defines Safety Integrity Levels (SIL) for safety-related electrical, electronic, and programmable electronic control systems. SIL levels range from SIL 1 (lowest) to SIL 3 (highest for machinery).
ISO 13849-1 defines Performance Levels (PL) for safety-related parts of control systems. PLs range from PL a (lowest) to PL e (highest).
Both standards achieve the same goal but use different methodologies. In practice:
- SIL is commonly used in process industries (oil and gas, chemical, pharmaceutical)
- PL is commonly used in machinery and factory automation
The Safety Lifecycle
Designing a functional safety system follows a structured lifecycle:
- Hazard analysis and risk assessment: Identify all hazards and determine the required safety level (SIL or PL) for each safety function
- Safety requirements specification: Define what each safety function must do, its required response time, and its integrity level
- System design: Select safety-rated components and design the safety logic
- Implementation: Programme the safety PLC and wire the safety circuits
- Verification and validation: Test every safety function to confirm correct operation
- Documentation: Maintain complete documentation including the safety manual, validation report, and maintenance procedures
- Operation and maintenance: Regular proof testing and inspection throughout the system lifecycle
Safety PLCs and Controllers
Modern safety PLCs combine standard automation and safety functions in a single controller:
- Siemens S7-1500F: Fail-safe CPU that runs both standard and safety programmes in TIA Portal
- Safety-rated I/O modules: F-DI, F-DO, F-AI modules that meet SIL 3 / PL e requirements
- Dedicated safety relays: For simple safety functions such as emergency stop and guard monitoring
Common Safety Functions
- Emergency stop (E-stop): Immediately removes power to hazardous motion when activated
- Guard monitoring: Detects when safety guards are opened and stops dangerous machinery
- Light curtains: Optical barriers that detect human presence in hazardous zones
- Two-hand control: Requires both hands on controls to initiate dangerous operations
- Safe speed monitoring: Ensures motors do not exceed safe speed limits during maintenance
- Safe torque off (STO): Prevents the drive from generating torque, used as a safe shutdown function
Safety Programming in TIA Portal
Siemens TIA Portal provides a dedicated safety programming environment:
- Safety programmes use a restricted instruction set for deterministic behaviour
- F-blocks (safety function blocks) are certified and cannot be modified
- Safety signatures ensure programme integrity through automatic checksums
- Access protection prevents unauthorised modifications to safety programmes
EDWartens UK Safety Training
Functional safety training at EDWartens UK covers risk assessment, safety system design, and safety PLC programming using Siemens F-CPUs. Our courses prepare engineers to design and commission safety systems that comply with UK and European regulations.