Comprehensive guide to understanding the critical differences between Safety PLCs and Standard PLCs, and when to implement each system for optimal industrial automation control.
Understanding Safety PLCs vs Standard PLCs
At Sull Systems, we help clients implement the right control solution for their specific requirements. Understanding the fundamental differences between Safety PLCs and standard PLCs is essential for ensuring both regulatory compliance and operational efficiency in industrial automation systems.
What is Standard PLC?
A standard Programmable Logic Controller (PLC) is a strengthen industrial computer that controls manufacturing processes, such as assembly lines, machine functions, or robotic devices. Standard PLCs are the workhorses of industrial automation, handling everyday control tasks.
Standard PLCs are designed for reliability and performance in normal operating conditions but are not certified for safety-critical functions. They can fail in unpredictable ways and don’t have the necessary redundancy for safety applications.
What is Safety PLC?
Safety PLCs are specialised industrial controllers designed to execute safety-related control functions without failing in a dangerous way. Unlike standard PLCs that handle normal automation tasks, Safety PLCs are built to meet stringent safety standards and can replace discrete safety relays and hardwired circuits.
Key Differences
Between Safety PLCs and Standard PLCs
Internal Architecture
Safety PLC: Uses redundant processors, memory, and I/O pathways with constant cross-checking
Standard PLC: Utilises single processing channels without built-in safety verification
Programming Approach
Safety PLC: Requires formal verification, access control, and version management
Standard PLC: Programs can be modified without strict change control procedures
Monitoring Capability
Safety PLC: Continuous self-diagnostics with the ability to detect its own failures
Standard PLC: Limited self-diagnostics, mainly focused on operational status
Fault Response
Safety PLC: Consistently fails to a predetermined safe state when any fault is detected
Standard PLC: May continue operating with undetected faults or fail unpredictably
Certification
Safety PLC: Certified to standards like IEC 61508 (SIL3) and ISO 13849 (PL e)
Standard PLC: No safety certification required for standard operation
Cost Comparison
Safety PLC: Higher initial investment due to redundant components and certification
Standard PLC: More economical for standard control applications
When to Use Safety PLCs vs Standard PLCs
Selecting the appropriate PLC system depends on your application’s risk assessment and the level of safety required. Here’s a guide to help you decide:
Use Standard PLCs for:
- General automation processes
- Non-hazardous equipment control
- Systems where failure doesn’t pose safety risks
- Process optimisation and monitoring
- Cost-sensitive applications with low risk
Use Safety PLCs for:
- Emergency stop systems
- Guard door monitoring
- Light curtain and safety scanner integration
- Two-hand controls for presses
- Safety-rated speed monitoring
- Robot cell protection
- Applications requiring SIL2/3 or PL d/e certification
Where to Use Safety PLCs
Robotics & Automation
Robot cells, high-speed handling systems, and collaborative workspaces where humans and machines share operating spaces.
Hazardous Machinery
Press machines, injection molding, high-energy equipment, and mechanical systems with significant pinch points or crush hazards.
Regulated Industries
Food production, pharmaceutical manufacturing, and other environments with strict safety validation requirements and regulatory oversight.
Complex Safety Logic
Applications requiring zone control, conditional safety muting, safe motion control, or systems with multiple interconnected safety functions.
Benefits of Implementing Safety PLCs vs Traditional Safety Relays
- Reduced Wiring Complexity: Safety PLCs eliminate the need for complex relay-based safety circuits, reducing panel space and simplifying troubleshooting.
- Comprehensive Diagnostics: Quickly identify the exact cause of safety stops with detailed information showing which specific safety device triggered, reducing downtime.
- Flexible Safety Logic: Easily implement complex safety relationships that would be difficult or impossible to achieve with hardwired safety relays.
- Simplified Documentation: Generate safety logic documentation and validation reports directly from the Safety PLC programming software.
- Scalable Architecture: Add safety devices or zones without rewiring the entire safety system – simply update the Safety PLC program as needed.
- Integration with Standard Control: Safety and standard control can communicate efficiently, enabling coordinated machine responses to safety events.
Safety PLC Integration with Safety Sensors
While Safety PLCs form the core of modern machine safety systems, they must connect with appropriate safety-rated sensors to create a complete safety solution. At Sull Systems, we integrate various sensor technologies to create comprehensive safety systems
Presence Detection
Light curtains, laser scanners, and safety mats detect personnel entering hazardous areas and trigger appropriate safety responses.
Guard Monitoring
Safety interlocks, gate switches, and locking devices ensure machine access points are secured before operation is allowed.
Motion Monitoring
Safety encoders and speed monitors ensure machine motion stays within safe parameters, enabling functions like safe limited speed.
Emergency Stop Systems
Properly designed e-stop circuits with dual-channel monitoring and appropriate stop categories based on risk assessment.
Our End-to-End Design Process
Safe Stop Categories
Category 0: Immediate removal of power (uncontrolled stopping)
Category 1: Controlled stop maintaining power to achieve stopping, then power removal
Category 2: Controlled stop maintaining power to the machine (for monitored standstill)
Advanced Safety Functions
SLS (Safely Limited Speed): Ensures motion stays below a defined safe speed
SOS (Safe Operating Stop): Monitors position to ensure standstill while maintaining power
SS1/SS2 (Safe Stop 1/2): Controlled deceleration before Safe Torque Off or monitored standstill
SBC (Safe Brake Control): Ensures safe application of mechanical brakes
Why Choose Sull Systems?
PLC Integration Expertise: Our engineers are certified specialists in all major PLC platforms including Allen-Bradley, Siemens, Schneider, and Mitsubishi.
Multi-Platform Knowledge: We work with diverse automation systems and can recommend the best solution for your specific industrial application needs
Industry Experience: We understand the unique requirements of manufacturing, energy, water treatment, and pharmaceutical control systems.
Full-Service Approach: From initial concept and design through implementation, commissioning, and ongoing technical support
Standards Compliance: We implement systems that meet IEC 61131, ISA-95, and other critical industry standards for automation integration.
Scalable Solutions: Our systems grow with your business, from single-controller applications to plant-wide integrated control networks.
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