Best Practices for Machine Safety: Emergency Stop Circuit Design and Compliance with Standards

Best Practices for Machine Safety: Emergency Stop Circuit Design and Compliance with Standards

Machine safety is a critical concern in industrial automation, especially when it comes to emergency stop (E-Stop) functions. Plant engineers and maintenance professionals often face uncertainty about whether auxiliary contacts on contactors can be safely used for E-Stop circuits. Additionally, navigating the complex Performance Level (PL) requirements under standards like ISO 13849-1 adds to the challenge. This case study explores a real-world scenario where a manufacturing facility needed to evaluate and redesign their emergency stop system to meet safety compliance without compromising operational efficiency.

---

The Problem: Uncertainty in Emergency Stop Circuit Design and Compliance

A mid-sized manufacturing plant specializing in packaging machinery was experiencing frequent safety audits highlighting potential non-compliance in their emergency stop circuits. Their existing system used auxiliary contacts from standard contactors to signal emergency stops. However, concerns arose:

• Can auxiliary contacts of contactors be reliably used for E-Stop functions?
• Does the existing single-channel design meet Performance Level c (PL c) requirements?
• How to properly evaluate and upgrade the emergency stop setup to ensure compliance with safety standards?

The plant’s maintenance team lacked clarity on these points, risking regulatory fines and, more importantly, operator safety.

---

Approach: Expert Evaluation and Standards-Based Redesign

The plant engaged Gross Automation to conduct a thorough review and redesign of their emergency stop circuit. The approach included:

1. Assessment of Current Emergency Stop Circuit

The existing setup used ABB CE4T-10B-11 contactors with their auxiliary contacts wired into the E-Stop circuit. While convenient, auxiliary contacts on contactors are primarily intended for control feedback, not as safety-rated devices. The team reviewed:

• Contact reliability and mechanical wear over time
• Potential for contact welding or failure modes
• Compliance with EN ISO 13849-1 and IEC 60204-1 standards

2. Understanding Performance Level Requirements

ISO 13849-1 defines Performance Levels (PL a to PL e) based on risk assessment. For single-channel E-Stop circuits, achieving PL c requires:

• Use of safety-rated components designed for emergency stop functions
• Diagnostic coverage to detect faults
• Proper architecture to prevent dangerous failures

The team identified that auxiliary contacts on standard contactors do not inherently meet these criteria.

3. Selecting Appropriate Safety Components

To meet PL c requirements, the team recommended replacing the auxiliary contact-based E-Stop with dedicated safety components such as:

• ABB’s KXTAAUX safety auxiliary contacts designed for reliable feedback
• Safety-rated contactors like the ABB OA1G10 series with integrated safety functions
• Safety relays and monitoring devices compatible with these components

4. Designing a Compliant Emergency Stop Circuit

A single-channel emergency stop circuit was redesigned using ABB’s CSW30H-BEPI3D61 safety relay modules, which provide:

• Positive-guided contacts for safe switching
• Diagnostic feedback for fault detection
• Compliance with PL c requirements

The new design incorporated ABB BC11ML-CSW30H safety contactors, ensuring mechanical and electrical compatibility with the safety relay.

---

Solution: Implementing a Safety-Compliant Emergency Stop System

Key Components Used

• ABB CSW30H-BEPI3D61 Safety Relay Module: Provides reliable monitoring of emergency stop buttons and safety contacts.
• ABB BC11ML-CSW30H Safety Contactor: Designed for safety applications with positive-guided contacts.
• ABB KXTAAUX Safety Auxiliary Contacts: Supplement contactors with safety-rated feedback contacts.
• ABB OA1G10 Safety Contactors: Optional upgrade for contactors with integrated safety functions.

Circuit Design Highlights

• Emergency stop pushbuttons wired in series to the safety relay inputs.
• Safety relay monitors the circuit and controls the safety contactor coil.
• Safety contactor interrupts power to the machine’s drive components upon E-Stop activation.
• Use of positive-guided contacts ensures that any welding or failure is detected.
• Diagnostic feedback loop alerts operators to faults or degraded conditions.

Practical Implementation Steps

1. Remove auxiliary contacts from standard contactors for E-Stop signaling.
2. Install safety relays (CSW30H-BEPI3D61) to monitor E-Stop buttons and safety contacts.
3. Use safety contactors (BC11ML-CSW30H) to break power circuits safely.
4. Integrate safety auxiliary contacts (KXTAAUX) for reliable feedback and diagnostics.
5. Test the system for proper operation and fault detection.
6. Document the new circuit design and update safety manuals.

---

Results: Enhanced Safety and Compliance with Reduced Downtime

After implementing the redesigned emergency stop circuit:

• Compliance Achieved: The plant met PL c requirements under ISO 13849-1, passing subsequent safety audits without issues.
• Improved Reliability: Safety-rated components reduced the risk of contact welding and undetected failures.
• Faster Fault Detection: Diagnostic feedback from safety relays enabled quicker maintenance response.
• Operator Confidence: Clear, tested emergency stop function improved operator trust in machine safety.
• Reduced Downtime: Reliable E-Stop operation minimized unplanned stops and maintenance delays.

The plant’s safety team now has a clear, standards-compliant emergency stop design that can be replicated across other machines.

---

Best Practices for Emergency Stop Circuit Design

Based on this case study and industry standards, here are actionable best practices:

Use Safety-Rated Components

• Avoid using auxiliary contacts on standard contactors for emergency stop signaling.
• Choose safety relays like ABB CSW30H-BEPI3D61 designed for emergency stop monitoring.
• Select safety contactors such as ABB BC11ML-CSW30H with positive-guided contacts.

Understand and Apply Performance Level Requirements

• Conduct risk assessments to determine required PL (a to e).
• For single-channel E-Stop circuits targeting PL c, ensure diagnostic coverage and safe failure modes.
• Follow ISO 13849-1 and IEC 60204-1 standards for design and validation.

Design for Diagnostics and Feedback

• Incorporate safety auxiliary contacts (e.g., ABB KXTAAUX) for reliable feedback.
• Use safety relays with built-in diagnostics to detect faults early.
• Implement regular testing and maintenance protocols.

Document and Train

• Maintain clear documentation of emergency stop circuit design and components.
• Train maintenance and operations staff on the function and testing of E-Stop systems.

---

Conclusion: Partner with Gross Automation for Machine Safety Solutions

Designing an emergency stop circuit that is both safe and compliant with Performance Level requirements can be complex. This case study demonstrates that auxiliary contacts on standard contactors are not suitable for safety-critical E-Stop functions. Instead, using safety-rated components from ABB, such as the CSW30H-BEPI3D61 safety relay and BC11ML-CSW30H safety contactor, ensures compliance and reliable operation.

If you’re facing challenges with your emergency stop circuit design or need expert evaluation to meet safety standards, Gross Automation is your trusted partner. We offer a wide range of ABB safety components and expert technical support to help you implement best practices in machine safety.

Contact Gross Automation today to discuss your machine safety needs and ensure your emergency stop systems are designed for maximum protection and compliance.

---