In today’s high-risk manufacturing environments, machine safety is not just about guarding—it’s about intelligent, fail-safe control systems that protect human life. Among the most critical components of a machine safety system is the safety interlock switch. Specifically, Keyence’s safety switches offer advanced functionality that supports both power-to-lock and power-to-unlock configurations. But which is safer? When should you use one over the other? And how do these choices impact access during operation or in emergencies?
This guide will clarify the differences between power-to-lock and power-to-unlock safety switches, evaluate which is safer for personnel, and outline best practices for access control during machine operation and emergency escape solutions.
What Are Keyence Safety Interlock Switches?
Keyence, a global leader in factory automation, offers advanced interlock switches designed to meet international safety standards (ISO 14119, ISO 13849-1, IEC 61508). These switches are engineered to monitor and control access to hazardous areas of machinery, ensuring that access is only permitted under safe conditions.
Keyence interlock switches typically feature:
Solenoid locking mechanisms
Electromagnetic actuation (power-to-lock or power-to-unlock)
High locking force (up to 3000 N)
Status indicators
Escape release and override options
PL e / SIL 3 compliance when integrated properly
Understanding the Two Main Types: Power-to-Lock vs Power-to-Unlock
1. Power-to-Lock (PTL) Switches
Definition: In a power-to-lock system, the solenoid must be energized to lock the actuator (keep the guard door closed). When power is lost or shut off, the switch automatically unlocks.
Advantages:
Fail-safe for emergencies: If there is a power outage, the door unlocks automatically, enabling workers to escape or enter safely.
Easy access during shutdowns or faults
Lower risk of entrapment
Disadvantages:
Unintended access risk: In the event of an unintentional power loss, the system could unlock and expose moving parts.
Not ideal for processes that require guard doors to remain locked under all conditions
Applications:
Appropriate for non-inertia or low-hazard systems
Used where quick egress in a power-down event is critical
2. Power-to-Unlock (PTU) Switches
Definition: In a power-to-unlock system, the solenoid must be energized to unlock the actuator (allow the guard door to open). When power is off, the system remains locked.
Advantages:
Fail-to-lock behavior: During power loss, the guard remains locked, maintaining protection from hazardous motion or inertia.
Ideal for high-risk or rotating machinery
Best for heavy or hazardous processes with run-down time
Disadvantages:
Risk of operator entrapment if inside the cell during power loss
Requires additional emergency escape or trapped key systems for safety
Applications:
Perfect for hazardous machines with dangerous inertia
Ideal for robotic cells, CNC machines, presses, and conveyors
Which System is Safer?
The safer system depends on the machine’s risk profile.
| Criteria | Power-to-Lock (PTL) | Power-to-Unlock (PTU) |
|---|---|---|
| Fail-Safe Access | Unlocks during power loss | Remains locked |
| Guard Integrity | Unlocks during faults | Remains locked |
| Risk of Entanglement | Reduced during power-off | Increased unless managed |
| High-Inertia Hazards | Not safe | Ideal |
| Emergency Escape Required? | Recommended | Mandatory |
Verdict:
Use Power-to-Lock switches for low-inertia, low-risk systems where safe escape is a priority.
Use Power-to-Unlock switches for high-risk, hazardous applications where guard containment is critical, but ensure emergency escape solutions are implemented.
Providing Safe Access During Operation
Whether you're using a PTL or PTU safety switch, access during operation must never compromise operator safety or machine integrity. Here are a few best practices for controlled access:
1. Request-to-Enter System
Integrate a two-step access protocol:
Operator requests access via HMI or push button
PLC initiates machine rundown and verifies safe conditions before unlocking
2. Time-Delay Unlocking
Use a time-delay release on a PTU switch to allow dangerous motion to stop fully before permitting access.
3. Guard Lock Monitoring
Ensure the switch supports lock status monitoring and ties back to the safety PLC or controller. Keyence’s GS and GS-M series include safety-rated lock monitoring contacts.
4. External Device Monitoring (EDM)
Confirm the safety relay or PLC monitors the entire chain:
E-stops
Guard doors
Lock/unlock status
Reset conditions
Emergency Escape Release: A Life-Saving Feature
Any safety system that could trap a person inside a hazardous area must include a means of emergency escape.
How Escape Releases Work:
Installed inside the guarded area
Mechanically overrides the locking mechanism
Can be illuminated or labeled for visibility in low-light conditions
Some versions are tamper-resistant to avoid misuse
Options for Keyence Switches:
GS-M Series supports built-in emergency release
Use a separate mechanical emergency release handle for basic models
Escape releases may be resettable from outside the guard
Real-World Application Example
Scenario: Robotic Welding Cell
Machine has high-speed moving parts with high run-down time
Operators occasionally perform cleaning inside the cell
Access must only be granted after shutdown is confirmed
Recommended Solution:
Keyence Power-to-Unlock switch
Integrated with time-delay logic to account for run-down time
Emergency escape release installed inside the cell
Access request HMI triggers lock release only when all safe conditions are verified
System Integration with PowerSafe Automation
At PowerSafe Automation, our TÜV-certified safety engineers specialize in designing and installing turnkey safety systems using power-to-lock and power-to-unlock technologies.
We provide:
Custom risk assessments to determine appropriate locking logic
Integration with Keyence safety switches
Installation of emergency escape releases
Functional safety validation to ISO 13849-1 / IEC 62061
Lockout/tagout integration and access controls
We also supply:
Guarding (aluminum extrusion, steel panels)
Electronic safety devices (light curtains, scanners)
Seal cabinets and conduit for IP-rated environments
Safety relays, PLCs, and status indicators
Conclusion
When deciding between a Keyence power-to-lock and power-to-unlock safety switch, you must consider:
The hazard level of your machine
Whether fail-safe access or fail-safe guarding is the top priority
The presence of personnel within a guarded area
Whether the application requires emergency escape options
There is no universal “safer” option—the correct switch depends on the risk context. However, PowerSafe Automation can help you determine the right solution, install it properly, and ensure you stay compliant with OSHA and ANSI standards.
