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Safety relay
- Hard-wired and certified for a specific function.
- Simple to install and easy to understand.
- Best for a small number of inputs and outputs. Adding functions means adding relays.
Safety PLC or configurable controller
- Programmable logic with many safety inputs and outputs in one device.
- Detailed diagnostics and, on many models, safety communication over a fieldbus.
- Easier to change when the machine or its zones change, but needs software and validation.
How to decide
- Do a risk assessment and determine the required performance level or SIL for each safety function.
- Count the safety inputs (emergency stops, gates, light curtains, scanners) and outputs (contactors, drive safe-torque-off).
- If you have only a handful of functions, a safety relay is usually the simplest.
- If you need many functions, zones, muting or diagnostics, use a safety controller.
- Validate the final solution and document it.
Important
Machine safety is a regulated area. Selection and validation must follow the applicable standards, such as ISO 13849-1 or IEC 62061, and be carried out by a competent person.
Frequently asked questions
Can I use a standard PLC for safety functions?
No. Safety functions need components certified for the required performance level or SIL. Use safety-rated relays or controllers.
What does safe torque off mean?
Safe Torque Off (STO) is a drive safety function that removes the energy that can produce torque in the motor, so it cannot start unexpectedly.
Who should validate a safety circuit?
A competent person following the applicable machine safety standards should validate it.
This guide is general engineering guidance. Always confirm part-specific details, ratings and compatibility against the manufacturer’s datasheet. We confirm price, stock and specifications for the exact part in a written quotation. See the glossary for the terms used here.