Safety Relays: Working Principle and Configuration Guide

1. Why Safety Relays Are Needed

Ordinary relays can fail the safety function when contacts weld, springs fatigue or coils fault. A safety relay uses redundant design, force-guided contact structures and internal self-checking circuits to achieve PL e per EN ISO 13849-1 / SIL 3 per IEC 62061, ensuring that the safety output reliably opens when any single element fails.

2. Working Principle

Take a typical dual-channel E-stop circuit: the two NC contacts of the E-stop button feed two input channels of the safety relay. In the normal state both channels close synchronously and the internal safety output contacts pull in, allowing the machine to run. When the E-stop is pressed, either channel opens and the safety output releases within 20–50 ms, cutting the contactor or VFD enable. Reset can be automatic or monitored manual (the reset button is pressed only after the fault is confirmed cleared).

3. Typical Monitored Objects

• E-Stop buttons: the most basic safety function
• Safety/guard door interlocking: machine stops when the door opens, e.g. machining center guard doors
• Safety light curtains / safety mats: stop when personnel enter a hazardous zone
• Two-hand controls: prevent accidental operation of presses and punch machines
• Safe speed / safe limits: cranes and robot workstations

4. Selection Points

1. Number of safety functions: single-function modules (such as the Pilz PNOZ X2 series dedicated to E-stops) are low-cost; configurable multi-function modules (such as PNOZmulti) can monitor E-stops, safety doors and light curtains simultaneously and suit complex machines.
2. Input channels: single channel suits simple equipment; dual channel with short-circuit detection achieves higher safety levels.
3. Output contact rating: when safety contacts drive contactors directly, verify contact current (typically 5–8 A); high-power circuits should be expanded through auxiliary contactors.
4. Reset mode: automatic reset suits stations where guard doors open and close frequently; manual reset is safer and suits E-stop circuits.
5. Certification consistency: exported equipment needs CE/UL certification; recommended brands are Pilz, Schneider XPS and Siemens 3SK.