Learn Automation with MINATA #46: Safety Relays and Light Curtains — Guarding a Zone by Function

Safety Relays and Light Curtains: Guarding a Zone by Function

Safety relays and light curtains are layers of protection designed around a specific safety function. They reach the intended level of safety only when they are selected, wired and verified according to the risk assessment of that machine. Fitting one does not, by itself, make a zone safe.

What a light curtain does, and what it does not

A light curtain projects a field of beams that detects a person or an object entering a hazardous zone. When a beam is broken, the safety outputs change state to stop the hazardous motion. The curtain detects the intrusion; it does not bring the machine to a safe stop on its own if the rest of the circuit has not been designed correctly.

Two parameters decide how effective it is: resolution — whether it detects a finger, a hand or a body — and safety distance. The curtain has to sit far enough from the hazard that the machine can stop before a person can reach it, calculated from the stopping time actually measured on that machine.

A safety relay monitors; it does not merely switch

A safety relay, or a safety PLC, monitors the safety devices — emergency stops, light curtains, guard-door switches — and only permits energy to reach the actuators when every safety condition is satisfied. What matters about it:

Two channels: two independent paths, so a single fault such as a welded contact or a broken wire is detected.
EDM, external device monitoring: the relay checks that the contactors or final elements really have returned to the safe state before it will allow a reset.
Deliberate reset: in a zone where people are at risk, no automatic reset. The operator resets on purpose, after confirming the zone is clear.

Never bypass a channel to keep production running

The most common and most dangerous shortcut is bridging one channel, hard-wiring an output, or switching to automatic reset for convenience. Each of those destroys exactly the fault detection the safety device was designed to provide. Devices, distances and configuration follow the risk documentation, not habit.

A worked engineering situation

When the light curtain is broken, the contactors feeding the actuators are dropped out through the dual-channel safety circuit. The HMI only displays the cause — "loading-zone light curtain broken" — to help the operator deal with it. It is never allowed to override or bypass the protective function. Once the zone is clear, the operator resets deliberately, and only then presses Start again.

The device type, the PL or SIL level, the safety distance and the EDM configuration all follow the risk assessment and the standards applicable to the machine.

Common mistakes

Placing the light curtain too close to the hazard, so the machine cannot stop in time.
Wiring a single channel, or using automatic reset where people are at risk.
Leaving out EDM, so a welded contactor goes unnoticed.
Using a light curtain as a counting or process sensor rather than as a safety function.
Letting the HMI or the ordinary PLC override a safety signal.

Safety relay and light curtain checklist

[ ] Resolution and safety distance are calculated from the measured stopping time.
[ ] The safety circuit is dual channel, with positively guided contacts where required.
[ ] EDM monitors the final elements before a reset is permitted.
[ ] Reset is deliberate; no automatic reset in a zone with risk to people.
[ ] The PL or SIL level and the devices follow the applicable standards.

Guarding by function means every layer of protection has a defined role and can be verified. #47 turns to the layout of the control panel: leaving room for heat, for cable and for maintenance.

Read more automation knowledge at MINATA: https://minatavn.com/en/blog/industrial-automation

Previous — #45: Machine safety and the emergency stop: https://minatavn.com/en/blog/automation-45-machine-safety-estop

Next — #47: Control panel layout: https://minatavn.com/en/blog/automation-47-control-panel-layout

View all MINATA technical articles