Wiring 42 — Light Curtain OSSD Pair into a Safety Relay

Light curtain OSSD pair into a safety relay

Training material. Follow your site procedures, local electrical code and the manufacturer's instructions. Lockout/tagout and a qualified person are required for real equipment.

A light curtain guards an opening with a row of infrared beams. Instead of a mechanical contact it reports through two solid-state safety outputs, called OSSD1 and OSSD2 (output signal switching devices). Both are on while the beams are clear and both switch off when a beam is interrupted. A safety relay reads the pair, and a start/reset button lets the operator re-arm the system after the area is confirmed clear.

What to notice while you wire

• The curtain needs its own supply. It has +24 V and 0 V terminals like any powered sensor, plus a functional-earth (FE) terminal that goes to the ground bar.
• Two outputs, two inputs. OSSD1 lands on relay input S12 and OSSD2 on S22. Keeping them on separate inputs lets the relay catch a fault that affects only one output (for example a cable short between the two).
• Two output contacts, two contactors. Relay output 13/14 drives KM1 and 23/24 drives KM2. Either contactor alone is enough to remove power from the hazard, and a fault in one does not defeat the other.
• Start/reset. A normally open button on the relay reset terminals. Clearing the beams alone must not restart anything; a deliberate press is needed.

Wiring list

1. Supply to the +24 V and 0 V strips; supply PE and curtain FE to the ground bar.
2. Curtain: +24 V and 0 V.
3. Relay: A1 to +24 V, A2 to 0 V.
4. Inputs: +24 V to S11 and S21; OSSD1 to S12; OSSD2 to S22.
5. Start/reset: +24 V to NO-IN; NO-OUT to S33; S34 to 0 V.
6. Outputs: +24 V to 13 and 23; 14 to KM1 A1; 24 to KM2 A1; both coils A2 to 0 V.

Real light curtains have their own wiring diagrams, cable length limits and alignment procedures. This lab only practises the signal topology, and it leaves out the contactor auxiliary feedback (external device monitoring) contacts that real installations usually wire into the reset loop, so a welded contactor would not be caught here.