Safety Light Curtain Wiring: OSSD Outputs, Muting, and Integration

What Is a Safety Light Curtain?

A safety light curtain (Type 4 AOPD — Active Opto-electronic Protective Device per IEC 61496-1) consists of a transmitter and receiver bar, each housing multiple infrared LEDs and photodetectors. The transmitter cycles through its beams; the receiver detects any blocked beam and commands an immediate machine stop.

Light curtains are classified by Type under IEC 61496:

Most industrial machine applications require at least PLd / SIL 2 (risk assessment dependent), which requires a Type 4 light curtain with dual-channel safety integration. Type 2 is only appropriate for lower-risk applications.

Leading light curtain manufacturers: SICK (deTec4, C4000), Pilz (PSENopt II), Rockwell (440L Guardmaster), Omron (F3SG), Banner Engineering (EZ-SCREEN).

OSSD Outputs

All Type 4 light curtains provide two solid-state PNP safety outputs: OSSD1 and OSSD2.

Normal Operation (Detection Zone Clear)

• OSSD1 = HIGH (24 V DC)
• OSSD2 = HIGH (24 V DC)

Trip Condition (Object Detected or Fault)

• OSSD1 = LOW (0 V)
• OSSD2 = LOW (0 V)

Test Pulses

Both OSSDs regularly generate test pulses — brief low-going voltage dips (~250 µs at a frequency of several hundred Hz). These test pulses allow the safety receiver (safety relay or safety PLC input) to detect cross-connections between OSSD1 and OSSD2, or a short circuit to 24 V. Without test pulse monitoring, a short-circuit fault would cause both outputs to read HIGH even when the curtain should be OFF — a dangerous failure mode.

The safety relay or safety PLC input must be designed to accept and process these test pulses. Standard DI inputs will interpret the pulses as high-frequency noise and may filter them out, losing the fault-detection capability.

Each OSSD output is rated for a maximum load current — typically 250 mA to 500 mA. If you need to drive multiple relays or higher loads, use the OSSD to drive a safety relay, which then provides heavier-duty output contacts.

Wiring to a Safety Relay

The most common integration is OSSD → standalone safety relay → contactors:

The safety relay monitors OSSD1 and OSSD2 simultaneously. Both must be HIGH for the relay to energise its output contacts. If either OSSD goes LOW, the relay de-energises and opens the output contacts, cutting power to the downstream contactor(s).

The output contacts of the safety relay (13–14, 23–24, 33–34 in a 3-contact module) are wired in series with the machine's safety-rated contactors. All safety output contacts should be forcibly guided (direct-opening) contacts — these open mechanically even if the coil fails, and their position can be monitored for cross-circuit detection.

Wiring to a Safety PLC

Modern safety PLCs (Siemens S7-1500F, Allen-Bradley GuardLogix, Pilz PNOZmulti 2, Beckhoff EL1904) have dedicated safety input terminals that process OSSD test pulses correctly.

Wiring is straightforward:

• OSSD1 → Safety DI channel A (e.g., DI 0.0)
• OSSD2 → Safety DI channel B (e.g., DI 0.1)
• Configure the input channel pair as "OSSD-compatible" or "test pulse tolerant" in the safety CPU configuration

The safety PLC's F-DI module detects discrepancies between the two channels. If OSSD1 and OSSD2 differ for more than the allowed discrepancy time (typically configurable 10–100 ms), the safety PLC raises a fault and initiates a safe state.

Safety PLC output to the machine's contactors is via the safety PLC's F-DO module, which also performs output test pulses and cross-circuit detection on its output channels.

External Device Monitoring (EDM)

EDM (also called contactor monitoring, feedback loop, or Y1/Y2 monitoring) verifies that the downstream contactors and safety relays are actually opening and closing as commanded.

A normally closed (NC) auxiliary contact from each contactor/safety relay is wired in series back to the light curtain or safety relay's EDM input. Before the light curtain can re-activate (allow machine restart), the EDM circuit must confirm:

1. When machine is stopped: EDM = open (NC contacts in series with a de-energised coil → open circuit)
2. When machine restarts command: EDM input monitors for correct contact state before re-enabling

EDM prevents a welded contact from causing a silent failure — if a contactor welds closed, its NC auxiliary opens when the contactor should drop out. The safety device detects the missing EDM transition and locks out. Without EDM, a welded contactor failure would go undetected until the next demand on the safety function.

IEC 60204-1 Clause 9.2.3 and ISO 13849-1 Category 3/4 requirements mandate cross-monitoring of redundant output contacts — EDM fulfils this requirement.

Manual Reset

After a light curtain trip, most safety applications require a manual reset — an operator must press a reset button outside the guarded zone to re-enable the machine. This ensures the operator has visually confirmed the zone is clear before restart.

Requirements for the reset button (IEC 62046 Clause 7.1.4):

• Must be located outside the guarded zone with a clear view into it
• Must require a deliberate action (no automatic restart)
• Wired as a normally open (NO) momentary contact to the safety relay's reset input (S34 on Pilz PNOZ, or equivalent)
• Must not be reachable from within the guarded zone when pressed

The safety relay's reset logic: OSSD inputs must be HIGH (zone clear) and then the reset button pressed momentarily before the output contacts close. This prevents an unintended restart if the reset button is pressed while someone is in the zone.

Some applications allow automatic reset (machine restarts as soon as the zone is clear, with no button press). This is only permissible where the risk assessment confirms that re-activation cannot cause a dangerous situation — typically only low-risk applications with PLc or Category 2 requirements.

Muting

Muting allows material (pallets, products, vehicles) to pass through the light curtain's protected zone without triggering a machine stop. During muting, the safety function is temporarily suppressed.

IEC 62046 Clause 7.2 governs muting. Key requirements:

• Two independent muting sensors — both must activate within a time window (typically ≤4 s) to initiate muting. A single sensor cannot activate muting alone.
• Muting sensors must be different types or positioned to prevent simultaneous manual defeat
• Muting status must be clearly indicated — a muting lamp (typically yellow) visible from outside the guarded zone
• Maximum muting duration — muting is limited by the passage of material; if material does not pass within the time window, muting times out and the safety function is restored
• A person must not be able to pass through while muting is active — muting sensor placement must ensure material is present and the person would interrupt at least one muting sensor before the light curtain

Common muting sensor arrangement: two retro-reflective or through-beam sensors placed at 45° to the conveyor, positioned so pallet triggers them before it reaches the light curtain. Both sensors must be HIGH simultaneously within the time window to authorise muting.

Fixed and Floating Blanking

Blanking permanently disables one or more beams in the light curtain's detection array to accommodate a fixed object (e.g., a conveyor belt or fixture) that passes through the protected zone.

• Fixed blanking — a defined set of beams is permanently suppressed. If the object is removed, the blanked beams do not trigger a stop. Minimum resolution is reduced to the beam pitch × number of blanked beams.
• Floating blanking — a defined window of adjacent beams (e.g., 1–3 beams wide) can be blocked anywhere along the curtain without triggering a stop. Used for objects that shift position laterally.

Blanking reduces the effective protection height. The minimum object detection size must be recalculated for the blanked configuration. Blanking is configured in the light curtain's configuration software (DIP switches or USB tool) and the settings must be documented in the safety validation file.

Minimum Safety Distance

The light curtain must be positioned far enough from the hazard that the machine stops before a person can reach the hazard point. The minimum safety distance S is calculated per ISO 13855:

S = K × (t₁ + t₂)

Where:
  K  = approach speed (mm/s): 2000 mm/s for hand/arm approach (default)
  t₁ = light curtain response time (ms) — from datasheet
  t₂ = machine stop time (ms) — measured from safety output de-energising to full stop

Example:
  t₁ = 10 ms (typical Type 4 light curtain)
  t₂ = 200 ms (machine stop time, measured)
  K  = 2000 mm/s

  S = 2000 × (0.010 + 0.200) = 2000 × 0.210 = 420 mm

The light curtain must be placed at least 420 mm from the nearest hazard point in this example. If the detection zone is larger than 40 mm (hand detection), an additional distance must be added per ISO 13855 Table 2. The calculation must be documented as part of the safety validation.

Achieving PLd / SIL 2

For a PLd / SIL 2 light curtain installation, the complete safety function (light curtain + wiring + safety relay/PLC + output contactors) must meet:

Use the manufacturer's SISTEMA library files (IFA, Germany) to calculate PFHd and verify PLd in SISTEMA software. Most major manufacturers (SICK, Pilz, Rockwell, Omron) provide free SISTEMA libraries for their safety components.

Frequently Asked Questions

What are OSSD outputs on a safety light curtain?

OSSD (Output Signal Switching Device) outputs are a pair of PNP 24 V DC safety outputs (OSSD1 and OSSD2). Both outputs are ON when the detection zone is clear. Either output switches OFF when an object is detected or a fault occurs. The outputs include regular test pulses (low-going spikes ~250 µs) to detect short circuits to 24 V — the safety device monitors these pulses to detect wiring faults.

What is EDM in safety light curtain wiring?

EDM (External Device Monitoring, also called contactor monitoring) is a feedback circuit from the contacts of the downstream safety relay or contactor back to the light curtain's or safety relay's monitoring input. Before the light curtain can re-enable the machine, EDM verifies that the downstream device (contactor/relay) has actually opened and closed correctly — detecting welded contacts.

Do I need a safety relay with a safety light curtain?

Not always. If your safety PLC or safety controller has dedicated OSSD-compatible safety inputs with cross-circuit detection, you can wire OSSD outputs directly to it. However, if using a standard PLC or non-safety relay, a standalone safety relay module (e.g., Pilz PNOZ, SICK UE410) is required to process the OSSD signals correctly and provide dual-channel output.

What is muting in a safety light curtain?

Muting temporarily suppresses the light curtain's safety function to allow material (e.g., a pallet or product) to pass through the guarded zone without triggering a stop. Muting is activated by two independently wired muting sensors placed at 45° angles to the material flow. The muting sensors must both activate within a defined time window (typically <4 s) and deactivate once material has passed. IEC 62046 governs muting requirements.