The Rule Buyers Keep Trying to Negotiate
Reset tells truth.
When a person can cross the sensing plane, stand inside a robot cell, reach behind a fixture, disappear behind a press frame, or crouch inside a palletizer opening, the reset decision becomes a legal and engineering statement about who is allowed to recreate motion. Who wants that statement written by a rushed panel builder?
Here is the hard answer: manual reset is required after whole-body access through a light curtain when a person can pass through the protective field and remain in the hazardous area after the beams clear. In that condition, automatic reset is not “efficient.” It is a blind assumption dressed up as productivity.
I’ve reviewed enough machine layouts to say this plainly: many light curtain projects fail not because the emitter and receiver are weak, but because the reset logic is treated like a terminal-block choice. Wrong mindset. A safety light curtain manual reset is part of the safety function, not a decoration on the enclosure door.
If you are still selecting the sensing device itself, start with the actual safety light curtain product range and then design the reset sequence around access behavior, stopping performance, and the worst credible misuse.
Table of Contents
Whole-Body Access Changes Everything
Whole-body access means the operator, technician, cleaner, die setter, or maintenance worker can pass completely through the light curtain plane and occupy a hazard zone without continuously blocking the beams.
That is different from a narrow hand-feed opening.
Very different.
A 14 mm finger-protection light curtain on a small insertion point may stop hazardous motion when a finger enters the field. A 30 mm hand-protection curtain may make sense for certain point-of-operation tasks. But an 80 mm, 300 mm, or 400 mm body-detection arrangement around a walk-in cell is a different animal. Once the person is past the detection plane, the curtain may see nothing.
And if the control system says, “beams clear, outputs on, machine ready,” then the machine has learned the wrong lesson.
OSHA’s mechanical power press rule gives the old but still useful backbone: a presence-sensing point-of-operation device must be interlocked into the control circuit to prevent or stop slide motion when a hand or body part is in the sensing field, and OSHA’s formula still uses the famous 63 inches/second hand-speed constant for safety distance calculations in press applications.
That 63 in/s number is not trivia. It is a warning. Humans move faster than optimistic safety files.
The Pass-Through Hazard: Where Auto Reset Becomes Reckless
A pass-through hazard exists when a person can interrupt the light curtain, walk beyond it, and then stand in a place where the light curtain no longer detects them.
That is when manual reset light curtain requirements stop being theoretical.
The reset should normally happen only after the protected area is verified clear. The reset control should be outside the hazardous area. It should be positioned so the person pressing it can see the hazard zone. It should not be reachable from inside the protected space. And it should not directly start hazardous motion.
Reset is not start.
Say it again in the design review. Reset is not start.
A proper light curtain restart interlock does this:
- Light curtain detects entry and OSSD outputs switch off.
- Hazardous motion stops or is prevented.
- Person exits the zone.
- Operator verifies the zone is empty.
- Operator performs a deliberate manual reset.
- A separate start command is required before hazardous motion resumes.
If your reset button can restart the machine by itself, I would call that a weak design until proven otherwise.
For a deeper design discussion, the internal guide on manual reset, auto reset, and auto-restart design for safety light curtains is the page I would connect to from this topic because it separates reset mode, restart interlock, OSSD behavior, and auto-restart risk.
The Evidence: Injuries, Legal Files, and the Ugly Details
The U.S. Bureau of Labor Statistics reported 5,070 fatal work injuries in 2024, with one worker dying every 104 minutes from a work-related injury. The fatal injury rate was 3.3 per 100,000 full-time equivalent workers, down from 3.5 in 2023, according to BLS fatal work injury data.
That is not a light-curtain-only statistic. But it is the background noise behind every guarding decision.
BLS also reported 2.5 million nonfatal workplace injuries and illnesses in private industry in 2024, with a total recordable case rate of 2.3 cases per 100 full-time equivalent workers, according to its 2024 nonfatal injury and illness report.
Now bring it closer to presses. NIOSH’s Current Intelligence Bulletin 49 on mechanical power presses concluded that press work carried a high injury and amputation burden despite OSHA standards; the CDC/NIOSH summary is available at CIB 49: Injuries and Amputations Resulting from Work with Mechanical Power Presses. The historical dataset is old. The physics are not.
The OSHRC Matsu Ohio decision is worth reading because it shows investigators care about more than catalog claims. The record discussed multiple presses, light curtains, e-stops, safety relays, clutch energy, and whether an employee standing in or penetrating the light curtain could be exposed to unexpected startup. In the Matsu Ohio OSHRC decision, the details around light curtain penetration, safety relay behavior, and reset impossibility are exactly the kind of ugly, practical facts that matter after an incident.
And then there is the Gestamp West Virginia variance file. OSHA’s 2021 grant of permanent variance for a TRUMPF TruLaser 8030 cell described a multi-step restart process after access, including visual inspection, pressing a reset switch, closing a door within 3–4 seconds, turning a key switch on the HMI, and enabling continuous operation. Read the Gestamp West Virginia OSHA variance. That file is a blunt reminder: when whole-body access exists, serious employers do not wave at reset logic. They document it.
Manual Reset vs Auto Reset vs Auto-Restart
| Reset / Restart Design | What It Does | Where It May Fit | Main Failure Mode | My View |
|---|---|---|---|---|
| Manual reset light curtain | Requires deliberate reset after the sensing field clears | Whole-body access, robot cells, presses, palletizers, walk-in zones | Reset button mounted blind or reachable from inside | Best default for personnel access |
| Auto reset light curtain | Returns safety output to ready when beams clear | Constrained openings where body entry is not credible | Assumes no one remains in the hazard zone | Useful, but abused |
| Auto-restart | Allows hazardous motion to resume automatically after conditions clear | Rare engineered cases with validated no-person exposure | Unexpected motion after access | Treat as guilty until proven safe |
| Restart interlock | Prevents restart until reset/start sequence is completed | Any pass-through hazard | Reset confused with start | Non-negotiable for whole-body access |
| Supplemental presence sensing | Detects people inside the zone after entry | Large cells, multi-sided access, blind zones | Dead zones and poor validation | Often needed with light curtains |
Where Manual Reset Is Required in Real Machines
Manual reset is required when clearing the light curtain does not prove the danger zone is empty.
That includes:
Robot Cells
If a person can walk past the light curtain and stand behind the robot base, fixture, weld table, guarding post, or material rack, use manual reset with restart interlock. Add internal presence detection if visibility is poor. A reset button outside the cell is not enough if the operator cannot see the whole zone.
Hydraulic Presses and Stamping Equipment
Presses are unforgiving. A safety light curtain reset function around a press should be tied to stop time, safety distance, OSSD response, clutch/brake behavior, valve state, EDM feedback, and whether the person can remain inside the protected area.
For rougher, wider, higher-energy machines, review heavy-machine light curtain options instead of forcing a small general-purpose model into a job it was not meant to defend.
Palletizers, Conveyors, and Packaging Lines
A conveyor with a walk-in opening is not a small part chute. If someone can step through the light curtain to clear a jam, pick up fallen cartons, adjust a sensor, or clean a roller, manual reset belongs in the design.
Multi-Sided Machine Access
The more sides you protect, the more chances you create for blind spots, confusing reset stations, and mistaken assumptions. A single light curtain may not cover the real access pattern. Use multi-sided access protection light curtains where the geometry supports it, and read the practical guide on protecting multi-sided machine access without overbuilding before adding hardware just to feel better.
Maintenance and Cleaning Tasks
Cleaning is where safety shortcuts like to hide. A line is down, scrap is piling up, and somebody says, “Just reset it.” No. If cleaning requires whole-body access, the reset sequence must assume a real human may still be inside.
The Reset Button Location Is a Legal Exhibit Waiting to Happen
A reset button is a witness.
It tells investigators where the operator stood. It tells them what the operator could see. It tells them whether the design expected verification or blind faith. It tells them whether reset and restart were separated.
The reset device should usually be:
- Outside the hazardous area
- Not reachable from inside the hazard zone
- Positioned with clear view of the protected area
- Protected against accidental operation
- Separate from the machine start control
- Connected through a safety relay or safety PLC as part of a validated safety function
- Tested during commissioning and periodic inspection
This is where ISO 13849-1, IEC 61496, PL d, PL e, Category 3, Category 4, SIL 2, SIL 3, OSSD, EDM, STO, and stop-time measurement stop being acronym soup. They become evidence.
If your engineering team is debating start, stop, reset, and safety-related control behavior, connect the discussion to what ISO 13849 really requires for start, stop, and reset functions.
The Design Pattern I Would Defend
Here is the pattern I trust for whole-body access light curtain applications:
Use the safety light curtain for detection. Send dual OSSD outputs into a safety relay or safety PLC. Use EDM feedback where contactors, relays, or valves control hazardous energy. Place the reset button outside the hazard zone with full viewing ability. Require manual reset after beam interruption. Require a separate start command after reset. Use internal presence sensing, interlocked gates, or safety scanners when the operator can hide from the light curtain after entry.
Simple works.
But simple must be validated.
I would also log status in the HMI, but I would not let standard PLC logic become the authority for personnel safety. A status bit is not a safety function. A dashboard is not a reset station. And an Ethernet packet does not prove a person is not inside a robot cell.
If your project includes remote reset, HMI reset, Ethernet diagnostics, safety PLC tags, or networked safety device monitoring, the page on Ethernet reset and PLC status monitoring limits should be required reading.
FAQs
When is manual reset required after whole-body access through a light curtain?
Manual reset is required after whole-body access through a light curtain when a person can pass beyond the sensing field, remain inside the hazardous area, and be hidden from the light curtain after the beams clear, because automatic reset could allow unexpected restart while that person is still exposed.
In practical terms, this applies to robot cells, large presses, palletizers, conveyors, packaging lines, and multi-sided machine openings where the light curtain is an entry boundary rather than continuous presence detection inside the danger zone.
What is a light curtain restart interlock?
A light curtain restart interlock is a safety control function that prevents hazardous motion from resuming automatically after the sensing field has been interrupted, requiring a deliberate reset and a separate start command before the machine can run again, usually from a verified viewing position.
The point is to separate “the field is clear” from “the machine may move.” That separation matters most when whole-body access or pass-through hazards exist.
Where should the safety light curtain reset button be installed?
A safety light curtain reset button should be located outside the hazardous area, positioned so the operator can verify the danger zone is empty, and designed so it cannot be reached from inside the protected space or used as a direct start command.
If the reset station cannot see the full hazard zone, the design needs additional viewing, additional reset stations, internal presence detection, or a different guarding architecture.
Is auto reset allowed on a safety light curtain?
Auto reset can be acceptable for a safety light curtain when the risk assessment proves that a person cannot pass through, remain undetected, become trapped, or face unexpected hazardous motion after the beams clear and the OSSD outputs return to ready.
I would consider auto reset only for constrained openings, material transfer points, or process interruptions where whole-body access is not credible. It is not the default for walk-in access.
What does whole-body access mean in light curtain guarding?
Whole-body access means the guarded opening is large enough, or the layout is permissive enough, for a person to step through the light curtain plane and occupy the hazardous area without continuously interrupting the sensing field, unlike simple hand insertion at a constrained point-of-operation.
This is the pass-through hazard that makes manual reset and restart interlock necessary. If the light curtain cannot prove the person has left, the control system must not assume it.
How do you reset a safety light curtain correctly?
You reset a safety light curtain correctly by clearing the sensing field, verifying the hazardous area is empty, correcting any fault condition, pressing the approved reset device, and then using a separate start command only after the safety relay or safety controller has accepted the reset.
Do not train operators to treat reset as a production shortcut. Train them to treat it as a safety verification step.
Your Next Steps: Stop Letting Reset Logic Be a Panel Detail
If your machine allows whole-body access through a light curtain, do not approve automatic reset until the risk assessment proves a person cannot remain inside the hazard zone. Check the access geometry, stopping time, safety distance, OSSD wiring, EDM feedback, reset button location, restart interlock, and validation record.
Send your machine layout, access opening size, stop-time data, required protection height, and control architecture to a safety light curtain supplier that will discuss the reset function before quoting hardware. If the answer starts and ends with beam spacing, keep asking harder questions.