Two-Hand Control (ISO 13851) vs Light Curtains on Presses — Which Safeguard Do You Need?

On a press, “safeguarded” is not a yes/no answer — it depends on who can reach the danger zone and how. The two-hand control and the light curtain are both legitimate point-of-operation safeguards, both have their own product standard, and both turn up on the same shortlist constantly. They are not interchangeable. One controls the operator's hands; the other detects a body. Confuse the two and you can end up with a press that is fully compliant for the operator and wide open to the person standing next to them.

This comparison explains the different protection limits. Consult themachine safety standards index for device and positioning references, and the press photoelectric protector range after the permitted safeguarding method and requirements are defined.

What is a two-hand control device under ISO 13851:2019?

A two-hand control device makes the operator hold down two separate actuators, positioned so both hands are demonstrably outside the danger zone, for the whole hazardous closing movement. Release either actuator and the control output terminates within its specified response; the machine then needs its actual stopping time, not an instantaneous stop. The governing standard is ISO 13851:2019, Safety of machinery — Two-hand control devices — Principles for design and selection, which superseded EN 574 and the 2002 edition. Two-hand controls on power-driven metalworking presses are required to comply with EN ISO 13851 (DIN EN ISO 13851:2019).

ISO 13851 sorts these devices into Type I, Type II and Type III, with Type III split into IIIA, IIIB and IIIC. The classes are ranked by the safety-related requirements each must meet: use of both hands, the input/output signal relationship, termination of the output signal, prevention of inadvertent operation and bypass, re-generation of the output signal, and synchronous operation. The higher you go, the more of those are mandatory and the better the fault tolerance has to be.

*Per ISO 13851, re-generation of the output signal is required for Type I only conditionally, following a careful risk assessment. The A / B / C sub-types of Type III differ in their fault-tolerance and diagnostic requirements, not in the row above.

Synchronous operation is the row that does most of the real anti-defeat work. For every Type III sub-type, both actuators must be operated within a time lag of no more than 0.5 s. Miss that window and the output is not generated; the operator has to release both actuators and start over before the press will run again. That is what stops someone taping one button down and slapping the other with a free hand. On the functional-safety side, verify exact device data and the complete control-system design. Type IIIC does not by itself certify the press to PL e or SIL 3 or establish its diagnostic coverage.

What is a presence-sensing light curtain, and how is it different?

A light curtain is a different animal: it doesn't care where the operator's hands are, it watches a region of space. It projects a plane of infrared beams across the point of operation; break a beam and the safety outputs drop and stop the stroke. Light curtains are electro-sensitive protective equipment (ESPE) standardised by the IEC 61496 series — Part 1 for general requirements, Part 2 for active opto-electronic protective devices (AOPDs). They are classified Type 2 or Type 4: Type 2 uses periodic self-testing and suits applications up to SIL 1 / PL c with model evidence; Type 4 provides self-monitoring with high diagnostic coverage and a redundant architecture for the highest-risk work, up to SIL 3 / PL e. For a power press, the applicable machine rules and risk assessment decide whether ESPE is permitted and what performance is required.

The defining difference is right there in the name. A light curtain is presence-sensing. It stops or prevents the stroke whenever any person breaks the field — operator, loader, a maintenance tech who leaned in. It covers reach-through and access by additional personnel, and because the operator no longer has to keep both hands pinned to two buttons, it frees their hands to load and unload, which usually means faster cycles. Its recognised weakness against a fixed guard is that it provides no impact or ejection protection: it detects a body, it doesn't catch a slug flying out of the die.

How do the safety distances compare (S = K × T + C)?

Both devices have to be positioned far enough from the hazard that the press has stopped before a hand reaches it, and both use the same shape of formula — S = (K × T) + C — but with different constants, because they are guarding against different motions.

• Two-hand control: the approach-speed constant K = 1600 mm/s (a gripping/approach reach), T is the total system stopping time, and the extra distance C must be greater than 250 mm for a device without masking — with masking, C can be zero. These figures come from the ISSA/BGHM Two-hand controls on presses guidance (2020).
• Light curtain (vertical / normal approach): per ISO 13855, K = 2000 mm/s where the calculated S works out below 500 mm, with C = 8 × (d − 14) mm, where d is the detection capability (resolution). Finger detection uses d ≤ 14 mm; hand detection is around 30 mm.

A worked light-curtain example makes the second one concrete: K = 2000 mm/s, a stop time T of 0.05 s (50 ms), resolution d = 14 mm gives C = 8 × (14 − 14) = 0, so S = 2000 × 0.05 + 0 = 100 mm as arithmetic only, not an installation approval. T must include the complete response and stopping time; geometry, minimums and additional reach requirements can demand more distance. Coarsen the resolution and the 8(d − 14) penetration term adds back in, pushing the curtain further from the die. If you want to run your own numbers, our ISO 13855 light-curtain positioning guide walks through the mounting and distance practice.

Does a two-hand control protect a second person at the press?

No — and this is the single fact that decides most of these cases. A two-hand control forces the operator to keep both hands outside the danger zone during the closing movement. That is all it does. It does not detect, and cannot protect, a second person — a loader, a helper, someone reaching in from the side — who puts a hand into the point of operation while the operator runs the press. Presses with automatic upward movement, and any press where a bystander can be exposed, therefore need additional protection such as an ESPE (a light curtain) or an inherently safe, closed-tool design.

A light curtain inverts that limitation. Because it senses presence rather than hand position, it stops the stroke for whoever breaks the plane. That is precisely why OSHA, in 29 CFR 1910.217, notes that presence-sensing devices protect not only the operator but other employees in the area, while a two-hand control is credited with protecting the operator who uses it. If your press has more than one person near the die, a two-hand control alone leaves a real gap.

What do OSHA 1910.217 and the ISO 16092 press series say?

Both safeguards are recognised by the regulators — the standards do not pick a winner, they pin requirements onto each. In the US, mechanical-press point-of-operation safeguarding sits under OSHA 29 CFR 1910.217, which accepts both two-hand controls and presence-sensing devices within their permitted scopes. Paragraph (c)(5) invokes control reliability and brake monitoring for the specified part-revolution press operations where the operator hand-feeds or removes parts at the point of operation using the listed safeguards. Paragraph (b)(14) defines that brake monitor; PSDI also has its own requirements. Check the actual operating mode instead of treating every press application as identical. Our OSHA 1910.217 light-curtain buyer's guide covers that US picture in full.

In Europe, press safety is given by the ISO 16092 series — ISO 16092-1:2017 (general safety requirements), ISO 16092-2:2019 (mechanical presses), ISO 16092-3 (hydraulic) and ISO 16092-4 (pneumatic). ISO 16092-1 (clause 5.4.5.3) even addresses indication of emergency-stop active/inactive status for two-hand control stations. The product standards — ISO 13851 for the two-hand device, IEC 61496 for the ESPE — sit underneath the press-machine standard, and the positioning of either is governed by ISO 13855. None of these forces a single choice; they require that whatever you choose matches the risk.

So which one do I actually need — and when do I use both?

Start with machine stopping capability, exposed people and access routes, and the protective methods permitted by the applicable machine standard. Then compare two-hand control, ESPE and enclosing or locking guards as parts of the complete safeguarding plan.

Combining devices requires a validated control sequence. Two-hand actuation must not be treated as mere initiation where maintained actuation is required, and a clear optical field does not prove the area behind it is empty. For product selection, evaluate the DQS press photoelectric safety guard and the finger/hand-resolution DQC hand-guard curtain are candidates to assess against required sensing and output specifications. Product names do not establish console compatibility or the achieved press performance.

If you are retrofitting an older mechanical press rather than speccing a new line, the sequencing of all this — stop-time measurement, brake monitor, device choice, mounting distance — matters as much as the device. Our stamping-press retrofit guide walks through that order of operations. Whatever you land on, the rule underneath does not move: the safeguard has to match who can reach the danger zone, not just the person holding the buttons.