APPLICATION CASE · PRESS SAFEGUARDING · 2026-06-14 · ~12-min read

Safeguarding a Mechanical Press: Why Two-Hand Control Alone Is Not Enough, and How a Type 4 Light Curtain Closes the Gap

A representative power-press point-of-operation cell. The operator runs the stroke from a Type IIIC two-hand control; that protects the operator's hands — and nobody else's. The instant a loader or inspector can reach the dies, the standards demand a presence-sensing device. Here is how the two-hand control, a Type 4 light curtain and a safety relay are made to work as one system.

A DAIDISIKE DQC Type 4 light curtain on the open access plane of a press — it trips a stop before a reach-in hand can get to the point of operation.

The representative cell: one operator, one open side, two different hazards

This is a representative power-press cell — not a named customer or plant, just the layout we see again and again. An operator stands at a fixed station and runs the press from a Type IIIC two-hand control: two guarded buttons that must be pressed concurrently and synchronously to initiate the slide. That arrangement is genuinely effective for the operator: with both hands committed to the buttons, neither hand can be in the point of operation during the hazardous stroke.

The problem is the other side. The point of operation also faces an open loading and inspection plane where a second worker — a loader, a helper, a QC inspector — can approach and reach toward the dies. The two-hand control on the operator's station does nothing to stop a stroke the operator legitimately initiates while that second person's hand is in the die area. That open access plane is exactly what a vertical Type 4 light curtain is for: break the field and the press stops, before the hazard is reachable.

Layout diagram of a representative press cell: operator two-hand control station, vertical Type 4 light curtain on the open access plane, both feeding a safety relay that controls the press stop — The cell layout: the operator's two-hand control path and the bystander light-curtain path both terminate in one safety relay that commands the press stop — one common safe state, two protective devices.

Why is two-hand control alone insufficient when others are nearby?

This is the load-bearing point of the whole case, and it comes straight from the standard. ISO 13851 explicitly states a two-hand control device only offers protection for the person using it. A two-hand control forces both of the operator's hands onto actuators away from the danger zone, so neither of their hands can be in the point of operation during the stroke. That is the entire scope of its protection. It has no way to know whether anyone else is near the dies.

So the classic, dangerous error is to treat “we have two-hand control” as “the press is safeguarded.” It protects the button-pusher and nobody else. The moment your risk assessment identifies that a loader, helper or inspector can reach the point of operation, a presence-sensing device is mandatory, not optional. The light curtain is what extends protection to the person the two-hand control structurally cannot see. (OSHA's machine-guarding guidance makes the same point from the other direction: where more than one operator is needed, each must have a separate two-hand control and all must be actuated concurrently — two-hand control is per-person by design.)

Field note — Engineer Cai: I ask one question on every press: “Who else can reach the die while the operator hits the buttons?” If the honest answer is “the loader” or “the inspector,” then two-hand control is doing its job and still leaving a person unprotected. The curtain is not a nice-to-have on that cell — it is the device covering the people the buttons were never able to protect.

The standards, and what each one actually governs

It helps to keep the lanes straight, because each standard answers a different question:

ISO 13851 (two-hand control devices) — defines the THCD types. Type IIIC is the highest: both hands (simultaneous), synchronous actuation within a ≤ 0.5 s simultaneity tolerance, self-monitoring, realized to Category 4 / PLe. And the clause that drives this case: it protects only the user.
IEC 61496 (ESPE) — the light curtain's product standard. Type 4 uses redundancy with continuous self-monitoring, suitable up to SIL 3 / PLe; Type 2 is capped at about PLc and is not appropriate for a press point-of- operation hazard.
ISO 13855 (positioning / safety distance) — where to put the curtain: S = K·T + C, including reach-over / reach- under height tables.
ISO 13849-1 (SRP/CS) — the control-system integrity: Performance Levels a–e and Categories. A press stop is typically PLd or PLe, realized with Category 3 or 4 (dual-channel, single-fault tolerant, fault detected before the next demand).
IEC 62046 (application of presence-sensing) — selection, positioning, configuration, commissioning, and the muting and blanking rules (2018 edition updated them).
ISO 3691-4 (driverless trucks / AGVs) — relevant only if the cell is AGV-served, where a horizontal safety laser scanner becomes the appropriate device (with IEC 61496-3 for the scanner).

One honesty note that matters for your declaration: IEC 61496 is not treated as giving an automatic presumption of conformity, so a type examination from an independent body is expected — do not assume the standard alone carries you.

How do you calculate where the light curtain goes? S = K·T + C

ISO 13855 gives the minimum distance between the detection field and the hazard as:

S = K · T + C

K = approach speed. 2000 mm/s for hand/arm detection; 1600 mm/s is permitted only in defined whole-body cases where S ≥ 500 mm. For a hand reaching in at a die, use 2000 mm/s — using 1600 here is a classic under-sizing mistake.
T = total stopping time. This is press stopping time + curtain response time + relay response time added together — not just the curtain. A device mounted too close to the hazard leaves residual danger precisely because someone forgot the press's own run-down in T.
C = intrusion distance from detection capability d. For a normal-approach vertical curtain, C = 8·(d − 14) mm (never below 0).

The biometric detection data behind d (per EN ISO 13855): finger = 14 mm, hand = 30 mm, leg up to 70 mm, body over 70 mm. So a 14 mm finger curtain gives C = 0 mm, while a 30 mm hand curtain adds C = 128 mm. Pick the resolution to match how the body part actually reaches in, then carry its real C into S.

We are deliberately not publishing a single computed S for this fictional press — T depends on the real machine's measured stopping time. Use the formula and your own measured numbers. For a full worked method, see our ISO 13855 safety-distance guide.

What does the resolution table look like? (the C term, side by side)

Detection capability d	Body part (EN ISO 13855)	C = 8·(d − 14) mm	Typical use
14 mm	Finger	0 mm	Fine point-of-operation reach-in; mounts closest
30 mm	Hand	128 mm	Hand reach-in; must mount further back than a finger curtain
> 40 mm	Leg / body	Larger C; whole-body / access detection, not fine reach-in	Perimeter / whole-body access guarding

Get the resolution wrong — choose a coarse curtain and under-count C — and the device ends up mounted closer than it is allowed to be. That is one of the most common findings on a press audit.

The other three ways a hand gets in: reach-over, reach-under, reach-around

A vertical curtain only guards the plane it covers. ISO 13855 height tables and good installation practice close the rest:

Reach-over — if the top beam is too low, a person reaches over the field. Set the top beam high enough per the ISO 13855 height tables.
Reach-under — if the bottom beam is too high off the floor, a person ducks or reaches under. Keep the bottom beam no more than about 300 mm (12 in) off the floor unless the gap is hard-guarded.
Reach-around — side gaps let a person reach around the field. Close the remaining sides with hard guards.

Each of these defeats the curtain while it still reports “guarded.” They are installation failures, not device failures — and they are entirely on the integrator to get right.

The convergence point: a DA31 safety relay with EDM

Both protective paths — the Type IIIC two-hand control output and the curtain's OSSD pair — have to command the same press stop, through a control-reliable (Category 3/4) architecture. They converge on a safety relay whose two normally-open safety contacts are wired into the press stop/clutch-brake control. Per DAIDISIKE, the DA31 takes the curtain OSSD, switches the power/stop circuit through a pair of safety contacts, and performs the EDM feedback check.

Why EDM is non-negotiable: External Device Monitoring reads back the state of the final switching elements (the contactors) and refuses to allow the next stroke if one is welded or stuck. Wire a curtain or two-hand output to a relay without EDM and a welded contactor goes undetected — the next demand fails dangerously and your realized PL drops below the required level. The DA31 is the single place where the operator's path and the bystander curtain path are combined and where single-fault detection lives.

DAIDISIKE DA31 safety relay module, the convergence point for the two-hand control and light-curtain OSSD with EDM feedback — The DA31 safety relay: both the two-hand path and the curtain OSSD feed it; its dual safety contacts drop the press stop, with EDM catching a welded contactor before the next stroke.

Signal flow — the Type IIIC two-hand control (synchronous, ≤ 0.5 s) and the Type 4 curtain OSSD both reach the safety relay; a broken curtain field must prevent a stroke regardless of the buttons.

Where DAIDISIKE products fit this cell

Framed vendor-neutrally, by role — no invented specs:

DQC series — the finger/hand-guard Type 4 line for the point-of-operation access plane. Use a finger- or hand-resolution model so the C term and approach assumptions match a person reaching in.
DQS / DQSA / DQV — the perimeter-guarding / high-protective-height line. Use these for whole-body access guarding of the surrounding zone, not for fine point-of-operation reach-in.
DA31 safety relay — the safety control core that takes the curtain OSSD, switches the stop circuit through a pair of safety contacts, and performs EDM feedback checking. The convergence point for both protective paths.
DLD-series LiDAR scanner — only if the cell is AGV/AMR-served or needs horizontal floor-zone presence detection (approach to the press footprint). It is an obstacle-avoidance / navigation / perimeter sensor and is not a substitute for the vertical point-of-operation light curtain — that vertical reach-in plane stays the curtain's job.

The close-to-run sequence and the failure modes it tolerates

Functionally the system behaves like this:

Operator presses both buttons concurrently and synchronously (≤ 0.5 s tolerance) → two-hand path permits a stroke.
The curtain field over the open access plane must be clear → OSSD high. A broken field prevents a stroke regardless of the buttons.
Both conditions reach the DA31; its dual safety contacts allow the press cycle.
Anyone breaks the curtain → OSSD drops → relay drops the press stop within the total stop time S accounted for.
EDM + a deliberate manual reset are required before the press can run again — clearing the field alone does not restart it.

The design must tolerate or detect: a welded output contactor (caught by EDM before the next stroke); a single channel fault on either path (Cat 3/4 redundancy detects it before the next demand); curtain optical fault or loss of alignment; short-circuit between OSSD lines (Type 4 cross-monitored wiring); blanking/muting misuse; and the human failure mode of a second person entering during a stroke the operator initiated — which is exactly the gap the curtain closes.

Common mistakes engineers make on this exact cell

Assuming two-hand control = full safeguarding. It protects only the button-pusher; a presence-sensing device is mandatory once others can reach in.
Mounting the curtain too close — forgetting that T in S = K·T + C includes the press's own stopping time, not just the curtain response.
Wrong K or wrong C / wrong resolution — using 1600 mm/s when a hand reaches in (must be 2000), or a coarse curtain with an under-counted C.
Reach-over / reach-under / reach-around defeats — top beam too low, bottom beam > 300 mm off the floor, or open sides.
No EDM / no monitored final elements — a welded contactor goes undetected and drops the realized PL.
Muting/blanking abuse — muting the curtain to feed material instead of designing it per IEC 62046 re-opens the bystander hazard.
Using a Type 2 curtain, or a laser scanner where a vertical curtain is required — Type 2 caps at ~PLc; a horizontal LiDAR zone does not protect a vertical reach-in plane.
Two independent stops with no common safe state — both paths must command the same Cat 3/4 press stop; a stroke must be impossible while the curtain field is broken.

How does this compare to other brands' kit?

The device classes here are standard across the industry: a Type 4 curtain like the SICK or Banner deTec / EZ-Screen families, two-hand control modules from Banner or IDEC, and safety relays such as the Pilz PNOZ or Allen-Bradley (Guardmaster) 440R/MSR families all do the same jobs to the same IEC 61496 / ISO 13851 / ISO 13849-1 device classes. DAIDISIKE's DQC (Type 4 curtain) and DA31 (PLe / SIL 3 safety relay with EDM) are built to the same standards, factory-direct, MOQ 1 set. If you are cross-shopping the relay, see our Pilz PNOZ alternatives and Allen-Bradley Guardmaster alternatives write-ups. Confirm holding/utilisation ratings, response times and PL against your risk assessment before swapping any brand — the standards govern, not the logo.

DAIDISIKE production facility manufacturing safety light curtains and safety relays in Foshan, China — DAIDISIKE manufactures the DQC light curtains and DA31 safety relays in this case factory-direct from Foshan, China.

The takeaway

On a press where the operator runs the stroke from a Type IIIC two-hand control and a second worker can reach the dies, the two-hand control is doing its job and still leaving a person unprotected — because ISO 13851 protects only its user. Add a Type 4 light curtain on the open access plane, positioned with a real S = K·T + C using the machine's measured stop time and the correct resolution; close reach-over / reach-under / reach-around; and converge both protective paths on a DA31 safety relay with EDM and manual reset to a Cat 3/4 press stop. Two complementary devices, one common safe state — that is point-of-operation safeguarding done honestly.

For Engineers

The gap: Two-hand control (ISO 13851) protects ONLY its user — not a bystander reaching the die
The fix: Add a Type 4 light curtain (IEC 61496) on the open access plane
Operator device: Type IIIC two-hand control: both hands, synchronous ≤ 0.5 s, self-monitoring, Cat 4 / PLe
Curtain position: ISO 13855: S = K·T + C; C = 8·(d − 14) mm (14 mm → 0 mm, 30 mm → 128 mm)
Approach speed K: 2000 mm/s hand/arm; 1600 mm/s only whole-body with S ≥ 500 mm
Total stop time T: press stop time + curtain response + relay response — NOT just the curtain
Curtain class: Type 4 (up to SIL 3 / PLe); Type 2 (~PLc) is NOT acceptable for a press
Convergence: DA31 safety relay: dual safety contacts + EDM, Cat 3/4 press stop, manual reset
Laser scanner: DLD LiDAR = horizontal/AGV zones (ISO 3691-4) — NOT a substitute for the vertical curtain

AuthorEngineer Cai

Reviewed bySenior Application Engineer · IEC 61496 / ISO 13855 / ISO 10218 specialty

Last verified2026-06-14

Foshan DAIDISIKE Optoelectronics Technology Co., Ltd.

Frequently asked questions

Why is two-hand control alone not enough to safeguard a mechanical press?

Because a two-hand control device protects only the person operating it. ISO 13851 states this plainly: forcing both of the operator's hands onto control buttons away from the danger zone keeps the OPERATOR'S hands out of the point of operation, but it does nothing for a second person — a loader, helper or QC inspector — who can reach the dies while the operator initiates a stroke. The moment another worker can approach the point of operation, the risk assessment requires a presence-sensing safeguard (a Type 4 light curtain) in addition to the two-hand control, not instead of it.

What is a Type IIIC two-hand control device under ISO 13851?

Type IIIC is the highest functional type of two-hand control device in ISO 13851. It requires use of both hands (simultaneous actuation), synchronous actuation with a simultaneity tolerance of no more than 0.5 s between the two buttons (releasing one button resets the requirement), self-monitoring, and realization to Category 4 / PLe per ISO 13849-1. The synchronous-actuation rule is what defeats the classic cheat of taping one button down: if the second button is pressed more than 0.5 s later, the stroke is not initiated.

How do you calculate where to mount the light curtain on a press (ISO 13855)?

ISO 13855 gives the minimum safety distance S = K·T + C. K is the approach speed: 2000 mm/s for hand/arm detection, or 1600 mm/s in defined whole-body cases where S is at least 500 mm. T is the TOTAL stopping time — the press stopping time plus the light-curtain response time plus the safety-relay response time, all added together. C is the intrusion distance from the curtain's detection capability d: for a normal-approach vertical curtain, C = 8·(d − 14) mm, never less than 0. A 14 mm finger-resolution curtain gives C = 0 mm; a 30 mm hand-resolution curtain adds C = 128 mm. Use the real measured press stop time for your machine — never a fabricated number.

What resolution light curtain do I need, and how does it change the safety distance?

For a person reaching IN at the point of operation, use a finger- or hand-resolution Type 4 curtain so the approach speed and the C term match the hazard. The biometric detection data in EN ISO 13855 is finger = 14 mm, hand = 30 mm, leg up to 70 mm, body over 70 mm. Because C = 8·(d − 14), a 14 mm finger curtain adds 0 mm of intrusion distance while a 30 mm hand curtain adds 128 mm — so a coarser curtain must be mounted further back. Picking a coarse resolution and then under-counting C is one of the most common reasons a curtain ends up illegally close to the dies.

Why a Type 4 light curtain and not a Type 2 one for a press?

IEC 61496 classifies electro-sensitive protective equipment (ESPE) by type. A Type 4 device uses redundant architecture with continuous self-monitoring and high diagnostic coverage and is suitable up to SIL 3 / PLe. A Type 2 device is limited to about PLc, which is below the PLd/PLe typically required at a press point of operation. So for a power-press reach-in hazard, a Type 4 curtain is the correct class and a Type 2 is not acceptable. (Note that IEC 61496 is not treated as giving an automatic presumption of conformity, so a type examination from an independent body is expected for the declaration.)

What does the safety relay do, and why is EDM essential?

The safety relay is the convergence point: it takes the two-hand control output and the light-curtain OSSD pair, and switches the press stop/clutch-brake circuit through two normally-open safety contacts. External Device Monitoring (EDM) feeds back the state of the final switching elements (the contactors) so that a welded or stuck output contact is detected BEFORE the next stroke is allowed. Without EDM, a welded contactor goes undetected and the next demand fails dangerously, dropping the realized Performance Level below what ISO 13849-1 requires. A DAIDISIKE DA31 takes the curtain OSSD, switches the stop circuit through a pair of safety contacts, and performs that EDM feedback check.

Can a safety laser scanner (LiDAR) replace the vertical light curtain on the press?

No — not for the vertical point-of-operation reach-in plane. A horizontal laser-scanner floor zone protects an area of floor (approach to the machine footprint) and is the right device when the press cell is fed or served by an AGV/AMR, governed by ISO 3691-4 plus IEC 61496-3 for the scanner. It does not protect a person reaching IN through a vertical access plane at the dies; that is the vertical Type 4 light curtain's job. Treat the scanner as complementary horizontal-zone protection, never as a substitute for the vertical curtain.

Is muting or blanking the press curtain to feed material safe?

Only when it is designed strictly per IEC 62046:2018, with correct muting-sensor geometry and timing, or fixed/floating blanking used only where it is justified by the risk assessment. Muting the point-of-operation curtain casually to push material through re-opens the exact bystander hazard the curtain was added to close — it lets a body part through while the curtain still reports 'guarded'. IEC 62046 governs the selection, positioning, configuration and commissioning of presence-sensing equipment and contains the muting and blanking requirements; its 2018 edition updated those rules. If you cannot satisfy IEC 62046, do not mute.

Safeguarding a press point of operation? Foshan DAIDISIKE Optoelectronics Technology Co., Ltd. ships the DQC Type 4 light curtain and the DA31 safety relay factory-direct — CE self-declared, IEC 61496, ISO 9001, TUV third-party testing available per order, MOQ 1 set. Send us your press stopping time, access geometry and resolution requirement and our engineering team will help you size S = K·T + C for your machine.

Contact DAIDISIKE — +86 15218909599

This is a representative engineering application scenario, not a named customer, plant or project. Brand names (SICK, Banner, Pilz, Allen-Bradley, Guardmaster, IDEC) are the trademarks of their respective owners and are used here only for nominative comparison. This article is general engineering guidance, not a substitute for a competent machine-safety risk assessment. Confirm the ESPE type, resolution, approach speed, measured stopping time and the ISO 13855 distance for your specific press before installation.