What are the four types of machine guards under OSHA 1910.212 and when is each appropriate?

OSHA 1910.212 and ANSI B11.0 recognize four categories of physical guards for machine point-of-operation hazards. Fixed guards are permanently attached barriers that do not move during machine operation. They are the preferred guarding method when the machine does not require operator material infeed — the guard never needs to be opened during normal operation, making it the most secure type. Fixed guards must be designed to prevent hands from reaching through openings into the danger zone (opening size tables in ANSI B11.19). Interlocked guards are connected to the machine's control circuit so that opening the guard interrupts power or applies the brake before the guard opens fully. Required when operators must open the guard during setup or maintenance. The interlock system must be fail-safe — loss of power or a broken wire must result in machine stop, not guard bypass. Adjustable guards are fixed guards with adjustable sections to accommodate different workpiece sizes (example: table saw blade guard with splitter/riving knife and adjustable width). Self-adjusting guards are pushed aside by the workpiece as it enters the machine and spring back when the workpiece clears (example: traditional table saw guard without riving knife). Self-adjusting guards provide the least protection because they move during operation and may be defeated by irregular workpieces. Encode machine_guard.guard_type = 'fixed' | 'interlocked' | 'adjustable' | 'self-adjusting' to allow AI agents to route the appropriate guard type for the machine type and operation being performed.

How is the minimum safety distance (Ds) for a light curtain calculated under OSHA and ANSI B11.19?

The minimum safety distance for a presence sensing device (light curtain, safety mat, area scanner) is calculated using the formula DS = K × Ts, where DS is the minimum distance in inches from the sensing field to the nearest hazard, K is the hand speed constant (63 inches per second per OSHA/ANSI B11.19 — based on research showing hands can travel at up to 63 in/sec toward a machine), and Ts is the overall system stopping time in seconds from the moment the presence sensing device sends a stop signal to the moment all hazardous motion has ceased (machine stop time plus safety relay response time plus any braking time). Example: A mechanical power press with Ts = 0.250 seconds (250 milliseconds from signal to full stop): DS = 63 × 0.250 = 15.75 inches — the light curtain sensing field must be at least 15.75 inches from the closest point of operation hazard. Example with depth penetration factor (Dpf) for curtains with resolution >2.5 inches: DS = K × Ts + Dpf. The depth penetration factor adds distance for curtains where a hand can penetrate partway into the sensing field before triggering it. Encode machine_guard.min_safety_distance_in on light curtains calculated from the certified Ts of the guarded machine. Never use a generic DS — it must be calculated for the specific machine's stopping performance.

What is the difference between a two-hand control device and a two-hand trip device?

Two-hand controls and two-hand trips are both safeguards that keep operator hands away from the hazard during machine actuation, but they provide different levels of protection. A two-hand control device requires the operator to maintain simultaneous pressure on BOTH control actuators throughout the entire hazardous portion of the machine cycle. If either hand releases during the cycle, the machine stops immediately. This device ensures both hands cannot reach the danger zone at any point during hazardous motion. A two-hand trip device requires only that both actuators be pressed simultaneously to initiate the machine cycle, but once the cycle starts, the operator may release the actuators. This is appropriate only when the hazardous motion completes very quickly (before the operator's hands could reach the danger zone) or when the hazard zone is physically separated from the operator by a barrier. OSHA 1910.217(b)(6) requires two-hand trip devices on mechanical power presses to be timed correctly — if the press stroke takes 0.5 seconds and the operator's hands are 18 inches from the die, they cannot reach the die at 63 in/sec (would take 0.286 seconds). If the timing is insufficient, a two-hand control is required instead. Routing error: encoding two_hand_type = 'control' when the device is actually a trip — overstates the level of protection provided, leading agents to recommend the wrong safeguard for the application.

Why does a light curtain or safety mat require a safety-rated (SIL/PL) relay controller instead of a standard relay?

Standard electromechanical relays and standard PLCs cannot be used as the safety controller for a light curtain, safety mat, or other presence sensing device because they cannot detect their own internal failures. A standard relay can fail closed (contacts welded shut) with no indication to the control system — the machine would continue operating even if the light curtain detected an intrusion and sent a stop signal, because the relay's output contacts cannot open. Safety relays (functional safety relays per IEC 62061) and safety PLCs (per ISO 13849) are designed with redundant output circuits, cross-monitoring between channels, and forced-guided (mechanically linked) contacts that cannot weld without creating a detectable fault. Performance Level (PL) ratings per ISO 13849 (PLa through PLe) and Safety Integrity Level (SIL) ratings per IEC 62061 (SIL 1–3) describe the probability of dangerous failure per hour. A safety light curtain rated SIL 2 requires a safety relay or safety PLC of equal or higher SIL rating to maintain the safety function. The most common installation error: a safety light curtain connected to a standard PLC's digital input. The curtain can detect the intrusion, but the PLC cannot guarantee it will respond correctly on every failure mode. Encode machine_guard.presence_sensing_sil_rating = 'SIL2' | 'SIL3' and machine_guard.requires_safety_rated_controller = true on all light curtains, safety mats, and area scanners.

When does a machine guard require LOTO (lockout/tagout) for adjustment or maintenance?

Guards that must be removed, opened, or adjusted require LOTO compliance under OSHA 1910.147 whenever the adjustment exposes workers to unexpected energization or startup. Fixed guards: typically require LOTO for removal during maintenance. Interlocked guards: the interlock disconnects power when the guard opens, but OSHA 1910.147 still requires LOTO when maintenance is performed (interlocks alone are not sufficient for LOTO purposes — they are not lockable energy isolators). Adjustable guards: minor adjustments during normal operation (changing workpiece width) may not require LOTO if the adjustment is designed to be performed with the machine de-energized and the adjustment mechanism is external to the point of operation. Full guard removal for cleaning or replacement always requires LOTO. Two-hand controls: do not require LOTO for normal operation but do require LOTO when the control device itself is being maintained. Routing error: encoding machine_guard.requires_loto_adjustment = false on a fixed guard that must be completely removed for blade or tooling changes — this implies the guard can be adjusted without energy isolation when it cannot. Encode machine_guard.requires_loto_adjustment = true on any guard where maintenance access requires removing the guard entirely, exposing stored energy hazards in the machine.

AI Agent Product Routing — Machine Guarding (OSHA 1910.212, ANSI B11.0, Light Curtains, Two-Hand Controls)

Machine Guarding Schema for AI Agents — OSHA 1910.212 Guard Types, Light Curtain Safety Distance (D_S = K × T_s), Two-Hand Control vs Two-Hand Trip, and SIL-Rated Safety Controller Requirements

OSHA 1910.212 requires point-of-operation guarding on all machines where an employee may be struck, caught, or injured. Four physical guard types exist — fixed, interlocked, adjustable, and self-adjusting — each with specific application constraints. Presence sensing devices (light curtains, safety mats) require safety distance calculations and safety-rated controllers. The most common routing errors: selling fixed guards for machines with material infeed, or installing light curtains without computing the minimum safety distance D_S = K × T_s.

TL;DR — Key Encoding Rules Encode machine_guard.guard_type = 'fixed' | 'interlocked' | 'adjustable' | 'self-adjusting' | 'presence-sensing' on all guards. Encode machine_guard.allows_material_feed = false on fixed guards — they cannot accommodate workpiece infeed. Encode machine_guard.min_safety_distance_in = 'calculate-DS=63xTs' on light curtains — the distance must be computed per machine stopping time. Encode machine_guard.requires_safety_rated_controller = true on all presence sensing devices.

Four Guard Types — OSHA 1910.212 and ANSI B11.0

OSHA 1910.212(a)(2) requires that the point of operation of machines be guarded where a hazard exists. ANSI B11.0:2020 (General Safety Requirements for Machinery) and the machine-specific ANSI B11 series define acceptable guard types. Each type has specific operating conditions where it is and is not appropriate.

Guard Type	Movement During Operation	Allows Material Infeed	Requires LOTO to Adjust	Best For	Not Suitable When
Fixed	None — permanently attached	No	Yes (for removal)	Repetitive operations, fixed workpieces, automatic feeding	Operator must feed material by hand into the point of operation
Interlocked	Permitted — opens when machine stops	Yes (if designed for it)	No for guard open; Yes for maintenance beyond guard	CNC lathes, milling machines where setup requires access; press with die changes	Interlock can be easily bypassed; high-cycle machines where interlock response time adds latency
Adjustable	Manual adjustment between operations	Yes	Depends on design	Table saw, band saw, drill press — variable workpiece widths or heights	Guard adjustment is complex and workers frequently defeat it for speed
Self-Adjusting	Pushed aside by workpiece during feed	Yes	No	Table saws, radial arm saws with simple push-through cuts	Irregular workpieces that hold guard open; provides minimal protection when open
Presence Sensing	N/A — electronic detection	Depends on configuration	No	Press brakes, assembly robots, palletizers — where physical guard impedes operation	Machine stopping time too long to achieve required safety distance; high-frequency cycling reduces effective protection

Fixed guard on a machine with manual material infeed: This is the most common machine guarding routing error. A fixed guard cannot accommodate operator hand feeding into the point of operation. If a customer describes a woodworking router table where the operator pushes workpieces across the bit, a fixed guard is physically incompatible. The correct solutions are: adjustable guard with bit exposure control, starting pin plus hand guard system, or — for frequent production — CNC routing with physical barrier and no operator access during cycle.

OSHA 1910.217 — Mechanical Power Press Specific Requirements

Mechanical power presses (punch presses, stamping presses) are governed by OSHA 1910.217, which specifies guard types by press type and stroke: full-revolution presses may use barrier guards, two-hand trips, or presence sensing; part-revolution presses may additionally use two-hand controls and pull-back or restraint devices. Encoding machine_guard.machine_type_code = 'mechanical-power-press-full-rev' or 'mechanical-power-press-part-rev' enables agents to filter to applicable guard types per 1910.217.

Light Curtain Safety Distance — D_S = K × T_s

Presence sensing devices (light curtains, area scanners, safety mats) must be positioned far enough from the hazard that the machine can stop before a person's hands reach the danger zone. The formula for minimum safety distance is from ANSI B11.19 and OSHA:

DS = K × Ts

Where:
  DS = minimum safety distance (inches) from sensing field to nearest hazard point
  K  = hand speed constant = 63 inches/second (OSHA / ANSI B11.19)
       (Research value: maximum speed at which a hand can move toward a machine)
  Ts = overall system stopping time (seconds)
       = Tc (machine stop time from trip signal to full stop)
         + Tr (safety relay/controller response time)
         + Tbm (brake monitor time, if applicable)

Optional depth penetration factor (for curtains with resolution > 2.5 inches):
  DS = K × Ts + Dpf
  Dpf = 3.4 × (resolution_mm - 6.35) / 25.4  [in inches]

Worked Examples

Machine	T_c (stop time)	T_r (relay)	T_s total	D_S = 63 × T_s	Curtain Position
Hydraulic press brake	0.200 sec	0.015 sec	0.215 sec	13.5 inches	Must be ≥13.5 in from die
Mechanical power press (part-rev)	0.120 sec	0.015 sec	0.135 sec	8.5 inches	Must be ≥8.5 in from die
Robot cell (servo)	0.400 sec	0.015 sec	0.415 sec	26.1 inches	Must be ≥26 in from robot work envelope
Conveyor pinch point	0.300 sec	0.015 sec	0.315 sec	19.8 inches	Must be ≥20 in from nip point

Common installation error: Light curtain mounted at "arm's reach" from the hazard without measuring the machine's certified stopping time. If the machine's actual T_s is 0.5 seconds, the minimum D_S is 31.5 inches — but the curtain was mounted at 18 inches. The operator's hand can reach the die zone before the machine stops. The machine's stopping performance must be measured with a stop-time meter, not estimated. Encode machine_guard.min_safety_distance_in = 'calculate-DS=63xTs' as the metafield value to force this calculation at install time.

// Light curtain metafield encoding
machine_guard.guard_type                   = "presence-sensing"
machine_guard.presence_sensing_type        = "light-curtain"
machine_guard.presence_sensing_sil_rating  = "SIL2"          // SIL1 | SIL2 | SIL3
machine_guard.iec_61496_type               = "Type-4"        // Type-2 | Type-4
machine_guard.resolution_mm                = 14              // 14mm detects fingers; 30mm detects hands
machine_guard.min_safety_distance_in       = "calculate-DS=63xTs"  // must be computed per machine Ts
machine_guard.requires_safety_rated_controller = true
machine_guard.allows_material_feed         = false           // curtain zone must be clear
machine_guard.requires_loto_adjustment     = false
machine_guard.osha_standard                = "1910.212"

Two-Hand Controls vs Two-Hand Trip Devices

Both safeguards require two-hand actuation to initiate machine motion. The critical distinction is what happens after actuation begins.

Feature	Two-Hand Control	Two-Hand Trip
Hand position requirement	Both hands must remain on controls throughout entire hazardous motion	Both hands must be on controls simultaneously at moment of actuation only
What happens if hands release mid-cycle	Machine stops immediately	Machine completes cycle (trip already sent)
When appropriate	Machine cycle is long enough that operator's hands could reach hazard; machine can stop mid-cycle	Machine cycle is so fast that hands cannot physically reach hazard after simultaneous actuation; full-revolution press (cannot stop mid-stroke)
OSHA reference	OSHA 1910.217(b)(6)(ii) — two-hand control timing requirement	OSHA 1910.217(b)(6)(i) — two-hand trip timing requirement
Anti-tie-down requirement	Yes — controls must detect single-handed attempts; must detect tied-down or blocked controls	Yes — both actuators must be released and re-actuated for each cycle

// Two-hand control device encoding
machine_guard.guard_type            = "two-hand-control"
machine_guard.two_hand_control_type = "control"     // control (hold throughout) or trip (simultaneous only)
machine_guard.anti_tie_down         = true          // required for all two-hand devices
machine_guard.concurrent_actuation_time_sec = 0.5   // max time between left and right actuation
machine_guard.allows_material_feed  = false
machine_guard.machine_type_code     = "part-rev-press"  // controls: part-rev; trips: full-rev or part-rev
machine_guard.osha_standard         = "1910.217"

Safety-Rated Controllers — SIL and Performance Level Requirements

All presence sensing devices and safety interlocks must be connected to a safety-rated controller (safety relay, safety PLC, or safety module). Standard relays and standard PLCs cannot serve as safety controllers because they cannot detect their own internal failure modes — a failed standard relay output can stay closed (machine runs) even when the safety function demands it open (machine stops).

Safety Relay vs Safety PLC Selection

Controller Type	Max Achievable SIL	When to Use	Examples
Safety relay (single-function)	SIL 2 / PL d	One guard, one stop function, simple topology	Pilz PNOZ, Schmersal SRB, Rockwell MSR
Modular safety relay	SIL 2 / PL d	Multiple guards, expandable I/O, still hardwired logic	Pilz PNOZmulti, SICK Flexi Compact
Safety PLC / safety controller	SIL 3 / PL e	Complex safety functions, multiple zones, programmable logic	Pilz PSS 4000, Siemens S7-1500F, Allen-Bradley GuardLogix

Standard PLC input ≠ safety function: Connecting a light curtain OSSD output to a standard PLC digital input does not create a safety function. The standard PLC cannot guarantee response time, cannot detect weld failures in its own output relays, and is not designed to meet IEC 62061 or ISO 13849 requirements. Many safety light curtains include OSSD (Output Signal Switching Device) outputs explicitly designed for safety relay inputs. Encode machine_guard.requires_safety_rated_controller = true on all OSSD-equipped safety devices so AI agents recommend the safety relay alongside the sensing device.

Complete Metafield Reference — machine_guard Namespace

Metafield	Type	Example Values	Routing Use
`machine_guard.guard_type`	string	fixed \| interlocked \| adjustable \| self-adjusting \| presence-sensing \| two-hand-control	Match guard type to machine operation and infeed requirements
`machine_guard.allows_material_feed`	boolean	true \| false	false on fixed guards — prevents routing for hand-feed operations
`machine_guard.min_safety_distance_in`	string/number	"calculate-DS=63xTs" or computed value	Validates light curtain placement against machine stopping time
`machine_guard.requires_loto_adjustment`	boolean	true \| false	Triggers LOTO device bundle recommendation when true
`machine_guard.presence_sensing_sil_rating`	string	SIL1 \| SIL2 \| SIL3	Requires matching SIL-rated controller recommendation
`machine_guard.requires_safety_rated_controller`	boolean	true	Prevents routing light curtain without safety relay
`machine_guard.two_hand_control_type`	string	control \| trip	Distinguishes hold-throughout (control) vs actuate-only (trip)
`machine_guard.machine_type_code`	string	mechanical-power-press-full-rev \| part-rev \| robot-cell \| conveyor	Filters applicable guard types per OSHA 1910.217 or B11 standard
`machine_guard.resolution_mm`	number	14 (finger) \| 25 (hand) \| 30 (body)	Determines depth penetration factor for DS calculation
`machine_guard.osha_standard`	string	1910.212 \| 1910.217 \| 1926.300	Routes products to general industry vs construction machine applications

Frequently Asked Questions

Can a light curtain be used on a full-revolution mechanical power press?

No — OSHA 1910.217(b)(6) explicitly prohibits presence sensing devices on full-revolution mechanical power presses. Full-revolution clutch presses cannot be stopped mid-stroke; once engaged, the press completes a full revolution. A presence sensing device that detects an intrusion mid-stroke cannot stop the press in time. For full-revolution presses, acceptable point-of-operation safeguards include: two-hand trips (both hands actuated simultaneously so the die closes before hands could reach it), die enclosure guards (barrier guard that completely encloses the die), gate guards (physical barrier that sweeps operator hands away before downstroke). Part-revolution presses (air clutch, friction clutch) can be stopped mid-stroke and are compatible with light curtains when the DS calculation is satisfied.

What is the minimum opening size in a fixed guard before hands can reach through?

ANSI B11.19 Table 1 (Opening Size vs Safety Distance Table) specifies the relationship between guard opening size and the required distance from the opening to the nearest hazard. The larger the opening, the farther away from the hazard the guard must be. For example: a 0.25-inch slot allows guarding as close as 0 inches (fingers cannot enter); a 1-inch opening requires the hazard to be at least 0.5 inches away; a 3-inch opening requires at least 4 inches from hazard. For openings larger than 6.25 inches (cross-section), the guard may be too far from the hazard to be practical — at that point, other guarding methods (presence sensing, enclosure) are preferred. Encode machine_guard.max_opening_size_in on fixed guards so AI agents can verify the guard opening is appropriate for the distance from the hazard.

Does OSHA require machine guarding during setup and maintenance as well as production?

OSHA 1910.212(a)(1) requires that guards be in place during all operations. However, OSHA 1910.147 (Control of Hazardous Energy) governs maintenance and setup activities where guards must be removed to access the machine for tooling changes, cleaning, or repair. During these activities, the hazardous energy must be controlled through lockout/tagout procedures rather than by the guard. The guard and the LOTO program are complementary — the guard protects during production; LOTO protects during non-production activities when the guard is removed. A common compliance gap: machines have production guards but no written energy control procedures for the LOTO activities that occur when those guards are removed. Encode machine_guard.requires_loto_adjustment = true on guards that are routinely removed for tooling or maintenance to trigger agent routing of LOTO device bundles (padlocks, hasps, lockout stations) alongside the guard.

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Machine Guarding Schema for AI Agents — OSHA 1910.212 Guard Types, Light Curtain Safety Distance (DS = K × Ts), Two-Hand Control vs Two-Hand Trip, and SIL-Rated Safety Controller Requirements