For perimeter access guarding, 2-beam, 3-beam and 4-beam safety light grids are the three stock configurations specified under IEC 61496-1 and IEC 61496-2, with beam spacing typically running 300 to 500 mm between adjacent beams [S1][S2].
A 2-beam grid places its two beams at 300 mm and 900 mm above the floor (300 mm beam-to-beam), a 3-beam grid adds a middle channel, and a 4-beam grid closes the gaps so a torso, limb, or crawl cannot defeat detection [S8]. Type 4 versions of these grids reach SIL CL 3 and PL e, while Type 2 versions cap at SIL CL 1 and PL c, which dictates which applications each may legally guard [S1][S3].
Beam count, beam position, and what each actually detects
Safety light grids are a separate device class from light curtains: a curtain uses many closely spaced beams (14 to 90 mm apart) for finger, hand, or body detection at the point of operation, while a grid uses 2 to 4 beams spaced 300 to 500 mm apart and is dedicated to body / perimeter access detection only [S1][S5]. Banner Engineering's SGS Safety Grid System ships in 2, 3, or 4 beam variants with sensing range up to 60 m and defined area heights from 500 mm to 1200 mm [S8].
A 2-beam grid only sees two horizontal planes, so a person crouching below the lower beam (300 mm) or rolling under the upper beam (900 mm) can defeat it on installations where reach-over or crawl-under was not separately mitigated. A 3-beam grid inserts a middle channel (commonly near 700 mm) that catches upright torso intrusion better; a 4-beam grid tightens spacing to roughly 300 mm between beams across a 1200 mm column, which closes the crawl-under path and adds an upper channel for step-over protection [S1][S8]. Euchner's LCA-series literature likewise describes 2 to 4 beam grids as the standard menu for access control to large work areas [S6].
Grid mounting height, not just beam count, is the dominant variable: EN ISO 13855 sets the safety distance from the hazard based on protective device height, resolution, and machine stop time, and that calculation must be redone any time the grid is moved or the machine stop time changes [S2].
Type 2 vs Type 4: fault detection changes the answer
The headline difference between a Type 2 and a Type 4 light grid is not beam count or housing size, it is fault detection: Type 4 continuously self-monitors via dual-channel cross-check and tighter ±2.5° effective aperture angle (EAA), while Type 2 runs a periodic self-test with a wider ±5° EAA and tolerates optical short circuits near reflective surfaces [S1][S3][S7].
On the safety metrics, Type 4 grids reach SIL CL 3 and PL e with PFHd in the 1×10-8 to 1×10-7 per-hour window, while Type 2 grids cap at SIL CL 1 and PL c with PFHd between 1×10-6 and 3×10-6 per hour [S3]. IEC 61496-1 / -2 is the standard that defines these Type requirements for electro-sensitive protective equipment (ESPE), and ISO 13849-1 is the standard that defines the resulting Performance Level [S1][S3].
For point-of-operation guarding on power presses, robot cells, and other high-energy machinery, ISO 13849-1 risk graphs and the machine's Type-C harmonized standard will normally force Type 4 / PL e. Type 2 grids remain legal for lower-risk auxiliary equipment and perimeter assist roles, and that is where the hardware cost gap of roughly 15 to 30% above Type 2 of comparable height matters to procurement [S3].
Selection criteria: pick the beam count from the access path, not the catalog
Use this four-criterion filter when choosing between a 2-beam and a 4-beam grid for a given access opening: the lowest authorized user posture (standing, kneeling, crawling), the height of the bottom beam above floor, the maximum sensing range to the opposite column, and the required Type / PL band for the hazard being guarded. [S2]
Decision matrix for access guarding on a perimeter where a person can walk, kneel, or crawl to the line:
Criterion 1, detection of crawling-under: a 2-beam grid with its lower beam at 300 mm can be defeated by a person who can present a flat profile below that line, so it is only acceptable where reach-into the hazard is impossible or where floor-level mechanical railing closes the gap. A 3-beam or 4-beam grid with a beam at or below 300 mm plus intermediate channels defeats the crawl-under path without a fence [S1][S8].
Criterion 2, step-over / climb-over: a 2-beam grid leaves a large open window between the upper beam (about 900 mm) and the hazard, so a person who steps over the lower beam and over the upper beam enters the field undetected only briefly. A 4-beam grid at 1200 mm column height reduces that open window to about 300 mm [S8].
Criterion 3, range and footprint: Banner's SGS grids go to 60 m on 2, 3, and 4 beam versions, so range alone does not force a beam-count choice; what changes is the column cost, because each added beam adds emitter and receiver optics plus alignment tolerance [S8].
Criterion 4, safety integrity: a 2-beam grid is available in both Type 2 and Type 4, and the same is true of 3-beam and 4-beam versions, so the Type decision is made on the hazard, not on beam count [S1][S3][S7].
Use cases matched to beam count
A 2-beam Type 4 grid is the typical pick for personnel-only access detection across a conveyor pass-through, a palletizer cell entry, or an automated guided vehicle (AGV) loop, where the hazard is low-energy and the access path is always upright walking. Banner, Schmersal, Euchner, and ifm all list 2-beam grids in their standard perimeter-guarding catalogs with the lower beam at 300 mm and the upper beam at 900 mm [S1][S2][S6][S8].
A 3-beam grid is the common compromise for robotic palletizing cells and CNC tending cells where the operator may kneel to change a fixture or lean into the work envelope, and where a middle beam at roughly 700 mm catches that intrusion without paying for a fourth channel [S8].
A 4-beam grid is the default for press-shop perimeter guarding, large press-brake envelopes, and any opening where the safety distance calculated under EN ISO 13855 needs the uppermost beam at or above 1200 mm to keep the safety distance within the cell footprint [S2][S8].
For a spec-driven comparison of detection resolution across these access-guarding devices versus point-of-operation curtains, the safety light curtain reference page lays out the finger 14 mm, hand 30 mm, and body 40 mm bands that separate the two product classes [S1][S3][S5].
Integration and standards wiring
A 2-beam or 4-beam safety light grid is an active opto-electronic protective device (AOPD) under IEC 61496-1 and -2, and its OSSD outputs must be wired into the machine's safety control system (safety relay, safety PLC, or safety controller) so that beam interruption produces a stop signal and a controlled restart [S2][S4].
For muting applications, where a conveyor must pass a pallet through the grid without stopping the line, Banner and other vendors offer integral muting models in the 2 to 4 beam range, and the muting logic must satisfy the same EN ISO 13855 safety distance calculation as the unmated guard [S8]. When grids and curtains coexist on the same line, the OSSDs from both feed the same safety zone and the higher-integrity channel (Type 4 / PL e) governs the resulting stop category [S3][S4].
For the broader electronic test and measurement context that surrounds commissioning these devices, stop-time analysis (at least 10 measurements of machine + safety-device response time) is the standard input to the EN ISO 13855 safety distance formula [S2].
Limitations and common failure modes
Two practical failure modes bite 2-beam and 4-beam grids in the field: optical short circuit on Type 2 grids mounted near reflective surfaces, and reach-over on any grid whose upper beam sits below the operator's shoulder when they stand close to the column [S1][S3]. The mitigation for the first is to specify Type 4 (±2.5° EAA) where shiny stainless steel or polished tooling is in the field of view; the mitigation for the second is to mount the columns far enough from the hazard that the EN ISO 13855 distance absorbs the reach, or to add a third or fourth beam [S1][S2].
Grid columns also have a finite alignment tolerance: a 60 m range grid demands accurate optical alignment, so vendors supply alignment aids and laser alignment tools as standard accessories, and a misaligned column can present as a nuisance trip or, worse, a beam that does not fully cover the protective field [S4][S8].
For plant engineers comparing grids to physical access control hardware like interlocked gates, the trade-off is well defined: a light grid detects presence but does not physically prevent entry, so the safety distance calculation must fully account for the time between beam break and safe stop, whereas an interlocked gate can be placed much closer to the hazard because the guard itself prevents reach [S2][S7].
Decision summary: when 2-beam is enough, when 4-beam is required
Specify a 2-beam Type 4 grid for upright-walk-only personnel access on low-to-medium energy machinery, where the bottom 300 mm beam catches the ankle and the top 900 mm beam catches the torso, and where reach-into the hazard is impossible. Specify a 3-beam grid where operators kneel or lean at the work envelope. Specify a 4-beam grid for primary safeguarding on high-energy machinery, for any opening where crawl-under or step-over must be defeated without a fence, and for any installation where the EN ISO 13855 safety distance calculation forces the uppermost beam to 1200 mm [S1][S2][S3][S8].
Track for 20 January 2027: EU Machinery Regulation (EU) 2023/1230 takes full effect, which tightens conformity assessment for safety components including ESPE devices and may push more perimeter applications from Type 2 to Type 4 hardware [S3]. Track also for incremental Type 3 product launches under IEC 61496-3 (AOPDDR), which mostly affects safety laser scanners rather than 2-beam or 4-beam light grids [S3].
Background reading: Marine Universal Joint Selection: Spec Gates for Stern-Drive and Auxiliaries.