EN ISO 13857: Reach Distances, Openings, and Whole-Body Access

EN ISO 13857 safety distances are the minimum separations that must exist between a person and a machine’s hazard zone, calculated from how far human limbs can physically reach over, through, around, or under a protective structure. The 2019 edition of the standard publishes these distances as lookup tables keyed to guard height, opening size, and the height of the hazard above the floor. A machine designed to these tables is presumed to meet the corresponding essential health and safety requirements of the EU Machinery Directive 2006/42/EC, for which the standard has been harmonized since March 2021.¹

Classify the Risk Before Opening a Table

Every hazard zone must first be classified as low risk or high risk under an ISO 12100 risk assessment. That classification decides which set of distances applies, and the gap between the two sets is not trivial. A hazard reached upward with no guard has to sit at least 2,500 mm above the floor to be out of reach in a low-risk scenario, and at least 2,700 mm in a high-risk one.² The same split runs through every reaching-over table: high-risk zones need taller barriers or greater horizontal distances than low-risk zones at identical hazard heights.

Low risk means both the severity of possible harm and the probability of that harm are low. If either factor is elevated, the zone defaults to high risk and the more conservative table applies. Picking values without doing this step first is the most common design error, because it produces a guard that looks adequate on paper but is undersized for the actual hazard.

Reaching Over a Guard

The core tables map three variables against each other: the height of the hazard above the floor, the height of the protective structure, and the minimum horizontal distance from the guard’s inner face to the nearest point of the hazard. A few points from the low-risk table (Table 1) show how the geometry works. With a guard 1,000 mm tall and the hazard sitting at 2,400 mm, only 100 mm of horizontal separation is needed, because the hazard is well above arm’s reach. Drop that same hazard to 1,000 mm and the horizontal distance jumps to 1,400 mm. Raise the guard to 1,400 mm while keeping the hazard at 1,000 mm, and the required horizontal distance shrinks to 900 mm.³ A taller guard blocks more of the reaching arc, so the hazard can sit closer.

The tables assume a flat reference plane. If a platform, step, or climbable surface could raise the standing height, the effective floor level has to be adjusted upward before you enter the table. Structures shorter than 1,000 mm are not in the tables at all, because they don’t meaningfully restrict body movement. Structures between 1,000 mm and 1,400 mm appear in the tables but should not be used alone without additional protective measures.

Reaching Through Openings

Slots, squares, circles, and mesh openings must be small enough, or set deep enough behind the guard face, that a limb cannot contact the hazard. Table 4 covers persons aged 14 and older. For slot-shaped openings, the key thresholds are:

4 mm or smaller: safety distance of 2 mm (an opening this small blocks even a fingertip).
Over 4 mm up to 6 mm: at least 10 mm.
Over 6 mm up to 8 mm: at least 20 mm (finger up to the knuckle).
Over 8 mm up to 10 mm: at least 80 mm.
Over 10 mm up to 12 mm: at least 100 mm (hand begins to fit).
Over 12 mm up to 20 mm: at least 120 mm.
Over 20 mm up to 30 mm: at least 850 mm; if slot length is 65 mm or less, the thumb acts as a stop and the distance can be reduced to 200 mm.
Over 30 mm up to 120 mm: at least 850 mm (the full arm can enter).³

The step from 120 mm to 850 mm between the 12–20 mm range and the 20–30 mm range is the steepest cliff in the standard. A slot just slightly wider than 20 mm forces roughly seven times more guard depth, which is expensive to fix after fabrication.

Irregular Openings

For openings that aren’t clean slots, squares, or circles, use a three-step method. First, work out the diameter of the smallest circle, the side of the smallest square, and the width of the narrowest slot into which the irregular opening could be fully inserted. Then look up the safety distance for each of those three shapes. Finally, use the shortest of the three distances. That matches the opening to whichever body-part geometry gives easiest access.

Reaching Around a Guard

When a guard doesn’t fully enclose a hazard, someone may reach around its edge. Table 3 ties the required distance to how much of the arm the guard actually blocks, measured along the path the arm would travel:

Shoulder and armpit only: 850 mm or more (the full arm is free to swing).
Arm supported up to the elbow: 550 mm or more.
Arm supported up to the wrist: 230 mm or more.
Arm and hand supported up to the knuckle joint: 130 mm or more.³

For any of the reduced distances below 850 mm to apply, the obstacle limiting arm movement must be at least 300 mm long. A thin edge or narrow flange doesn’t count, because the arm slips past rather than being genuinely supported. Side panels and wing extensions earn their keep here: a guard wide enough to block the arm at the elbow can cut the required distance nearly in half compared with one that only blocks at the shoulder.

Reaching Under a Guard

The gap between the bottom of a guard and the floor lets feet and legs slide underneath toward a hazard. For a ground clearance of 200 mm or less, the horizontal distance from the guard’s inner face to the hazard must be at least 665 mm to keep the foot out of contact. Larger gaps push the distance requirement up substantially, because the leg can enter further.

This matters most for conveyors, presses, and other machines with low-sitting moving parts. Many designers close the gap entirely with a floor-height skirt or a flexible seal strip, which avoids the lower-limb table. Where a gap is necessary for drainage, ventilation, or cleaning access, the 200 mm threshold is the design line to watch.

Whole-Body Access

Openings large enough for a person to climb or squeeze through need a separate analysis. The standard sets ceilings on opening size beyond which whole-body access becomes possible:

Slots (vertical or horizontal): 180 mm maximum width.
Square or circular openings: 240 mm maximum dimension.

Any opening larger than these thresholds needs additional safeguarding to keep someone from entering the hazard zone entirely. A protective enclosure must be at least 1,400 mm tall to meaningfully discourage climbing and segregate the area. Structures shorter than 1,000 mm are considered ineffective under any circumstances. Between 1,000 mm and 1,400 mm, extra measures such as interlocked gates or presence-sensing devices are required to compensate.

Machines Accessible to Children

The main tables assume users are aged 14 or older. For environments where children aged 3 and older may be present, Table 5 gives more conservative distances, because smaller fingers penetrate narrower openings. A slot between 12 mm and 20 mm, for example, jumps to 900 mm rather than 120 mm, and the thumb-stop exception only applies when the slot is 40 mm or shorter, reducing the distance to 120 mm.³ Table 5 covers upper limbs only; the standard does not give lower-limb distances for children.⁴ Machines in public spaces or any location without controlled access should use Table 5 rather than Table 4.

Documenting the Calculation

Applying the tables correctly is only half the compliance picture. Under the Machinery Directive, manufacturers must document the safety distance calculations in the machine’s technical file, along with the risk assessment that drove the choice between low-risk and high-risk tables. The file needs detailed drawings showing guard dimensions, the hazard zone location, and the resulting safety distances, plus any test results confirming conformity, and it must be available to market surveillance authorities on request.⁵

Thorough records matter for liability too. If an injury later puts the guard dimensions in question, the technical file is the first thing an investigator asks for. A file showing that the designer identified the hazard, classified the risk level, selected the correct table, and verified the dimensions is strong evidence of due diligence. A missing or incomplete file shifts the burden the wrong way, even when the guard itself was correctly sized.

1
European Commission. Machinery Directive 2006/42/EC – Summary List of Harmonised Standards
2
ISO. ISO 13857:2019 – Safety Distances for Machinery Hazard Zones
3
ISO 13857:2019 Tables. ISO 13857:2019 – Safety Distances Reference
4
International Organization for Standardization (ISO). Safety of Machinery – Safety Distances to Prevent Hazard Zones Being Reached by Upper and Lower Limbs
5
Health and Safety Executive for Northern Ireland. Technical File Information