Danger Within Reach


Expertise Includes:

    • Fires & Explosions
    • Machine Design
    • Machine Safeguarding
    • Products Liability
    • HVAC Systems
    • Risk Assessment

How far away from a hazard should you stay?  Your parents or grandparents would probably have told you to stay far away, but what are you to do when a hazard is present, and you must work around or near the hazard?  And what exactly is a hazard?  ISO 12100 Safety of machinery – General principles for design – Risk assessment and risk reduction defines a hazard as a “potential source of harm.

Where would a designer of a machine or product start if they wished to protect the user from a known hazard?  Consensus standards are a great place to begin the quest for safety.

Designers have guides such as ISO 13857, Safety of machinery – Safety distances to prevent hazard zones being reached by upper and lower limbs.  The scope reads as follows:

This international Standard establishes values for safety distances in both industrial and non-industrial environments to prevent machinery hazards from being reached.  The safety distances are appropriate for protective structures, It also gives information about the distances to impede free access by the lower limbs.

The distances apply when adequate safety can be achieved by distance alone.

Here we see that the standard is created to prevent access to machine hazards by providing protective structure(s) (commonly called “guards”) to keep individuals from reaching a known hazard.  This works if distance alone would provide protection, however some hazards might not be mitigated by keeping a hand or other limb out of a known hazard.  Radiation or noise are two potential examples where distance alone might not provide protection from a hazard and other forms of protection (for example: shielding or noise absorbing insulation) may need to be integrated into the design.  However, a mechanical hazard, such as an in-running nip point or pinch point could potentially be effectively mitigated with the safe distance concept.  ISO 13857 gives examples of distances that will protect the majority of the people who might encounter a hazard.

Several key points must be understood with regard to the application of the standard.  These are listed in the “assumptions” section of the documents and include:

-the protective structures and any openings retain their shape and position

Here we see that the protective structure must be constructed in a manner that it cannot be “cheated” and deformed or pushed aside to allow access to a known hazard.

-safety distances are measured from the surface restricting the body or the relevant part of the body.

The designer must be sure that the measurements are applied in the correct manner.

-persons may force parts of the body over the protective structures or through openings in an attempt to reach the hazard zone

Some people will have the desire to defeat the protective measure.

-the reference plane is a level at which persons would normally stand, but is not necessarily the floor (e.g. a working platform could be a refence plane)

The layout of the equipment and working conditions must be considered.

-there is some contact with the refence plane while wearing shoes (use of high-soled shoes, climbing and jumping are not included)

The person is standing and not jumping or using artificial means to increase their height or reach.

-no aids such as chairs or ladders are used to change the reference plane.

As above, the person is not using some temporary means to increase their height or reach.,

-no aids such as rods or tools are used to extend the natural reach of the upper limbs.

The dimensions are for human limbs and not human limbs with the aid of tools.

Another key item to understand as it relates to this standard is the concept of “low” and “high” risk.  These concepts require that a formal risk assessment be conducted, and that the probability of occurrence and the foreseeable severity of the injury be considered when assigning a “low” or “high” risk.

The standard has numerous situations that are addressed with regard to the design of protective structures.  One example is the case of reaching through a rectangular opening as seen below:

Image Credit: ISO 13857

Another keynote is this table is for persons 14 years of age and above and covers 95 percent (or 95th percentile) of the population.  Thus, it will not cover every person alive, but will cover a majority of the population.

Take the case of a fingertip.  This is very applicable to the design of a lot of guards.  If the opening is 4 mm or less, then the opening needs to be 2 mm or greater from the hazard, regardless of the shape of the opening.  However, if the dimension of the opening is between 4 and 6 mm, the safety distance will need to be 10 mm or greater if the shape of the opening is a slot, while the safety distance will only need to be 5 mm or greater if the opening is square or round.

What about the human hand?  If the opening is between 20 and 30 mm and the shape of the opening is a slot, then the guard would need to be 850 mm or greater from the hazard.  However, if the opening is square or round in shape, the distance from the hazard drops to 120 mm or greater.  Another key point to be aware of is that if the length of a slot opening in a guard is 65 mm or less, then the thumb of most people will not pass through the opening and the safety distance can be reduced to 200 mm.

This standard was updated and confirmed to create a 2019 edition, however most of the changes were for clarity with other standards.  Dimensions are still based on the 95th percentile and did not appreciably change.

Proper application of this standard requires a thorough evaluation of the hazard being guarded against as well as the assumptions and limitations of the standard.  When properly applied, the concepts will assist the machine designer in the quest for machinery that provides the least risk to a user that is technologically and economically feasible.

Chad Jones, PE, CFEI, CVFI, CMSE has a Bachelor of Science in Mechanical Engineering from Clemson University. Chad has over 20 years of engineering experience including mechanical, process, and manufacturing engineering. This work has included equipment design, machine safeguarding, cost estimating and safety compliance. Chad also has over 10 years of commercial, industrial, and residential HVAC and plumbing design experience. A lifelong auto and motorcycle enthusiast, Chad is accomplished in the maintenance, repair, and modification of vehicles and engines. Chad is a Certified Fire and Explosion Investigator, Certified Vehicle Fire Investigator, and IFSAC certified Firefighter II in Greenwood County, South Carolina.

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