Hazard can be defined as a potential source of harm. Machine safeguarding seeks to protect people from these potential sources of harm. Often distance from a hazard will play a key role in providing a means of protection.
One would often think of distance as it relates to the location of a barrier guard from a hazard. ANSI B11.19, Performance Criteria for Safeguarding, defines safety distance as “the distance a safeguard is installed from a hazard such that individuals are not exposed to a hazard.” An example from ANSI B11.19 of the recommended distance of a slotted opening in a barrier guard from a hazard is shown in the table below:
Table 1: Minimum Slotted Opening vs Distance from Hazard – From ANSI B11.19
This information will help assess if the opening present in a barrier guard will meet the values established in a consensus standard such as ANSI B11.19. These distances and dimensions should be carefully considered when designing fixed barrier guards.
However, when more sophisticated means such as safeguarding devices are used to protect an individual from a hazard, distance takes on a different meaning. ANSI B11.19 defines a safeguarding device as “a device that detects or prevents inadvertent access to a hazard.”
A light curtain is a well-known presence-sensing device. ANSI B11.19 defines a presence sensing device as “a device that creates a sensing field, area or plane to detect the presence of an individual or an object.” An example of a light curtain is shown below.
If the individual utilizing a machine protected by the light curtain breaks the plane created by the sensing device, then the hazard behind must be rendered safe before it can be reached. For example, a hazardous motion must stop to prevent an injury to the individual that breaks the plane. Here the distances noted in Figure 1 above may not be applicable and a different method of determining the safety distance should be considered.
ANSI B11.19 states in section 6 General safeguarding requirements, 6.3 Safety distance:
“When required by this standard, the guard or safeguarding device shall be located a distance from its associated hazard such that individuals cannot reach the hazard before cessation of hazardous motion (or situation).”
Here we see that the hazard must be rendered safe before an individual can reach it through the presence sensing device and be injured.
Furthermore, section 8 Safeguarding devices, 8.3 Electro-optical, RF and area scanning presence-sensing safeguarding devices, 8.3.2.3 states:
“The presence sensing device shall be installed at a location so that the effective sensing field prevents individuals from reaching the hazard(s) during the hazardous portion of the machine cycle.
How do we determine this location or “safety distance”? Explanatory information in ANSI B11.19 notes:
“The safety distance calculation is dependent upon the:
Speed of approach of the individual
Total response time of the safeguarding device as stated by the supplier
Response time of the interface
Response time of the control system
Time it takes the machine to stop hazardous motion; and
Depth penetration factor of the safeguarding device.”
Here we see that with a presence sensing device, the value for a safe distance has many facets that must be considered to provide for safe operation by a user. ANSI B11.19, Annex D provides a method for determining what a safe distance should be based on factors mentioned above.
Safeguarding is often not a one size fits all activity. Careful consideration should be given to the safeguarding method chosen and proper attention paid to the specific design details. Careful selection and proper design details will lead to a safer machine.
Chad Jones, PE, CFEI, CVFI, CMSE has a Bachelor of Science in Mechanical Engineering from Clemson University. Chad has over 25 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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