Safeguarding employees in the workplace from the hazards of their machines has become commonplace, but it is still important that safeguards not interfere with production. As industry looks for machine safeguards that allow higher rates of production, the use of presence-sensing devices has become more common. Presence-sensing devices include safeguards such as radio-frequency field sensors and laser scanners. However, the most common presence-sensing devices by far are banks of photoelectric transmitters/receivers, more commonly known as "light curtains."
Light curtains have many advantages as safeguards. They allow unobstructed access to the parts of the machine they guard, and afford excellent visibility into the machine. When used correctly (meaning that the application of a light curtain has properly considered the ability of the machine to safely stop before a machine operator can get his/her body parts into the hazardous area of the machine), light curtains can provide safeguarding and allow for optimum operator productivity.
Light curtains and other presence-sensing devices have been used in machine safeguarding since at least the 1950s. It was not long after the introduction of light curtains as safeguards that some enterprising engineers noted that they could also be used to initiate (start) a machine cycle.
The use of a light curtain or other presence-sensing device to initiate the cycle of a machine is called "presence-sensing device initiation" (PSDI). In a typical PSDI application, a machine operator reaches through a light curtain to load a part into a machine: the interruption of the light curtain prevents the machine from starting. As the operator withdraws his/her hands from the machine (and the light curtain's sensing field), the completion of the light curtain immediately initiates the machine's start command, without the operator needing to press a control button or activate a foot switch to begin the cycle.
Operating a machine in this way would allow for increased productivity, as employees would no longer need to make "non-value added" movements to cycle the machines. Years later, as ergonomics began to receive more attention in the workplace, it was noted that by eliminating the need to press the cycle start switch(s), repetitive motions could be reduced.
OSHA's Chilling Effect
The advantages that PSDI would seem to have were not immediately realized. Various problems with the sensitivity of these devices and serious issues with the reliability of their control systems began to manifest themselves in employee injuries, including amputations. When OSHA came out with their Mechanical Power Press standard in 1973 (which was based on the 1971 ANSI standard), OSHA prohibited the use of PSDI for mechanical power presses.
The Mechanical Power Press standard (29 CFR 1910.217) is the most detailed machine safeguarding standard in OSHA's Subpart O (Machinery and Machine Guarding). Many of the machine safeguarding concepts and techniques used to safeguard other types of machines are gleaned from this standard, as well as from ANSI's B11 series of standards. OSHA's ban on PSDI did not (and does not today) apply to machines other than mechanical power presses, but it did have a chilling effect on the use of PSDI in the U.S., where PSDI usage is still uncommon.
In 1982, OSHA contracted with an expert, Trygve Hauge, and the National Institute for Occupational Safety and Health (NIOSH) contracted with Purdue Research Foundation to study PSDI. Their reports were widely circulated by OSHA and comments were received. The reports included information on the history of the safe use of PSDI on mechanical power presses and other machines in Canada and Europe, and led OSHA to reconsider the use of PSDI for mechanical power presses. In 1988, OSHA's Mechanical Power Press standard was amended to permit the use of PSDI, with some "limitations."
The 1988 OSHA limitations placed on PSDI were quite onerous, and make up more than half of the total number of pages of the 1910.217 standard (including all four appendices to 1910.217). One of the most burdensome requirements was for validation of PSDI by an OSHA-recognized third-party validation organization (nationally recognized testing laboratories or NRTLs). Even today, no NRTLs have requested permission from OSHA to validate PSDI applications, effectively continuing the ban on PSDI.
The benefits of PSDI have not gone unnoticed by U.S. manufacturers, and OSHA is again reconsidering its position. In 1998, OSHA began asking for comments on PSDI. Comment periods have been extended several times; the most current comment deadline was May 31, 2004 (this regards an extension of the Office of Management and Budget's Approval of Information-Collection Requirements). One problem that OSHA is experiencing is that there is not much data on the use of PSDI in the U.S.
An updated ANSI B.11.1-2001 standard on mechanical power presses was issued in 2001. This standard allows for PSDI (with common-sense considerations for reliability), but does not require NRTL certification. OSHA should consider adopting ANSI's approach to PSDI and perhaps even replace the existing OSHA standard with the new ANSI B11-2001 standard, as there are several other (non-PSDI) machine safeguarding issues with the current OSHA standard that also need to be addressed.
In today's globally competitive environment, where incremental productivity gains are important, PSDI is a subject that is sure to be revisited by the U.S. manufacturing community. Safety professionals should familiarize themselves with PSDI, as its use in the United States is certain to accelerate in the near future.
Thomas J. Martin, CIH, CSP, is manager, Occupational Health and Safety for the Detroit and Cleveland regional offices of Clayton Group Services. Martin has 20 years of experience as an occupational health and safety consultant. His primary focus at Clayton is OSHA compliance audits, safety-related evaluations, health and safety training course development and delivery, emergency response planning, accident investigations and industrial hygiene studies. He received a B.S. in occupational safety and health from Ferris State University. Martin co-presented the "Machine Safeguarding 101" Professional Development Course (PDC) at the American Industrial Hygiene Conference and Exhibition in 2001, 2002, 2003 and 2004.