In this article, Electrical Engineer, Kenneth J. Kutchek, P.E. provides an introduction to electronic machine safety guards. Designed to enhance the safety of industrial processes and machinery, the adequacy of these systems is frequently contested in casework involving industrial mishaps. This discussion provides an overview of the safety hierarchy of hazard control, and the various ways electronic machine safety guards are utilized to mitigate hazards.
Machine Guarding: Electronic Safety Guards
In forensic casework, our experts are commonly tasked with determining if a machine/process was designed in a manner that protected workers from exposure to hazards. To approach that question, it is important to first understand the safety hierarchy of hazard control, which is depicted below. The best and most effective option to address a hazard is to design the machine or process such that the hazard is eliminated. If the hazard is inherent to the machine or process and cannot be eliminated during the design process, then the next best option is to guard the hazard such that personnel cannot physically contact the hazard. This article focuses on electronic machine safety guards and is one aspect of the “guard” hierarchy shown in the illustration. If the hazard cannot be designed out of the machine or process, or guarded against, the last and least effective option in the safety hierarchy is to warn personnel of the hazard. The design, guard, and warn building blocks of the safety hierarchy form the fundamental principles and rules of practice for the safe and appropriate engineering of products and processes.
The purpose of a guard is to protect people from hazards associated with machinery. Guards are designed to keep the operator’s hands, arms and body from entering the hazard zone of a machine. Guards generally fall into one of two categories; fixed barrier guards and electronic machine safety guards.
Fixed barrier guards are permanently attached to equipment and are the most common type of guard because of their simplicity and effectiveness. These types of guards physically prevent personnel from contacting the hazards associated with machinery.
ELECTRONIC MACHINE SAFETY GUARDS
In some applications, a fixed barrier guard is impractical and would drastically limit the utility of a machine or render it altogether useless. Examples might include instances where operators need access to the machine to load parts or to perform routine maintenance. Interlocked guards and presence sensing devices provide an alternative to fixed barrier guards for these situations. The interlocked guards and presence sensing devices described in this section are typical electronic machine safety guards.
Interlocked guards are designed to be removed or opened to allow access to the hazard zone. Once the guard is opened or removed, the machine is designed to shut down automatically thru an electrical safety interlock to the machine control system. When the guard is closed or replaced, the electrical safety interlock is “made-up”, and the machine is ready to be re started. In some cases, the interlock is unreasonably easy to defeat, thus potentially presenting a hazardous condition to an unsuspecting machine operator.
PRESENCE SENSING DEVICES
Presence sensing devices are an advanced method of machine safeguarding which use electronic controls to detect the presence of a person’s body, arms, hands or fingers within a guarded area and stop or prevent machine operation if the sensing field is interrupted. Presence sensing safety devices are commonly used in applications where personnel need frequent and easy, yet safe access to hazardous areas such as robotic welding, machining, stamping, hydraulic presses, turrets, or packaging equipment. Presence sensing safety devices are reliable devices that should have redundant outputs for fail-safe operation. In some applications, an appropriately rated programmable logic controller (PLC) is utilized as the interface device between the presence sensing device and the starting/stopping of the machine.
Presence sensing devices are used during normal machine operation. They are not a substitute for a robust lockout-tagout policy. A variety of safety presence sensing devices exist for various applications including safety light curtains, safety mats and safety area laser scanners as discussed in the following paragraphs.
SAFETY LIGHT CURTAINS
Safety light curtains use an array of parallel light beams to detect a person or object in the sensing field. Safety light curtains can be used to protect a “point of operation” hazard near the machine opening, or to provide perimeter protection around a designated work cell. When properly configured, these arrangements will detect the presence of a finger, hand, arm, or body and send a signal to stop the machine before an injury occurs. Examples of safety light curtains are shown in the following illustration.
Safety mats are pressure-sensitive floor mats that detect the presence of a person or object on the sensing surface. Within an industrial setting, safety mats can be used in front of a machine or at the entry of a work area or throughout a work area to deactivate equipment.
SAFETY AREA LASER SCANNERS
Safety area laser scanners use an array of light pulses to provide area detection around a machine or inside the work cell. Safety area laser scanners are a non-contact alternative to pressure-sensitive safety mats and can be easily configured to scan irregular shaped areas. Safety area laser scanners can be configured for segmented sensing areas to provide a warning zone and a protective safety zone.
Presence sensing devices do not prevent a person from walking or reaching into a hazardous area, therefore, presence sensing devices must be mounted at a sufficient distance from the hazard so that the machine can be stopped before the person can contact the hazard. Calculating the minimum “safety distance”, represented by “S” in the illustration below, is critical to ensure worker safety. The safety distance calculation is based on the presence sensing device response time, control system response time, machine stopping time, and the hand speed constant of 63 inches/second.
Two-hand controls is a safeguarding method used in single cycle machine operations. It requires an operator to use both hands to activate a machine thereby keeping the operator’s hands away from the point of operation during the machine cycle.
Incorporating two-hand controls into machine operations requiring more than one operator, if properly configured, must provide each operator with separate two hand controls and require each set of hand controls to be engaged to complete a machine cycle. The removal of any hand from any control button will stop the machine. The control system must be designed with “anti-repeat” and “anti-tie down” features. Anti-repeat requires the release of all hand controls before the next machine cycle can be initiated. Anti-tie down requires both individual hand controls to be actuated concurrently. The premature actuation of either hand control will not allow the machine to cycle when the second hand control is actuated. An example of two-hand control is shown in the illustration below.
Safety relays monitor the signals from safety device inputs (such as presence sensing devices) and monitor the health status of the safety components. Safety relays quickly switch off output contacts when an input condition is triggered, or an internal or external fault occurs in a sensor or actuator.
INVESTIGATING INDUSTRIAL MACHINERY INJURIES
The engineers at Robson Forensic regularly investigate industrial machinery injuries and fatalities to determine their cause. These investigations typically include an evaluation of equipment designs and installations in accordance with industry standards, maintenance and repair issues, presence of defects, and the adequacy of warnings and instructions.
Each machine is unique including its function, layout, operational requirements, materials processed, and the hazards involved. Therefore, machine safeguarding decisions must be made on a on a case-by-case basis after a detailed risk assessment. Safeguarding may include fixed guarding, interlocked guard door or presence sensing devices with some designs requiring a combination of more than one of these methods. Robson Forensic Electrical Engineers are routinely involved when electrical safety components are integrated into the machine control system.
Electrical Engineer & Industrial Automation Expert
Ken Kutchek is an electrical engineer with more than 25 years of hands-on experience in electrical design, industrial automation, machine controls and workplace safety. Ken applies his expertise to forensic casework involving industrial automation, electrical controls, electric shock injuries, electrical related fires, failure analysis as well as various issues involving electrical systems safety and performance.
Several members of the Industrial Safety practice group regularly investigate industrial machinery incidents. Please contact Robson Forensic to discuss your case and determine which of our experts is best qualified to assist with the specific issues in your case.