In this episode of Control Intelligence, written by managing editor Anna Townshend, editor-in-chief Mike Bacidore discusses the features, locations and best practices of e-stops in industrial safety.
Transcript
An emergency stop, or e-stop is a safety mechanism used in industrial machines and automation systems. It is an easily accessible button or switch that, when pressed or activated, immediately halts the operation of a machine or an entire industrial system.
The e-stop’s primary purpose is to stop, as quickly and safely as possible, all potentially hazardous movements or processes in case of an emergency situation to prevent accidents, injuries or damage to equipment. When activated, most e-stops initiate a Cat. 1 stop in which the actuator or motor is brought to a controlled stop. “An emergency stop should be initiated by a single manual action,” says Mark Eitzman, safety and product manager at Integrated Mills Systems in Willoughby, Ohio.
Emergency stops are a critical safety feature in industrial automation and machinery, ensuring the protection of both workers and equipment in emergency scenarios. E-stops should be color-coded red and yellow, and they often feature a distinctive mushroom-shaped button designed to make them easy to identify and activate in high-stress situations.
Many standards guide the use and operations of e-stops, such as ISO 13850, NFPA 79 and IEC 60204-1.
Before operating a new e-stop application, be sure to consult the standards or a safety expert who understands the standards and the specific application.
E-stops are meant to operate as quickly as possible in an emergency situation, so visibility is key. The actuator of the emergency-stop device must be colored red, and the background area behind the actuator should be colored yellow. The actuation of an emergency-stop device must be from a single manual action, such as pressing a push button or pulling a cable. A mushroom-head actuator is a common device used for emergency-stop devices, but others are also acceptable.
The emergency stop of hazardous motion should not create new hazards, such as upsetting a load. This means using the proper stop category, Cat. 0 or Cat. 1.
Category 0 is an uncontrolled stop which immediately removes power to the machine actuators.
Category 1 is a controlled stop with power to the machine actuators available to achieve the stop. Power is removed when the stop is achieved. For example, for high-friction applications with unstable loads that could fall over, a Cat. 1 stop should be employed to extend the stop by ramping speed down to bring the machine to a stop. Conversely, a high-inertia load would include electrical or mechanical braking to bring the machine to a stop faster than it would if power were immediately removed from the actuator.
An emergency stop device should be located at each operator control station, and a risk assessment should determine other appropriate locations for the particular application. E-stops are commonly found at entrance and exit locations or where operators and machines work together.
Emergency-stop devices should be directly accessible to operators for easy actuation. The location should take into account the operator's line of sight and range of movement. Operators should not have to navigate obstacles to access e-stops. For larger machinery, multiple emergency-stop devices may be necessary. ”Wireless, on-person e-stops should be considered for maintenance personnel that troubleshoot live equipment,” says Integrated Mills Systems’ Eitzman.
The machines and application will determine an e-stop’s span of control. To determine that span through a layout analysis, Eitzman recommends the following considerations:
• the physical layout of the machine, based on the visible area of the machine
• the possibility to recognize hazardous situations through visibility, noise and odor
• any safety implications relating to the production process
• the foreseeable exposure to hazards
• and the possible adjacent hazards.
“Each span of control can cover sections of a machine, an entire machine or a group of machines,” Eitzman says. Task and hazard identification includes identifying the task zone, the physical area where people will do a task and the control zone for engineering controls, including interlock devices, presence-sensing devices, enabling devices, hold-to-run controls, resets and emergency stops. Spans of control may also overlap, Eitzman explains.
There are also recommended practices for restarting after an e-stop. “An emergency-stop actuator shall be a maintained mechanism,” Eitzman says. Once activated, the e-stop remains actuated until manually reset, so the machinery remains stopped until it is intentionally reset. “Once reset, the machine can be restarted with a separate, intentional, manual command,” he explains.
Other control functions or systems should never override an emergency-stop command. Once activated, the stop condition should take precedence. Deactivation should require a manual reset, so operators must take a deliberate action to restart. “The twisting and/or pulling out of the depressed actuator is a common feature used to reset the e-stop button,” says Eitzman. “Other means include a separate reset button, typically blue and labeled as reset. Other devices, such as some cable pull switches have a blue reset button right on the main switch body.”
Routine inspections should be part of the regular maintenance schedule for emergency-stop devices. Functional testing should periodically activate the e-stop function, according to the manufacturer’s instructions, and verify that the machinery stops. Test any maintained mechanisms and confirm the e-stop control resets.
“The frequency of the testing needs to be balanced between the stresses that e-stop actuation causes the equipment—mechanically, electrically or fluid power—and ensuring that the e-stop functions properly,” notes Eitzman. “Consider the level of risk that the e-stop measure is meant to reduce. In other words, if the risk is higher in severity and probability, then test more often.”
