Flipping the switch on safety
Nov. 1, 2011
by Shane Whitaker
Although Rick Rodarte, director of engineering, Stewart Systems, Inc., Plano, TX, said it’s difficult to say that any new or cutting-edge safety systems have been introduced in recent years, he pointed out that companies are continually coming out with devices that work in parallel with machines’ control systems, and now it’s possible to integrate safety devices and relays with PLCs. “For a long time, safety devices had to be hardwired, and you couldn’t go through any kind of electronic system or a PLC, because if a PLC lost power, the machine was considered unsafe,” he said.
Now, safety relays are integrated into PLCs, and plants can put them online either through a local Ethernet or the Internet. “A plant can have a master system to monitor,” Mr. Rodarte said. “If a piece of equipment goes down, they can determine whether it was a safety switch or a motor fault. They can categorize incidents as they monitor them for efficiency or for safety numbers in their working organization.”
The fact that processors cannot defeat safety switches as easily as they once could represents the most important change in safety systems, according to Mike Hall, director of engineering, Shaffer, A Bundy Baking Solution, Urbana, OH. Defeating safety switches wasn’t just an issue in the baking industry but in all types of manufacturing plants; operators would commonly defeat safety switches to keep machines operating as they liked.
They would use electrical tape or paper clips to defeat switches, Mr. Hall said, adding that he has seen many different methods employed. “Now, they cannot do that, or at least it’s a lot more difficult to do,” he said.
It’s more difficult because manufacturers have come out with new switches to meet the latest safety standards. “The technology for the switches has changed, so now you have true safety-rated switches,” Mr. Hall said.
Safety switches can provide a key line of defense to protect employees. For example, Stewart Systems uses safety switches on its oven doors that will set off an alarm and shut down the oven should someone open a door. “It goes into full exhaust,” Mr. Rodarte said. “You’re trying to pull heat out of there as much as you can because you can’t have someone walking into a 400°F environment.”
Approved safety devices are required when integrating electrical hardware into the safety circuit, according to Travis Getz, director of engineering at Reading Bakery Systems, Robesonia, PA. “The three categories of safety devices are optic, switching and stopping,” he said. “A combination of all three may be required on some machines.”
Optic safety devices are also referred to as light curtains, which will cause a piece of equipment to automatically shut down if tripped.
Switching devices such as keyed, magnetic or hinged interlocks and limit or RFID switches are used when a device switches states, according to Mr. Getz. “We use all of these devices on our equipment when interlocking guard doors and access points,” he said. “The device is determined by the type of guarding required. Noncontact switches, primarily RFID, have become more popular recenlty due to the ease of integration.”