When you've got to stop, you've got to stop. For machine builders, that's usually a job for safety switches. They allow safe access to dangerous machines by putting systems in a harmless state, which could mean cutting off power completely. In selecting a safety switch, it's important to keep in mind frequency and function, according to Roberta Nelson Shea, global marketing manager for Rockwell Automation. For something that needs to be opened only once a year, the best solution likely is to use simple guards, such as by completely encasing the area. More frequent access has to be easier. It's when access is frequent, but not constant that an interlocking safety switch comes into play.
"You expect somebody needs to get in there at least once a day, possibly more frequently," Shea says. "That's when we would make use of some sort of interlocking switch for that guard that would be connected with the machine control system, so that when you opened the guard, the machine would come to a safe state."
The simplest, often lowest-cost and most operationally robust solution is a tongue interlocking switch, Shea indicates. These switches have a tongue actuator on one door that forces apart contacts in a switch attached to another door. Pulling out the tongue signals that the door has opened, and the machine should be stopped.
As for function, trapped key systems are best for sequential access, which requires going through a series to steps to get to the area in question, according to Shea. Trapped key systems also could be the method of choice in a hazardous location because some require no voltage to operate. However, to minimize any danger, the interlocking typically takes place outside of the hazardous location itself.
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In different operating conditions, guard-locking switches are likely best. For instance, sometimes machines have a long run-down time, such as when a spinning or otherwise moving system takes some time to come to a halt. Consequently, there has to be some way to keep the door or other access points closed. This can be done through a switch that does not release a guard until certain conditions, such as a long enough period of time, have been met. Guard-locking switches also can handle cases in which a chance for serious bodily injury exists, and power is lost to a machine.
"You have to apply power to have it unlock. So, therefore, if there is loss of power, the guard is locked," Shea says.
In still other circumstances, non-contact safety interlocks are best. There are several technology choices, such as an array of coded magnets or a radio frequency identification (RFID) reader and tag. No matter the technology, such switches have built-in safeguards that prevent an easy defeat of the safety function.
"For example, if you have an RFID system, you can have a unique code, so that actuator will only work with that sensor," says Matt Dodds, product manager of safety switches, relays, controllers, edges and mats at Omron Automation (www.omron247.com).
Speaking of non-contact safety switches, Dodds adds, "These are good in wet environments because there are no exposed moving mechanical parts to be interfered with."
Other advantages of non-contact safety switches are that they're small, lightweight and easy to mount. A disadvantage is that actuation might be problematic, particularly for some of the more inexpensive switches, according to Dodds.
No matter what type of switch is used, it's important to recognize and account for the needs of engineering staff, maintenance personnel and operators to access a machine for upkeep and cleaning. Consider a setup with an interlocking switch that immediately kills power to a machine if a door is opened. When that happens, the result is a system on which maintenance technicians can't run diagnostics. Given that situation, maintenance has to bypass the safeguard to do its job, so a supposedly safe setup is actually not, Dodds points out. "The safety needs to be engineered into the system, so it's integral to the process and done at the onset of the machine design, not after the fact," he says.
For those times when nothing less than a complete power shut-off will do, consider an industrial safety switch. Built with a pull lever or handle, these are part of the standard electrical portfolio and have a basic function, according to Terri Vallera, senior product manager for safety switches at GE Industrial Solutions. "The main reason to use a safety switch is as a means to disconnect power to whatever device or machine you're feeding," she says.
Placed on every piece of piece of equipment in a facility, a safety switch should be easily visible and within 50 feet of the device it supports, Vallera adds. By cutting power, these switches allow servicing of machinery without risk of electrical injury and also provide an emergency shut-off.
The switches themselves are simple in operation, with a quick-make/quick-break technology that hasn't changed significantly in years. When contacts are closed, power flows. Power stops when contacts are open, either by someone moving a handle or, if the switch is fused, by an overcurrent situation.
Because such switches must meet a machine's requirements, the most important consideration is the needed voltage and amperage, or the horsepower. After that, the next consideration is the type of enclosure around the switch. Indoor applications in fairly clean environments can get by with a NEMA 1-rated enclosure, whose main job to protect against inadvertent contact. Switches located outdoors should go into a weather-protecting NEMA 3R enclosure. Most plant floors probably will have a dust-tight NEMA 5/12 or a NEMA 4X enclosure, with the latter corrosion-resistant and capable of withstanding a washdown.
If it's important to keep a machine fault isolated from other devices upstream, then a fusible switch should be used, Vallera indicates. If a motor or another component with a rush of starting current is involved, it might be best to go with a time-delay fuse to permit additional current draw during start-up.
No matter what safeguarding technology is chosen, a switch or other component mishap or breakdown can occur. Thus, ready product availability is important. Otherwise, what might be stopped could be more than a machine.
As Vallera says, "Factories and even OEM applications can be dependent on local availability to keep their operations running smoothly."
