A downside of safety relays can be the lack of diagnostics they provide. When the machine stops, you still have to go out and do some troubleshooting, find and repair the problem, and then restart the machine. But where do machine builders draw the line between diagnostic needs and safety?
"We use a lot of safety relays," says Jim Braun, vice president, product development and standardization for MAG Americas (www.mag-ias.com), Hebron, Kentucky. "We also have safety integrated on our machines, which you're doing through safe PLCs and specific logic. It adapts well on some machines and not on others. You see a lot of safety integrated on European machines."
Being a global company, MAG builds a lot of machines in and for Europe. "In the U.S., we still do a lot of safety relays," says Braun. "Our experience is that the relays are pretty reliable. If there's a problem, they're a little more difficult to troubleshoot. The trend in the industry is going more to safe PLCs and safe drives. You have this whole regimen of software you go through. Over time that will tend to eliminate the safety relays. Safe PLCs and safe drives can be more cost-effective."
Jack Chopper, chief electrical engineer, Filamatic (www.filamatic.com), Baltimore, believes the lack of diagnostic information will be a short-lived problem. "Diagnostic capability is always a time-saver, but the safety integrity and reliability must override the other information-related features," he says. "The safety-rated PLC is relatively young and still very pricey, but as those types of products mature, they will settle down in price."
Diagnosing a problem determines the root cause of a potential or pending failure, explains Hilton Hammond, product manager—ScopeMeter, Fluke (www.fluke.com). "Safety components, by design, are built into a system to ensure, in the event of a failure, something catastrophic does not occur that could present a hazardous condition to the operator or the surrounding environment," he explains. "Sensing and measuring critical variables, like voltage or current, and applying them to some form of a diagnostic algorithm, can provide the user feedback about the operating state of the machine or process."
Without a relay, interlock or other protective device built into the active circuit, a shutdown could be delayed, which might result in an unacceptably hazardous condition, says Hammond.
"My first safety relay system had more than 30 relays in a cabinet," recalls Wade Peterson, electrical engineer, CMD (www.cmd-corp.com), in Appleton, Wisconsin. "It took two days to fully commission that system. But we became a lot more proficient at it and got to the point where the same job would take only four hours. Better diagnostics would have helped in the first situation but would have been of less use toward the end after the initial learning curve was overcome. Diagnostics should be a concern when there are a variety of devices or if maintenance resources are scare. Product longevity and reliability are more important in safety circuits when component standardization and good training are in place."
Traditional safety relays can be problematic, says Helge Hornis, PhD, manager, Intelligent Systems Group, Pepperl+Fuchs (www.pepperl-fuchs.com). "AS-Interface offers a safety solution—Safety at Work—that enables very detailed diagnostics down to the contact level," he offers. "Using this solution for safety applications, it is easy to find sticky contacts. All it takes is a few rungs of PLC logic."
There are more downsides to safety relays than just the lack of diagnostics from the relay, warns Kurt Wadowick, I/O systems specialist at Beckhoff Automation (www.beckhoff.com). "With outdated relay technology, safety system designers also need accurate electrical drawings, a trained electrician who can read those drawings and at least a digital multimeter to test the circuits that are in doubt," he explains. "In contrast, distributed and automated safety components with proper configuration can point out faults automatically. This enables the electrician to immediately understand which component has failed and focus attention on repairing the actual failure point instead of spending excessive time trying to find that failure."