end up leaking flammable or toxic materials,” sums up Bill Goble, principal partner at Exida.com, a Sellersville, Pa.-based safety training and consulting firm.

The variety of Safety PLC architectures and differing approaches on the market can be confusing for end users, observes Scott, at integrator AE Solutions. Safety PLCs suitable for SIL 3 system applications are often configured in 1 out of 2D, 2 out of 2D, 2 out of 4D and triply redundant varieties, while simplex, single-processor units are often sold for use in lower-level SIL 1 or SIL 2 systems. Each approach has its advantages and disadvantages. “The vendors are all fighting architecture wars, saying, ‘Mine’s better than yours,’ ” Scott says.

The “Rolls Royce” of Safety PLCs for the process industries, according to Scott, are the systems from Triconex, in Irvine, Calif., that use three processors as well as triply redundant I/O. “If I have one processor die, it’s no big deal, I degrade to a one out of two scheme. And if another processor dies, I degrade to a one out of one and I’m still making product,” Scott says.

But Scott is quick to add that depending on a user’s application, the high-end Triconex system is not always the way to go. “You need to do a life cycle cost analysis to make sure that you can overcome the initial capital cost, because that Rolls Royce costs you a lot of money,” he says.

At Triconex, a unit of London-based Invensys, Marketing Vice President Bill Barkovitz concedes that his company’s triple modular redundancy (TMR) systems generally sell for a price premium over competitive products. “We would typically be in the range of 10 percent to 20 percent higher than a 1 out of 2D-type system,” he says.

But Barkovitz argues that the premium is minimal, given the overall cost of a typical process industry safety system. “When you include the design, the programming and commissioning, the installation and everything that goes with it, the cost of the hardware is maybe 10 percent of the whole project,” he says. Besides, Barkovitz adds, “we don’t really sell on price. Our customers buy our product because it’s high quality and very easy to use, and for all of the benefits they get with TMR technology.”

See sidebar to this article: Safety PLC market shares