Some designs push their operating specifications right up to the limit of the components in the system or device. (let’s not talk about the designs that try to spec a limit that is beyond the capability of one or more components… phooey on them!) And if you make a product that is not used in a critical application or in environmental extremes, this may not be a big deal. But in the safety world, we’re better than that! Not only should you keep the operating specs below the limits of the components, you should keep the specs at a comfortable margin below those limits. IEC 61508-2 suggests (but does not mandate) a two-thirds derating factor be applied to components of a safety certified product. And IEC 61508-2 requires justification for using components AT their rated specs. Test results are often cited as a justification for meeting the limits. (We tested at 85C, it works at 85C, and we spec it for 85C!) Yes, specs can be a competitive advantage. Yes, most customers will never approach the max limits of your product. But there are those customers who will push these operating specs and go past the limits on a regular basis, even if not intended.
It’s important to remember that most testing considers only one type (or perhaps two types) of stress in a given test sequence. When your device or system is operating out in the field, it is bombarded with stress from many directions… simultaneously! And you may not know the fine details of the component construction that led the vendor to adopt the particular limits. It is a well-known axiom that failure occurs when stress exceeds strength. You really owe it to your design, and your customers, to be conservative with component ratings and derate by 50% to 70%, especially for complex or high risk components. Give your products the edge against temperature, shock, vibration, EMC, corrosion, and the like. Remember that your product may be the last line of defense in a dangerous process.