Reliability may be defined as the probability, or degree of confidence that a product will perform a specified number of times under prescribed conditions. For example, the reliability of an electrical changeover switch may be defined as 0.9999. This may hold true only when it is operated with an input voltage of 440 volts AC, in an environmental temperature range of 0 to 80 degrees C, with humidity less than 90 percent, if its housing has never been opened, if it has been operated less than 1 million times, and if its is less than five years old. Even if any one of these conditions is violated, then the reliability concepts goes off. When parts are interdependent, and especially when there is a large number of them, achieving a high degree of reliability for a product presents and real challenge.
Maintainability refers to the ability of the user to keep the product or system in operating condition with a reasonable amount of effort. This ability to maintain operation may entail the availability of some required service from the manufacturer or authorized repair facility, provision of a stock of replacement parts available to the user, and written maintenance and repair procedures. Good product design for maintainability usually implies ease of product dis-assembly and case of access to within the product to facilitate routine service or replacement. Switches, valves, motors with brushes, oil fills, etc. should be located for ready across, with removable covers placed at convenient locations. The alternative to providing maintaining is to “over-design”. This approach reduces the need for repair but raises production costs.