Steady-state desires availability

In repairable systems, the steady-state availability is one of the more important factors in specifications and choice of hardware. Some (not all) books, and some expensive courses on reliability, suggest that the steady-state availability of a series system is found by the product rule as for reliability. This is wrong. When a series system fails due to the failure of any one of its components, all the other components take a rest and are therefore not at risk of failure. It might be thought that the error is very small. For small, highly available systems so it is, but for long series systems the difference can be crucial to the assessed economics of the system; that is, it can make the difference between project acceptance and rejection. The author's objective in this paper is to stimulate debate so that the assumptions and restrictions often made, will emerge, and some standard text-books will be challenged.