The Fail-Safe Feature of the Lapp & Powers Fault Tree

In response to the Editor's suggestion, 3 states of the Lapp & Powers fault-tree model are distinguished, `good', `failed safe' and `failed unsafe'. Both the logical and the probability functions are given separately for both failed safe and good events as well as the probability calculations. A simplified 2-component example is also discussed to illustrate the relationship between XOR gates in fault trees and fail-safe systems and subsystems.