Comment on: On the Availability of 1-out-of-2 Standby Systems

The approximation (a1 + a2)/(1 + a1 a2) for the availability of a 1-out-of-2: G standby system with unit availabilities a1, a2 is exact when no preference is shown between the two units for equal failure rates and different constant repair rates. the result was shown by Sherwin to be an approximation where preference is given for one unit over the other. The result is useful since it allows studies of system planning in terms of the simple expression for availability.