SRC Forum - Message Replies
Forum: Reliability & Maintainability Questions and Answers
Topic: Reliability & Maintainability Questions and Answers
Topic Posted by: Reliability & Maintainability Forum
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
Organization:Design Analytx Int'l.
Date posted: Wed Jul 26 17:04:27 US/Eastern 2000
Subject: Availability Calculations
Problem: In modeling for Operational Availability (Ao) in a single redundant system, when a failure occurs the redundant spare switches in after fifteen seconds. I want to model for the probability of a second failure while the first is being switched/active maintenance begins. The latter begins after the system is restored by the redundant spare.
What happens to my availability during the period of active repair? It is my contention that the Availability equation should be replaced by System Reliability equation once the failed unit is replaced by the redundant spare, thereby removing any further redundancy for any future event until a working spare is put in place.
Where should I go to research further. I have most texts and they don't treat this subject in enough detail to satisfy my need for in-depth details.
Subject: Availability Calculation
Reply Posted by: Bruce Dudley
Organization: Reliability Analysis Center
Date Posted: Thu Jul 27 13:22:17 US/Eastern 2000
Availability information, including model/equations, can be found in the Mil-Hdbk-338B "Electronic Reliability Design Handbook". In section 10 of this handbook, system models for inherent or achieved availability, operational availability, instantaneous/point availability and steady state availability are described and presented. For your conditions an operational availability that includes mean-interval-between-corrective and preventive maintenance, mean down time and ready time should allow you to calculate your system availability. this equation is:
Ao = MTBM + RT / MTBM + RT + MDT