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
Posted by: Jean Capmeil
Date posted: Mon Mar 5 15:45:04 US/Eastern 2007
I need some help.
MTBF = [total operating time]/[number of failure].
If I know the [total maintenance time] can I say that :
[total operating time] = [total time] - [total maintenance time]? If yes where can I find the equation in Reliability Toolkit, MIL-HDBK-338, etc? Thanks
Reply Posted by: Ken
Date Posted: Mon Mar 5 17:17:37 US/Eastern 2007
Your approach to total operating time assumes that the equipment in question is operating 24/7. If this is true, then your approach is acceptable with assumptions.
1. Total maintenance time includes all downtime to include technician availability delays, parts delays, delay due to travel to the repair site, etc.
2. The only non-operating time is due to maintenance and maintenance related non-operating periods. Include periodic/scheduled maintenance non-operating periods as well. However, a scheduled maintenance event should not be included in an MTBF calculation, but included in an MTBM or availability calculation.
I do not know of a specific reference to your approach. Logically, it works if the assumptions are true and followed.
If the system is not operating 24/7, then an accurate log of on and off times or installation of an ETI (elapsed time indicator) that records operating time will be necessary. Note that an ETI will be recording time during maintenance diagnostics and repair verification as well.