Definition:
Mean Repair Time (MRT) is the average time required to repair and restore a system to operational condition after a failure is detected. It does not include time to detect. Only the time to plan the repair, execute the repair, and restore.
- a – time to detect the failure
- b – time spent before starting the repair
- c – effective time to repair
- d – time after repair to put back in service.
- MTTR = a + b + c + d –> from the failure itself to the restoration
- MRT = b + c + d –> meaning MRT does not include time to detect.
- MTTR = a + MRT
- MDT = a –> mean detection time
MRT impacts PFDavg calculations in the event that proof testing is done in bypass. The logic is… a SIF is put in bypass (meaning the process is still running without protection), a defect is found, the defect is repaired while in bypass, and the item is restored.
Key Points:
- Critical input for system availability analysis.
- Affects operational downtime.
- This is a key input to the decision of an acceptable maximum permitted response time.
- MTTR is part of PFDavg for systems with diagnostics. MTR may be part of for systems with and without diagnostics.
Example:
A plant has a SIF with 2oo3 instruments. They have inventory in stock, technicians are always on call, and the repair is straightforward. However, they have a badly managed work order approval process where it takes days to approve, which would impact the “b” term. Thus, 3 days may be needed.
See Also: MTTR, MPRT, MTTR Blog Post
Cited Source:
- IEC 61511-1:2016, Clause 3.2.37.1
- Limble article – Guide to Understanding Failure Metrics