Module 8 - Random Failure Measures Flashcards
What is reliability?
The ability of a system to function properly over a given time period?
What is availability?
The probability of a system to function properly at a given instant
What is maintainability?
The ease with which a system can be maintained
What is dependability?
A measure of the system’s reliability, availability, and maintainability
What is failure rate? What is the symbol for average failure rate?
Failure rate is the number of failures per unit time. Symbol Lambda
Is failure rate constant?
No. Failure rate follows a bathtub curve
What are the two methods of finding faults within a component?
- Diagnostics
2. Proof tests
What are diagnostics?
High frequency automatic tests internal to the component
What are proof tests?
Low frequency tests usually run manually to evaluate a component
What are the two components contained within the average failure rate?
Safe and dangerous failures (L = Ls + Ld)
What is the difference between safe and dangerous failure?
Safe failures cause spurious trips, dangerous failures prevent safety functions
How do diagnostics impact the average failure rate?
They further divide the safe and dangerous failure rate into detected and undetected failures for each type (Ls = Lsd + Lsu, Ld = Ldd + Ldu)
What is diagnostic coverage?
The fraction of dangerous failures detected by diagnostics (DC = LDD/LD)
What is safe failure fraction?
A measure of how fail-safe a device is.
SFF = (Ls + Ldd)/(Ls + Ldd + Ldu)
SFF = (Ls+Ldd)/L
What is the purpose of safe failure fraction?
- To calculate Ldu from L
2. To determine hardware fault tolerance (architectural constraints) from IEC 61508 Route 1H
What are the two methods to calculate Ldu?
- SFF –> Ldu = (1-SFF)*L
2. DC –> Ldu = (1-DC)*Ld
Where do Lambda values come from?
From an FMEA performed by the manufacturer and supplied on a datasheet
What is formula for PFDavg?
PFDavg = Ldu * TI/2
What is the relationship between PFDavg and proof test interval?
Directly proportional
What is the relationship between L and MTTF?
Inverse of each other
What is the relationship between repair rate Mu and MTTR
Inverse of each other
What is an alternative statement of PFDavg?
The fraction of time the component is unavailable
PFDavg = MTTR/MTBF
What is an alternative statement of TI
MTTR = TI/2 + MRT
What are the three main sources of reliability data?
- Operating company experience
- Databses such as OREDA
- Manufacturers data
