High Availability Theoretical Basics - Schneider Electric

More documents

Recommendations

Info

Bathtub Curve High Availability Theoretical Basics The following figure shows the Bathtub Curve, which represents the Failure Rate (λ) according to the Lifetime (t): Consider the relation between Failure Rate and Lifetime for a device consisting of assembled electronic parts. This relationship is represented by the "bathtub curve". As shown in the previous diagram. In "early life," this system, exhibits a high Failure Rate, which gradually reduces until it approaches a constant value, maintained during its "useful life.” The system finally enters the “wear-out” stage of its life, where Failure Rate increases exponentially. Note: Useful Life normally starts at the beginning of system use and ends at the beginning of its wear-out phase. Assuming that "early life" corresponds to the ”burn- in” period indicated by the manufacturer, we generally consider that Useful Life starts with the beginning of system use by the end user. RAMS (Reliability Availability Maintainability Safety) The following text, from the MIL-HDBK-338B standard, defines the RAM criteria and their probability aspect: "For the engineering specialties of reliability, availability and maintainability (RAM), the theories are stated in the mathematics of probability and statistics. The underlying reason for the use of these concepts is the inherent uncertainty in predicting a failure. Even given a failure model based on physical or chemical reactions, the results will not be the time a part will fail, but rather the time a given percentage of the parts will fail or the probability that a given part will fail in a specified time." Along with Reliability, Availability and Maintainability, Safety is the fourth metric of a meta-domain that specialists have named RAMS (also sometimes referred to as dependability). The Bathtub Curve 10
Metrics High Availability Theoretical Basics RAMS metrics relate to time allocation and depend on the operational state of a given system. The following curve defines the state linked to each term: • MUT: Mean Up Time MUT qualifies the average duration of the system being in operational state. • MDT: Mean Down Time MDT qualifies the average duration of the system not being in operational state. It comprises the different portions of time required to subsequently detect the error, fix it, and restore the system to its operational state. • MTBF: Mean Time between Failure MTBF is defined by the MIL-HDBK-338 standard as follows: "A basic measure of reliability for repairable items. The mean number of life units during which all parts of the item perform within their specified limits, during a particular measurement interval under stated conditions." Thus for repairable systems, MTBF is a metric commonly used to appraise Reliability, and corresponds to the average time interval (normally specified in hours) between two consecutive occurrences of inoperative states. Put simply: MTBF = MUT + MDT MTBF can be calculated (provisional reliability) based on data books such as UTE C80-810 (RDF2000), MIL HDBK-217F, FIDES, RDF 93, and BELLCORE. Other inputs include field feedback, laboratory testing, or demonstrated MTBF (Operational Reliability), or a combination of these. Remember that MTBF only applies for repairable systems 11
Page 1: System Technical Note How can I...
Page 4 and 5: Introduction to High Availability 4
Page 6 and 7: Introduction to High Availability A
Page 8 and 9: Guide Scope High Availability Theor
Page 12 and 13: • MTTF (or MTTFF): Mean Time to F
Page 14 and 15: Availability Definition Mathematics
Page 16 and 17: Classification Reliability Definiti
Page 18 and 19: High Availability Theoretical Basic
Page 20 and 21: Serial RBD Reliability High Availab
Page 22 and 23: Calculation Example High Availabili
Page 24 and 25: High Availability Theoretical Basic
Page 26 and 27: Redundant System Reliability, over
Page 28 and 29: Step 1: Calculation linked to the S
Page 30 and 31: Conclusion Serial System High Avail
Page 32 and 33: Information Management Level Redund
Page 34 and 35: High Availability with Collaborativ
Page 36 and 37: . Clients: Operator Workstations Hi
Page 38 and 39: Target: I/O Device High Availabilit
Page 42 and 43: Plant Topology High Availability wi
Page 44 and 45: Ring Topology High Availability wit
Page 52 and 53: Fast HIPER-Ring High Availability w
Page 54 and 55: Control System Level Redundancy Pri
Page 56 and 57: In-Rack and Remote I/O Architecture
Page 60 and 61:
High Availability with Collaborativ
Page 62 and 63:
Distributed I/O Architectures Ether
Page 64 and 65:
Profibus Architecture High Availabi
Page 66 and 67:
Redundancy and Safety High Availabi
Page 68 and 69:
Controlled Switchover High Availabi
Page 70 and 71:
Switchover Latencies High Availabil
Page 72 and 73:
Conclusion Quantum Hot Standby Esse
Page 74 and 75:
Benefits Standard Offer Conclusion
Page 76 and 77:
Appendix Glossary Appendix Note: th
Page 78 and 79:
15) Hidden Failure FTA illustration
Page 80 and 81:
26) Non-Detectable Failure Appendix
Page 82 and 83:
Standards General purpose Appendix
Page 84 and 85:
Calculation Examples The calculatio
Page 86 and 87:
Redundant Architecture Calculation
Page 88 and 89:
Redundant PLC System, Using Shared
Page 90 and 91:
Calculation Examples For this Seria
Page 92 and 93:
Consider a common misuse of reliabi
Page 94:
Schneider Electric Industries SAS H
show all

High Availability Theoretical Basics - Schneider Electric

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?