lørdag den 16. august 2008

RAMS and how to control it


EN 50126 is all about controlling the RAMS parameters of a Railway system (e.g. a complete train, an LED lamp etc.).

It appears directly from the title: “Railway applications - The specification and demonstration of Reliability, Availability, Maintainability and Safety (RAMS)”.
The RAMS parameters are linked as shown at Figure 2:



Interpretation
The RAMS parameters are useful when categorizing different items e.g.:
  • the requirements to and specifications of the system
  • faults and findings during design and service.
This way, all parties (Operator, Supplier, Safety Authority, Assessor) know what we are talking about if e.g. an error is disclosed during testing: Is the fault a Reliability issue, an Availability issue, a Maintainability problem or a Safety problem.

Lets say we have a new train ready and approved for operation, but some errors exists. The errors have been categorized as Reliability issues, which are not directly safety-related. In this case Figure 2 above would look as shown on the left:

The yellow "Reliability" in the bottom will cause a Yellow "Availability" in the middle, which again will cause a yellow top level "Railway RAMS".


Since we have a green "Maintainability" in the bottom, it might be possible to increase the "Maintenance" work and hereby compensate for the yellow "Reliability", so we obtain a green "Availability", which again will cause a green top level "Railway RAMS". See the Figure below.

This is controlling RAMS!






Next chapter >> 2.2 The V-model






From the Source (EN 50126)

The links are described more detailed in chapter 4.3.2 and 4.3.3 in EN 50126:1999:

"Safety and availability are inter-linked in the sense that a weakness in either or mismanagement of conflicts between safety and availability requirements may prevent achievement of a dependable system. The inter-linking of railway RAMS elements, reliability, availability, maintainability and safety is shown in figure 2."
"Attainment of in-service safety and availability targets can only be achieved by meeting all reliability and maintainability requirements and controlling the ongoing, long-term, maintenance and operational activities and the system environment."

A more elaborated version of Figure 2 is given in Figure 5 (not shown here).

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