Finding fault in design work undertaken by others may make you less popular at your work place! But a good RAMS engineer should be able to scrutinise design and find design issues, which could affect reliability. RAMS assurance is not just about RAMS deliverable but also about ensuring robustness of the products. In this week’s blog we look at how to manage vibration requirements for the products and how to deal with the practical issues of it.
Hardware qualification tests demonstrate that the products can work in harsh conditions. Hardware qualification typically involves supplier to meet the EN50125 requirements for the products. EN50125 typically for Europe, specifies harsher ranges of key environmental parameters such as temperature, humidity, the products should meet vibration/shock that shall.
While working in one of the previous projects to assess the life expectancy of the ageing ATC system, I have observed that the environment conditions play a great role in the reliability performance of the products. In that project I have taken a closer look at the in-service failure data by estimating reliability of the various products. I have found that in certain Signaling Equipment Rooms (SER) the ATC train detection and field control equipment experienced frequent failures compared to that of the rest of the SERs. A closer analysis had revealed that the increased failures are attributable to the higher vibration experienced at the SERs
Any issues with the products selected to withstand the application environment ideally should be resolved at the design stage and expected to have no cost involved to the client. It is acceptable to claim compliance to EN50125 requirements based on application of the products under consideration from their application in previous projects provided there is actual testing carried out for the previous project. It could be the case that in the previous project all signaling equipment rooms are beyond 3m from the rail and hence the vibration effects are considered negligible as per the standard and no testing was required. But in the current project if some of the SERs are within 3m from the rail then evidence of application in a previous project will not be sufficient.
Then there could be a solution that the supplier can do installation for all the SERs beyond 3m with equipment, which do not need to undergo any vibration testing and only plan to do testing for the equipment installed in the SERs, which are within 3m. This strategy has implications that you are expecting maintainer to keep spares separately for vibration-tested equipment and non-vibration-tested equipment over its life. It is not pragmatic to keep spares separately for vibration/non-vibration tested equipment. The choices the supplier has are
a) demonstrate compliance to EN50125 requirements by doing the required vibration requirements and use the tested configuration of the product in all SERs
b) Use of anti-vibration mounting on the equipment without changing its configuration and demonstrate compliance to EN50125 which has an advantage that installation work can progress before testing.
Also there can be an interesting interpretation of the vibration requirement from EN50125. The engineer may justify not doing vibration testing on the basis that if you measure point to point, only the circuit connection boxes falls within 3m and all other equipment in the SER are beyond 3m. Generally the SER is a pre-fabricated building. If any portion of the SER is within 3m, that vibration is expected to be transmitted to any equipment within the SER regardless of where they are located in the SER. Hence the requirement shall be interpreted such that if the whole of the SER is not beyond 3m then any equipment in the SER should be vibration tested to EN50125 requirements. Though EN50125 specifies vibration limits in all directions, it is not very clear about the application of the requirements in a pre-fabricated cabin, ground, fixed building. Location of the equipment in fixed building/prefabricated building or ground significantly affects the transmission of vibration from the rail.
A RAM engineer constantly faces the challenge of ensuring future system reliability by influencing the design within the constraints of the delivery date and cost. A RAM engineer’s responsibility is to make sure that the right measure to ensure reliability of the system is taken at the correct stage of the project. The effort spent in RAM assessment during the early design stage and the resolution of the design issues can result in whole life cost savings once the system is operational.