201703 – McDonald – Is RAMS All BULL for Electromechanical Equipment?

Railways are required to operate safely and one of the methods to demonstrate this is type approval of signalling equipment. That approval must include documentation of high RAMS (Reliability, Availability, Maintainability and Safety) when applied in vital and even non-vital applications. Suppliers have provided such values, in some form or another, for electrical, electronic and programmable electronic equipment for many years. The limitations and applicability of these values have not always been well understood and they have often been misapplied. The decisions for product comparison or maintenance plans could therefore be compromised or invalid.

More recently, purchasers, and personnel assessing type approval are demanding values such as SIL (Safety Integrity Level) and MTBF (Mean Time Between Failure) for electromechanical equipment and systems. The standards currently used for programmable electronic systems clearly state that using them to derive values for electromechanical is inappropriate.

This paper delves into the importance of understanding and applying meaningful RAMS values for signalling equipment and addresses the inappropriateness of SIL and MTBF for Electromechanical Equipment. It continues to offer some suggestions for how RAMS can be used for Electromechanical Equipment.