## 4 Classification into RPN Groups by Analysis of Risk Priority Number (RPN)  
### RPN Groups  
To analyze and identify the cause of failure of each component for risk management via FMEA and quantify the scores of occurrence, sensitivity, and detection to demonstrate the necessity for group management by component, the risk priority number (RPN) was calculated as the product of occurrence, sensitivity, and detection, as shown in Figure 16. In Step 1, the failure frequency distribution, was calculated based on the probability number. Step 2 was to determine the sensitivity number, which represents the quantity according to the component by frequency. Step 3 involved the detection number, which is an evaluation and analysis of the data by function and cause of failure. In the rail industry, the procedure used is based on risk priority number (RPN)  
Figure 16: FMEA cycle. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Score** & **Meaning** & **Range (\%)** \\ \hline \hline  & Distribution of numerical scores of detection & \\
1 & methods such as visual inspection & 0 \(<\) Level of difficulty \(<\) 5 \\  & Low & \\
2 & Medium & 5 \(\leq\) Level of Difficulty \(<\) 10 \\
3 & High & 10 \(\leq\) Level of Difficulty \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Detection score estimation criteria. When selecting the RPN groups for each component, the results of the analysis of existing maintenance records data and survey results from the expert group were reflected in terms of weights. Table 8 shows the criteria for calculating the score for classification into RPN groups. Considering the criteria for scoring, the score for group C was set at 20 points, which was the minimum requirement, or to higher values. This group was set as the score group for the range within which the damage occurrence could be reduced by modifying and improving components to obtain efficiency in maintenance. For group B, the score distribution range was set to 10-20 points, which is the range of scores for the planning of improvement measures. This scoring range represents the scores that could be advanced to group A by improving and modifying the components. Finally, RPN group A refers to the group of components that do not need immediate corrective actions for improvement and for which maintenance can be performed through routine inspection. As shown in Table 9, the components that corresponded to group C, which had the highest priority for preventive maintenance, included the crossing, switch toes, joint bolt, and heel block bolt, with RPN scores between 24-75 points. Meanwhile, the components in group B included the normal rail, closure rail, stock rail, switching bolt, wooden tie, crossing baseplate, and check rail bolt, with RPN scores in the range of 12-18 points. Finally, group A comprised 12 components, including the check rails, with RPN scores between 2-8 points. Based on the efforts of the maintenance engineers in the inspections and improvements to reduce the failure occurrence of rails and bolts in RPN group C, in addition to the baseplate, wooden tie, and bolts in RPN group B, it was determined that groups B and C could be advanced to groups A and B. \begin{table}
\begin{tabular}{c c c c} \hline
**Category** & **C Group** & **B Group** & **A Group** \\ \hline Score range & 20 \(\leq\) RPN & 10 \(\leq\) RPN \(<\) 20 & 0 \(<\) RPN \(<\) 10 \\ Maintenance & Immediate corrective actions & Planning for improvement & \\  & for improvement & measures & Routine inspection \\ \hline \end{tabular}
\end{table} 
Table 8: Classification of RPN groups.