Information Technology Reference
In-Depth Information
line, the amount of transport, the track layout in the station, the initial cost
and so on, and decide on a system architecture. A signalling system based
on information technology will be introduced more actively in the future.
Therefore, a systematic method of evaluating a signalling system will be
more important in the development phase.
The relationship between Reliability, Availability, Maintainability and
Safety are described in the RAMS standard. According to the definition,
an availability and safety are the main indicators. As for safety, various tech-
nologies have been developed and applied, and signalling systems have an
excellent safety record. In recent years, safety analysis is generally carried
out by some systematic methods (e.g., FTA and FMEA), with appropri-
ate measures being applied to reduce risks. On the other hand, there is no
established concrete method applicable to comprehensive evaluation of the
signalling system concerning availability. Furthermore, there is a growing at-
tention to the stability of railway transport.
Therefore, we assume that any system based on proven safety technologies
and safety analysis achieves a sucient level of safety, and have focused on
availability, which consists of reliability and maintainability.
3
Evaluation Method
3.1
Basic Concepts
There are several measures to express an availability. For example, it is ex-
pressed as the ratio of Mean Time Between Failure (MTBF) to sum of MTBF
and Mean Time To Repair (MTTR). Although this definition expresses a per-
formance of each equipment, the relationship between equipment failures and
their impact on railway trac service is not clear. Therefore, we propose a
method to evaluate system availability based on the following concepts.
1. Estimating the impact on train operation caused by an equipment failure
(a component of signalling system) .
2. Converting the impact to a loss (cost).
The proposed method will be useful to support decisions on the system ar-
chitecture adapting to conditions of a line or station, making it possible to
evaluate the system's life cycle cost by adding the loss to the initial and
maintenance costs.
Among those three costs, initial and maintenance costs can be estimated
if the system architecture is defined. However, the loss caused by failures
should be derived considering the track layout in the station, train schedule
and other conditions. The method to estimate the loss is described in the
following section.
