2.1 Condition Monitoring Definitions and Capabilities
For the purposes of asset condition monitoring, a failure is defined as any unplanned event which results in an asset being unavailable for its designed purpose.
A fault is a condition of an asset which causes it to deviate from its normal behaviour. This may or may not cause a failure, depending on both the severity of the fault and the functional area of the asset affected by the fault. For example, a loose screw on a luggage shelf may cause rattling in the saloon of a passenger vehicle; this is a fault, because the components are behaving abnormally, but it will not necessarily cause a failure, because it does not stop the train from functioning properly or safely.
An incipient fault is a fault which develops gradually over a period of time. An example might be the loosening of non-threadlocked bolts or screws when subjected to prolonged vibration. Incipient faults are difficult to detect early using automatic methods, and therefore have a tendency to cause failures or only be detected at a late stage of development.
In condition monitoring, the terms fault detection, fault diagnosis and fault identification have distinct meanings. Detection means determining purely whether or not a fault is present in an asset. Diagnosis means to decide what fault may be present. Identification means to determine the severity of the fault.
These capabilities can be added in layers to provide progressively higher levels of condition monitoring. As further layers are added, automated systems can take on more responsibility and gradually improve asset availability by reducing time spent in reactive maintenance, diagnosis and by optimally scheduling maintenance tasks.
The flow diagrams in Figure 1 show how tasks can be split between human maintainers and automated condition monitoring systems for five levels of capability which have been defined as part of the INNOTRACK project (Silmon 2009). Table 1 categorises the capabilities of the condition monitoring system shown in Figure 1.
Figure 1. Capability levels and the interaction between maintainers and automated systems
Table 1. Capabilities of a condition monitoring system from Figure 1
| Capability Level | Description |
| 0 | Failure detection |
| 1 | Fault detection |
| 2 | Fault detection and diagnosis |
| 3 | Fault detection, diagnosis and time-to-failure calculation |
| 4 | Automatic maintenance scheduling |
Currently the most advanced condition monitoring systems available for railway applications have capability level 2. The capability depends partly on the availability of a suitable algorithm or model for the asset being monitored, and partly on the opportunity for analysis or training of the models, which must be done in co-operation with the asset manager.