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speed supervision, control of point position, complete and permanent loss of
the brake system, or loss of the traction cut-off function. Further explanations
are given by Braband (2007).
This means that formally RAC-TS relates only to potentially catastrophic
accidents. In order to apply it also to other severity categories, for example,
a risk matrix could be used. However, such a matrix would need sound jus-
tification.
This paper seeks to give formal justification in order to move a step for-
ward towards the harmonisation of risk matrices.
2
Risk Matrix
Risk matrices are a well-known tool in risk assessment and risk classification,
also in the railway domain (see for example EN 50126 (1999) or Braband
(2005)). Table 1 gives a typical example. The major drawbacks of such risk
matrices are:
- Risk matrices must be calibrated to their particular application.
- The results depend on the system level to which they are applied.
- The parameter classes must be concisely defined in order to avoid ambi-
guity and misjudgements.
- It must be defined which frequency is meant, e.g. accident or hazard
frequency.
However, if these drawbacks can be overcome, risk matrixes are a well-
accepted and easy-to-use tool, also for risk prioritisation (see the rank num-
bers in Table 1).
Tabl e 1 . Typical Risk Matrix
Frequency
Often
10
6
3
1
Probable
14
9
5
2
Occasional
18
13
8
4
Rare
21
17
12
7
Improbable
23
20
16
11
Unbelievable
24 22 19 15
Negligible Marginal Critical Catastrophic
Severity
3
System Definition
The discussion in this paper focuses on technical systems only. A technical
system is a product developed by a supplier including its design, implemen-
tation, and support documentation. It should be noted that:
 
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