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Probabilistic Safety Analysis of High Speed and Conventional Railway Lines

Table 2 Sets of frequencies used in the standards

Frequency Description

Frequent It may happen frequently. The threat will be played continuously.

Probable It will happen several times. The threat can often be expected to occur.

Occasional It may happen several times. The threat can be expected to occur several times.

It may occur during the system life cycle. It can be reasonably assumed that the
Remote
threat will occur.
Slight chance but possible. It can be assumed that the threat can occur
Improbable
exceptionally.

Incredible Extremely unlikely. It can be assumed that the threat will not occur.

For example, when referring to events that occur frequently or several times, it is not indicated
if this refers to one day, one month, one year or the life of the system being analyzed. Similarly,
the meaning of slight chance or extremely unlikely are not quantified, which is the only way of
avoiding a misinterpretation of the codes.
Once frequencies and severity levels of their consequences of given events are determined,
Table 1 allows to determine the risk levels, which can be classified as: Intolerable, Undesirable,
Tolerable, and Insignificant.
Table 3 shows the required action to be taken under any risk level, which has a relevant
role in the safety assessment of hazardous events. Thus, careful attention must be paid to
the different actions to be carried out for each risk level: “Intolerable”, “Undesirable”,
“Tolerable” and “Insignificant”, resulting from the combinations of the different frequencies
(“Frequent”, “Probable”, “Occasional”, “Remote”, “Unlikely”, or “Incredible”) and severity
levels (“Insignificant”, “Minimum”, “Critical” or “Catastrophic”).

Table 3 Required actions associated with the different risk levels

Risk level Required action

Intolerable It must be removed
It will be accepted only when the risk reduction is impracticable and in
Undesirable
agreement with the Railway Authority (ADIF)
Acceptable with proper control and in agreement with the Railway Authority
Tolerable
(ADIF)

Insignificant Acceptable without any agreement

It is surprising that in Table 1 the levels of “Critical” and “Catastrophic” risk associated with
an “Incredible” frequency are associated with an “insignificant” level of risk, that is, a risk
acceptable without any agreement. Even “Critical” and “Catastrophic” risk levels associated
with “Improbable” occurrence frequencies are associated with a “Tolerable” risk level, that is,

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