Page 67 - 捷運技術 第43期

捷運技術半年刊 第43 期 61

these requirements should ensure the identification and understanding of all known hazards and
their associated risks, and mishap risk eliminated or reduced to acceptable levels. The objective of
system safety is in fact to achieve acceptable mishap risk through a systematic approach of hazard

analysis, risk assessment, and risk management.
Typical activities detailed in an SSPP include the following:
h safety management addressing organisational and other aspects for ensuring compliance

with the safety program;
h hazard analyses and associated risk assessment to identify and assess hazards and their
relative risks in all phases of life cycle;

hsafety V&V to demonstrate the level of safety.
The hazard severity and hazard probability definitions in MIL-STD-882 are very similar to
those in EN 50126, but there are some slight differences. For example, MIL-STD-882 describes five
categories for hazard probability, while CENELEC has six. Some categories for both frequency and

severity levels do not fully mach in definition.

Table 1 Accident Hazard Indices / Safety Assurance Levels according to MIL-STD-882D
FREQUENCY LEVEL

SEVERITY LEVEL Frequent Probable Occasional Remote Improbable

Catastrophic High High High Serious Medium

Critical High High Serious Medium Medium

Marginal Serious Serious Medium Medium Low

Negligible Medium Medium Low Low Low

MIL-STD-882 is risk-based as are EN ones, but, unlike them, is not based on SILs.
On Hazard Tracking and Risk Resolution Process MIL-STD approach foresees the following steps:

a. Identification of Hazards;
b. Assessment of mishap risk;
c. Identification of mishap risk mitigation measures;

d. Reduction of mishap risk to an acceptable level.
e. Verification of mishap risk reduction.
In general MIL-STD-882 does not describe a definitive safety program, fixed set of activities,
fixed safety case or set of associated documentation, but rather is tailored to the needs of a specific

development program. However, the standard is very similar to EN 50126 with respect to the safety
management aspects addressed.