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Safety engineering

Safety engineering is used to assure that a system behaves as needed even
when pieces fail.

Fault modeling techniques

The two most common fault modeling techniques are called "failure modes and
effects analysis" and "fault tree analysis." UML activity diagrams can be
used as graphical components in a fault tree analysis. These techniques are
just ways of finding problems and of making plans to cope with failures.

Failure modes and effects analysis

In the technique known as "failure modes and effects analysis", an engineer
starts with a block diagram of a system. The engineer then considers what
happens if each block of the diagram fails. The engineer than draws up a
table in which failures are paired with their effects and an evaluation of
the effects. The design of the system is then corrected, and the table
adjusted until the system is not known to have unacceptable problems. Of
course, the engineers may make mistakes. It's very helpful to have several
engineers review the failure modes and effects analysis.

Fault tree analysis

In the technique known as "fault tree analysis", an undesired effect is
taken as the root of a tree of logic. Then, each situation that could cause
that effect is added to the tree as a series of logic expressions. When
fault trees have real numbers about failure probabilities (often unavailable
because of testing expense), computer programs can calculate failure
probabilities from fault trees. The classic computer program is the Idaho
National Engineering and Environmental Laboratory's SAPHIRE, which is used
by the U.S. government to evaluate the safety and reliability of nuclear
reactors, the space shuttle, and the International Space Station.

Safety certification

Usually a failure in safety-certified systems is acceptable if less than one
life per 30 years of operation (109 seconds) is lost to mechanical failure.
Most Western nuclear reactors, medical equipment, and commercial aircraft
are certified to this level.

Preventing failure

Adding equipment and systems

Once a failure mode is identified, it can usually be corrected by adding
equipment to the system. For example, nuclear reactors emit dangerous
radiation and contain nasty poisons, and nuclear reactions can cause such
high heat that no substance can contain them. Therefore reactors have
emergency core cooling systems to keep the heat down, shielding to contain
the radiation, and containments (usually several, nested) to prevent leakage.

Redundancy

For any given failure, a fail-over, or redundancy can almost always be
designed and incorporated into a system.

Fail-safe design

When adding equipment is impractical (usually because of expense), then the
design has to made inherently safe, or "fail safe". The typical approach is
to arrange the system so that ordinary single failures cause the mechanism
to shut down in a safe way. For example, in an elevator the cable supporting
the car pulls spring-loaded brakes open. If the cable breaks, the brakes
grab rails, and the car does not fall. Another common fail-safe system is
the pilot-light sensor in most gas furnaces. If the pilot light is cold, a
mechanical arrangement disengages the gas valve, so that the house cannot
fill with unburned gas. Fail safes are common in medical equipment, traffic
and railway signals, communications equipment, and safety equipment.

The safety engineer

Personality and role

Oddly enough, personality issues can be paramount in a safety engineer. They
must be personally pleasant, intelligent, and ruthless with themselves and
their organization. In particular, they have to be able to "sell" the
failures that they discover, as well as the attendant expense and time
needed to correct them.

Safety engineers have to be ruthless about getting facts from other
engineers. It is common for a safety engineer to consider software,
chemical, electronic, eletrical, mechanical, procedural, and training
problems in the same day. Often the facts can be very uncomfortable.

Teamwork

It is important to make the safety engineers part of a team, so that safety
problems cannot be discounted as due to the safety engineers' personality
problems or ignored by firing a single engineer.

It is a severe safety problem if an engineering team or management
discredits a safety engineer: either the manager apppointed a poor engineer
to the position, indicating that there may be numerous undiscovered safety
issues, or the team has inverted development priorities and considers safety
to be less important than upper management or government does.

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