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stage. If the deviation proceeds uncorrected, loss of control can lead to an accident event, such
as a vessel rupture explosion. Various protection systems, such as alarms, interlocks, and
emergency relief systems, may be employed to keep the accident event from occurring.
3.2.3 Review Previous Incidents
The PSM Rule requires all PrHAs to address "any previous incident which had a likely
potential for catastrophic consequences in the workplace," 29 CFR 1910.119(e)(3)(ii). An
incident is an unplanned event that may or may not result in injuries and/or loss. For example,
an incident might involve a flammable gas leak that does not ignite. An accident, on the other
hand, is an unplanned event that actually leads to personal injury, property damage,
environmental damage, and/or business interruption losses, such as the ignition of a flammable
gas leak resulting in burns and fire damage.
Previous accidents and incidents involving a process under study must be reviewed as part of
the PrHA. The importance of reviewing accident and incident records is discussed in the
anatomy of a process accident outlined in the preceding section (see Figure 3.3). Incidents can
indicate what could happen if protection systems, which are not totally reliable, do not work.
Thorough incident investigations may also indicate root causes of initiating events and
protective system failures and thus suggest action items to improve safety-management
systems. Incident records also help show the likelihood of failures and operational errors.
3.2.4 Analyze Controls and Control Failures
Process safety is the successful elimination or control of process hazards over the lifetime of a
process. Engineering and administrative controls must be in place to keep process parameters
within safe operating limits and to prevent challenges to system integrity. A PrHA addresses
engineering and administrative controls applicable to process hazards, as well as the
interrelationship of these controls, by identifying and documenting the process safety levels.
For example, the safety levels to keep a deviation from becoming an accident should be
documented in the protection (or safety levels) column of a HAZOP study worksheet when
that method is employed. The levels of protection to keep the accident from occurring are
included in a FTA as protective system branches which come together with initiator branches
at AND logic gates.
As examples of engineering and administrative controls, the PSM Rule lists "appropriate
application of detection methodologies to provide early warning of releases." For systems
handling toxic materials, detection methodologies are generally mitigation systems that reduce
the severity of consequences after an accident occurs.
Most PrHA methods study protection systems but do not explicitly study mitigation systems.
FTA looks at all events and combinations of events that could lead to a top event, such as

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