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something significant)
Step 5. Develop improved controls and implement as appropriate
Electrical
Kinetic (moving mass e.g. a vehicle, a machine part, a bullet)
Potential (not moving mass e.g. a heavy object suspended overhead)
Chemical (e.g. explosives, corrosive materials)
Noise and Vibration
Thermal (heat)
Radiation (Non-ionizing e.g. microwave, and ionizing e.g. nuclear radiation, x-rays)
Pressure (air, Hydraulic, water)
FAA System Safety Handbook, Appendix F
December 30, 2000
F-34
Figure 1.3.1C Example ETBA
Scenario: The supervisor of a maintenance facility has just investigated a serious incident
involving one of his personnel who received a serious shock while using a portable power drill in
the maintenance area. The tool involved used a standard three-prong plug. Investigation revealed
that the tool and the receptacle were both functioning properly. The individual was shocked when
he was holding the tool and made contact with a piece of metal electrical conduit (it one his drill
was plugged into) that had become energized as a result of an internal fault. As a result the
current flowed through the individual to the tool and through the grounded tool to ground resulting
in the severe shock. The supervisor decides to fully assess the control of electrical energy in this
area.
Option 1. Three prong tool. Electrical energy flow that is from the source through an insulated
wire, to the tool, to a single insulated electric motor. In the event of an internal fault the flow is
from the case of the tool through the ground wire to ground through the grounded third prong
through a properly grounded receptacle.
Hazards: Receptacle not properly grounded, third prong removed, person provides lower path of
resistance, break in any of the ground paths (case, cord, plug, and receptacle). These hazards are
serious in terms of the frequency encountered in the work environment and might be expected to
be present in 10% or more cases.
Option 2. Double insulated tool. The tool is not grounded. Protection that is provided by double
insulating the complete flow of electrical energy at all points in the tool. In the event of an internal
fault, there are two layers of insulation protection between the fault and the person preventing
shorting through the user.
Hazards: If the double layers of insulation are damaged as a result of extended use, rough
handling, or repair/maintenance activity, the double insulation barrier can be compromised. In the
absence of a fully effective tool inspection and replacement
program such damage is not an unusual situation.
Option 3. Grand Fault Circuit Fault Interrupters. Either of the above types of tools is used
(double insulated is preferred). Electrical energy flows as described above in both the normal and
fault situations. However, in the event of a fault (or any other cause of a differential between the
potential of a circuit), it is detected almost instantly and the circuit is opened preventing the flow
of dangerous amounts of current. Because no dangerous amount of current can flow the individual
using the tool is in no danger of shock. Circuit interrupters are reliable at a level of 1 in 10,000 or
higher and when they do fail, most failure modes are in the fail-safe mode. Ground Fault circuit
fault interrupters are inexpensive to purchase and relatively easy to install. In this case, the best
option is very likely to be the use of the circuit interrupter in connection with either Option 1 or 2,
with 2 the preferred. This combination for all practical purposes eliminates the possibility of
electric shock and injury/death as a result of using portable power tools.
FAA System Safety Handbook, Appendix F
December 30, 2000
F-35
1.3.2 THE FAULT TREE ANALYSIS
FORMAL NAME: The Fault Tree Analysis
ALTERNATIVE NAMES: The logic tree
PURPOSE: The Fault Tree Analysis (FTA) is a hazard identification tool based on the negative type
Logic Diagram. The FTA adds several dimensions to the basic logic tree. The most important of these
additions are the use of symbols to add information to the trees and the possibility of adding quantitative
risk data to the diagrams. With these additions, the FTA adds substantial hazard identification value to
the basic Logic Diagram previously discussed.
APPLICATION: Because of its relative complexity and detail, it is normally not cost effective to use the
FTA against risks assessed below the level of extremely high or high. The method is used extensively in
the acquisition of new systems and other complex systems where, due to the complexity and criticality of
the system, the tool is a must.
METHOD: The FTA is constructed exactly like a negative Logic Diagram except that the symbols
　
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