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Contact Voltage General
Considerations
The hazardous nature of Contact
Voltage conditions and a number
of recent injuries and deaths
among humans and animals has
raised safety concerns
throughout the public and
electric utility industry.
Regulatory agencies have
responded to these incidents
with increasing interest and
additional requirements
directing increased safety
testing.
Consideration should be given
first to identification of the
underlying hazards, areas of
highest risk, and efficient risk
mitigation methods. When
considering Contact Voltage, the
underlying hazard may not be
well defined as a function of
the measured potential at the
publicly accessible surface. In
field investigations of cases
where only a few volts were
found on the surface, further
investigation has shown
situations where damaged
insulation and high impedance
faults were the root cause. The
variability of the fault
condition should be considered
when considering the possibility
of injury. A low voltage
measured where there should be
no voltage is always
justification for further
investigation.
A publicly accessible energized
surface represents one leg of
the shock triangle. Shown in the
figure below, the shock
triangle, much like the fire
triangle, depicts the requisite
conditions for a shock to occur.
The probability a shock will
occur is a case of combined
probabilities. The legs of the
triangle are comprised of an
energized structure, a ground,
and a human or animal in
contact. There is variability in
each and an associated
probability of each at a given
location. Some consideration of
each leg is appropriate when
discussing risk reduction.
As operators, designers and
regulators of distribution
systems, we are responsible for
control and safety of structures
comprising the network. This leg
of the triangle is the only one
we control. If we were able to
achieve complete control of this
element, shocks could be
eliminated. The presence of a
human or animal, in contact with
an energized structure and a
ground is another probabilistic
element. Note that while efforts
to test for and reduce the
numbers of energized structures
are important, increasing or
concentrating those efforts
where large numbers of people
travel on foot will yield the
greatest reduction in shocks.
The remaining leg of the shock
triangle is the ground. While it
is true that lack of a
contactable ground near an
energized structure is an
important element in assessing
the danger of that structure, it
is important to consider all
possibilities. The “reach” of a
ground or energized structure
can easily be extended when
standing water, a conductive pet
leash or other variable is
introduced.
A conceptual understanding of
the elements in the shock
triangle and the probabilities
they occur in unison, form a
good basis for risk reduction
planning.
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