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Distribution System Geometry
There are a number of elements
that contribute to the numbers
of energized structures in the
landscape. Certainly
construction practices, design
practices, materials,
workmanship and maintenance all
factor into the safety
performance of a distribution
system. Ultimately, proximity to
the system is one of the largest
determinants in the occurrence
of accessible energized
structures. Proximity not only
affects the number of energized
structures, but also dictates
the appropriate test
methodology.
When considering distribution
systems and Contact Voltage, it
is helpful to consider overhead
and underground distribution
systems as two categories.
Consider the figures shown
below, and the pictorial
representation of the differing
systems.
In the case of overhead
distribution, much of the system
is far out of reach to the
public. The greatest proportion
of the equipment is
inaccessible. The system
elements that are accessible are
system endpoints such as utility
poles, conduits, and ground
leads. Street lighting,
conductors, and transformers,
are out of reach. This in itself
has two major implications.
First, many elements are out of
reach and therefore do not
impose a shock hazard. Any
energized structures will likely
be either the system endpoints
that reach ground level, or
structures nearby those
endpoints. Second, it is
appropriate to test those
endpoints and upon successful
test, it is reasonable to draw
conclusions about the safety of
the system.
Consider the case of underground
distribution. Nearly the entire
infrastructure is located below
the surface. In this case,
testing system endpoints, such
as manhole covers, service box
covers and lighting does not
provide accurate indication of
the system safety performance.
In underground distribution
systems, the public right of ways
are located directly above the
distribution system. Energized
surface level structures are the
frequent result of system
deterioration and construction
damage. Energized structures can
exist anywhere above the
distribution system, often
occurring on structures such as
sidewalks, gates, fences, metal
curbing, and storm drains. These
structures are not assets of the
utility company and are not
inspected or tested for contact
voltage as typical utility
practice. Merely testing the
utility owned assets or system
endpoints is inadequate to know
the safety performance of the
distribution system. A detection
system such as the SVD2000 is
the only method suitable for
finding energized structures in
underground distribution
regions.
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