i5000 Receiver - Operations Manual
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©Metrotech Corporation 2006 – All Rights reserved
4.4 Positive Line ID
Positive Line ID is an essential task of a modern locate receiver. The
i5000
family incorporates
three elements that aid the user in positively identifying the target utility from others that might be
carrying the signal because of bleedover or bleedoff. Bleedover is defined as a loss of signal due
to capacitive or inductive coupling between lines, and increases in significance with higher
frequencies. Bleedoff results when two or more lines are galvanically connected through a
common bond point, such that the signal in the target line is reduced as it branches out and finds
multiple paths to ground. Bleedoff occurs and has equal effect at all frequencies.
Both bleedover and bleedoff can cause signal distortion, since the magnetic field induced from an
applied AC current (the directly coupled signal) exists on multiple lines, and can constructively or
destructively combine. This, of course, depends on the relative positions and currents of the
utilities with respect to the target line. When distortion is present, the normal assumptions about
the field behaving as a cylindrical shell around the conductor can be (in various degrees) invalid.
Therefore, centerline, current, and depth estimates made in such distorted fields may also be
biased.
In the
i5000
receiver, Distortion Alert
TM
and Signal Select
TM
are used to define the degree of
distortion present and the direction of the signal in the targeted line, respectively. Both of these
methods are enabled by Signal Select modulation. Line Guidance™ composes the third leg of
the triad, and facilitates a “look-ahead” of the line direction. Curvature in the target line, or
galvanically connected junctions like "Y" and "T" connections create anomalies in the shape of
the field that are detected by the Line Guidance function.
All of these elements are combined into a single graphical component, called the Guidance
Compass
TM
, in the upper right-hand corner of the receiver display. The yellow arrow shows the
detected direction of the line at all times, including the look-ahead guidance feature. The ways
that Distortion Alert
TM
and Signal Select
TM
and Line Guidance combine is discussed in this
section.
