Figure 1 - Acoustic surge detection
No matter what method is used for fault locating on direct buried underground cable,
at some point an “x” must be marked on the ground to say “dig here.”
The most commonly used prelocation methods such as arc reflection or
current impulse will get reasonably close to the fault, but are
not accurate enough to define the exact fault location.
Before digging, in order to repair the faulted cable, some type of pinpointing technique must be used.
The classical methods all revolve around a way to zero in on the sound produced by the thump or discharge of energy at the fault created by a surge generator.
A simple and well-used method is the
fault-locator-ear-on-the-ground-butt-in-the-air technique. Under some
conditions such as after a rain or heavy morning dew this can be a
shocking experience, literally. Under certain conditions such as created
by a corroded neutral, when surging the cable, current will flow in the
earth itself rather than back to the generator through the neutral.
When this occurs, a voltage drop is produced between the spread hands of the fault locator
each time the surge generator discharges. Other less painful approaches
involve old reliable tools such as traffic cones, shovel handles, and
modified stethoscopes.
Slightly more sophisticated equipment uses an acoustic pickup or microphone placed on the ground, an electronic amplifier, and a set of headphones.
This
setup amplifies the sound and helps to zero in on the source at the
fault. An improvement on this technique is the addition of a
second pickup. See Figure 1 above. A switch
and meter on the amplifier allow comparison of the magnitude of the
sound from each pickup. The higher signal is from the pickup closest to
the fault and the sensors are moved in that direction. With pickups
straddling the fault, the sound levels are equal.
These acoustic techniques all assume that the sound produced at the fault travels directly to ground level
unimpeded and that the loudest sound is heard precisely above the
fault. If the cable happens to be in duct or conduit, under paving or
surrounded by tree roots, this assumption may not be valid. In duct or
pipe, the loudest sound occurs at either end or at a break.
If the
fault is under paving, the loudest sound may be at a crack or seam.
Root systems seem to carry the sound off in all directions.
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