General Machine Products RD7000 Traceable Rod Detection - Manual (Page 37)

RD7000 Operation Manual 31
Take the dB reading and keep it for reference. If there is a
single fault on the cable, it will be approximately the same
dB value as the reference reading.
NOTE. To establish how often to take readings on the
cable, move away from the ground stake and take
further readings until the arrow has difficulty in locking
and the dB reading is low. Measure the distance that
the receiver is now away from the ground stake. This
is the distance that you can safely use between taking
readings on the cable to ensure that you do not miss
the fault.
8.3 How to find a fault
Starting from the transmitter, walk along the cable route
pushing the A-Frame spikes into the ground with the
green spike pointing away from the transmitter. Where
there are no faults the dB reading will be low and the
direction arrow may flicker forward and back.
NOTE: Flickering arrows may also indicate that you may
be too far away from the fault or ground stake (or both)
for the receiver to lock on.
If you are trying to locate high resistance faults or there is
a long distance between faults, carry on with the survey
and the receiver will lock on when you get closer to
the fault.
Road
Path
Soil
Fault
line
Take readings at the survey intervals determined by the
reference reading. To locate the cable during a Fault-Find
survey, press the key once and the receiver will switch
to 8kHz null mode. Locate the cable using the normal
null mode technique. Press the key again to return to
Fault-Find mode.
As a fault is approached the Fault-Find direction arrow will
lock on to the fault signal and point forward and the dB
readings will increase. When the fault is passed the arrow
will point back towards the transmitter. Take readings at
smaller survey intervals to determine the exact point of
the fault.
When the A-frame is directly over the fault the dB reading
will drop as shown opposite.
NOTE: The values given are for illustration purposes
only and may not be the same as those obtained in
other situations.
To pinpoint the fault, turn the A-frame 90° to the cable
and Fault-Find until the exact point is found where the
arrow direction changes. The center line of the A-frame is
now directly over the fault.
Mark the ground to show the position of the fault. Find
the maximum dB reading in front of the fault by pushing
the A-frame into the ground at small intervals. Note the
dB reading. If the reading is approximately the same as
the reference reading, you can assume that there is only
one fault. If the reading is less than the reference reading,
keep surveying the cable for other faults.
Replace the bonds between the sheath and ground at
each end of the line once the fault-finding survey has
been completed.
NOTE: If the cable runs under a road, use the
equipment as normal on the road surface as it can
sometimes detect signals when working on blacktop,
concrete, or paved surfaces. If necessary, try wetting
the road surface. Pouring a very small amount of
water around the bottom of the A-frame spikes before
each Fault-Find will generally ensure a good ground
connection.
NOTE: If the cable runs under a paved surface, the fault
can often be pinpointed by fault finding in the grass/soil
adjacent to the paving. Reduce the distance between
placing the A-frame spikes in the ground to allow for
the increased distance to the actual fault position.
Figure 8.1: Cable sheath fault-
finding
Figure 8.2: Locating cable
sheath faults with the
receiver and A-frame