3M CablePipe Locating Techniques Manual

							51
2.2 To verify the fault location, insert the frameÕs red probe directly on the spot between the legs.
Pivot the frame in a circle around the red leg, reinserting the green leg in the ground every few
degrees of the circle.  The bar graph should always be on the left (red), indicating that the fault is
directly below the red leg. 
						

							52
A. Locating Faults Near Pedestals
2.3 The placement of the ground rod can have unfavorable effects on locating a fault that is very close
to a pedestal.  The pedestal is the access point for applying the Transmitter signal.  If the fault is
very close (a few feet, at most), and the ground rod is also very close, it may be impossible to find
the fault.  The returning currents from the fault are so distorted by close ground rod placement that
the earth contact frame cannot recognize the fault. A good practice is to use the ground extension
cable and always place the ground rod as far away from the pedestal as possible and in line with
the suspected cable path. 
						

							53
B. Locating Faults Under Pavement
2.4 When the cable is routed beneath and in line with a paved surface, you can locate the fault using
one of the following methods.
Perpendicular Method
2.5 Hold the frame parallel to the cable path but several feet to the side of the cable path.  Bar graph
reversal occurs when the frame center is directly perpendicular to the fault.
Triangulation Method
2.6 To check the accuracy of the Perpendicular method, move back several yards from the point of bar
graph reversal.  Probe in one spot with the frame, rotating it a few degrees between inserts, until
the bar graph reverses with less than one inch of frame movement. A line marked perpendicular to
the frame intersects the cable path at the fault.  Repeat this procedure a few yards ahead of the
Perpendicular method location to triangulate and confirm the location. 
						

							54
Extended Frame Methods
2.7 Where a cable passes under a roadbed or other narrow stretch of pavement, and a check on both
sides with the frame proves the fault to be under the roadway, triangulating the fault from both
sides of the road gives a general indication of the fault location.  A more accurate method is to
extend the distance between the probe tips:
Insert the green leg in earth on the far side of the fault, but do not insert the red leg.  Strip
about ten inches of an insulated conductor such as cross-connect wire and wrap it around
the red probe.  Pull out enough wire to equal twice the width of the road.  Strip another
ten inches of insulation from the far-end and wrap it around a screwdriver.  Ground the
screwdriver at a point in line with the cable path across the road from the Receiver.  The
screwdriver acts as an extension of the frameÕs red probe.
Return to the Receiver, making sure the red probe of the frame is held clear of the
ground, and note the bar graph. If the bar graph is indicating the striped (red) side, the
fault is nearest the grounded wire side of the road. If working alone, it helps to reverse
the positions of the frame and the screwdriver at this reading.
If the bar graph is indicating the green (solid) side, move the frame forward along the
cable path, probing with the green leg and keeping the red probe clear, until the bar graph
reverses.
Once the reversal point is established, pull the wire tight between the two contact points.
The fault lies exactly half the distance between the green probe and the screwdriver as
measured by the tight wire.  Fold the wire in half above the cable path for an exact fault
location from either contact point. 
						

							55
C. Finding a Fault without Tracing the Cable Path
2.8 Since the green leg of the Earth contact Frame points to the fault, tracing the path before or during
the fault finding process may not be necessary.  This technique uses 90 degree turns to pinpoint
the fault. It usually requires at least three such turns and is called the Ôthree-ninetyÕ method:
Start in the normal manner by holding the Receiver in one hand and the Earth Contact
Frame in the other with the green-banded leg towards the fault.  Near the location of the
ground rod, insert the frame probes fully into the ground.
The Receiver bar graph is visible on the right (green) side.  This indicates that the fault is
ahead of the operator in the direction of the green-banded leg of the earth contact frame.
Since the green leg of the frame points toward the fault, continue in that direction
reinserting the frame probes every few steps while watching the Receiver bar graph.
When the bar graph reverses and becomes visible on the left (striped or red) side, turn the
earth contact frame 90 degrees either right or left until the bar graph is again visible on
the right (green) side.  Continue in that direction reinserting the frame probes every few
steps until another reversal occurs.
Every time the bar graph reverses, turn 90 degrees and continue in the green direction.
By the third reversal the fault is very close, so make frame insertions every few inches.
At the fourth reversal, back up slowly, inserting the frame every few inches until the bar
graph returns to the green side.  The fault is located beneath the center of the frame when
the bar changes from green side to the red side. 
						

							56
D. Multiple Faults
2.9 The problem with multiple faults on a cable is finding the major fault.  Digging a hole to fix a
pinhole in the sheath when a fault that disrupts service is farther down the cable can be frustrating.
The procedure below will prevent this from happening:
When setting up the transmitter to find faults be sure to set the output level to high so that
the fault sensitivity will be highest.  You want to find all the faults on the cable. Do not
change the output level setting during fault locating.
Near the location of the ground rod (about one frame width away), insert the frame
probes fully into the ground with the green banded leg toward the fault.  Record the fault
level reference by pressing Gain/REF key.  This reading is a gauge of the signal returning
to the ground rod from all the faults on the cable.
When a fault is found and pinpointed, move the Earth Contact Frame about one frame
width away from the fault and insert it in the ground with the green-banded leg towards
the fault. Compare the numeric indicator reading with the ground rod reference number in
the lower left corner of the display. If they are close, the fault is a major one. If the fault
reading is 20 points or less than the reference, other faults almost surely exist and further
searching is necessary to locate a major fault. 
						

							57
3. Locating Aerial Faults
3.1 Shorted pairs, crossed pairs, and grounds on aerial cables can be pinpointed using an exploring
tone coil.  To locate a fault with the exploring tone coil, use this procedure.
3.2 Determine if the fault is a short or cross or if a conductor is grounded to the shield.  This will
determine how the Transmitter is attached to the faulted conductors.  If the resistance of the fault
is greater than 1800 ohms as measured between the faulted conductors or between the conductor
and ground, this method will not work.
If the fault is a shorted pair, connect the Red clip of the Direct Connect Transmitter Cable
to tip and the Black to ring (or vice-versa) of the shorted pair.
If the fault is a cross (two pairs involved), connect the Red clip of the Direct Connect
Transmitter Cable to the crossed conductor of one pair and the Black clip to the crossed
conductor of the other pair.
If the fault is a ground, connect the Red clip of the Direct Connect Transmitter Cable to
faulted conductor and the Black clip to ground.
3.3 Turn the Transmitter on and select the TONE mode and set the output level to the high.
Warning!  Potential for shock exists when handling connecting cables while the Transmitter is in the
        FAULT or TONE modes.  Turn the transmitter off before handling connecting cables.
3.4 Connect an exploring tone coil to the Receiver.  Turn the Receiver on and select the tone mode.
Select the 577 Hz frequency. Place the exploring tone coil on the cable and find a peak signal and
press the GAIN key.  Because of the twist or spiraling and the lay of conductors in the cable, there
will be fluctuations in signal strength along the cable so be sure and listen for a peak before
pressing the GAIN key.  Adjust the speaker volume and listen for the tone while probing.  Use
headphones if needed in noisy locations.
3.5 Follow the cable with the exploring coil.  The fault is located at the point where the signal stops or
drops off sharply. 
						

							58
4. Locating Earth Faults Near the Meter Box
4.1 When the meter box is the access point for applying the Transmitter signal to locate a fault in
secondary cable, be careful where the ground rod is placed.  If the fault is very close (a few feet, at
most), and the ground rod is also very close, it may be impossible to find the fault.  The returning
currents from the fault are so distorted by close ground rod placement that the earth contact frame
cannot recognize the fault.  A good practice is to always place the ground rod as far away from the
meter box as possible and in line with the suspected cable path.  However, it may not be possible
to place the ground rod in line with the path because the house or structure supporting the meter is
in the way.
5. Locating Earth Faults in Secondary Cable
5.1 Locating a fault in secondary cable requires the Transmitter to send a current down the conductor
to the damaged insulating jacket (the fault) and back to the ground rod.  The operator uses the
earth contact frame to find the point where the current stops traveling through the conductor and
starts traveling through the earth.  Since a grounded secondary neutral is also in the same trench
with the faulted cable, it is a good practice to disconnect the neutral at both ends.  If the neutral is
not disconnected and it is also faulted, transmitter current could be prevented from traveling
through the earth and the fault cannot be found, as in figure B. 
						

							59
Section 7  Locating Active Duct Probes
1. Introduction
1.1 The 3MÔ Active Duct Probe (ADP) is a self-contained, small, waterproof transmitter (sonde). It
can be attached to a push-rod or pull-tape and inserted into a buried nonmetallic sewer, duct,
drain, or pipe. A cable locator of the same frequency will find the exact surface location. There are
several ADPs available from 3M. Models 3236, 3237, and 3238 Active Duct Probes transmit at
frequencies detectable by 500A/573A cable locators. 3236 is for 500/573A, 3237 is for 500/
573AP, and 3238 is for 500/573AC. Model 3229 transmits at 33 kHz and is detectable by either
the 2210, 2250/2273, as well as the 500DL, 573DL, 2220L and 4420L.
2. Locating the Active Duct Probe
2.1 Unscrew the ADP cap. With the battery
terminals up (toward the cap), connect the
battery, then screw the cap back on and
hand tighten only. Refer to Battery
Replacement Figure.
2.2 The ADP cap has a female 1/4 X 20 thread and the body has a female 3/8X16 thread. Use these to
attach the ADP to the snake, rodding tool, or pull tapes. Always use a lock nut.
2.3 Set the cable locatorÕs receiver to the same frequency as the ADP. Set the receiver to PEAK mode.
Make sure the ADP is working by placing the receiver 6 feet (2 m) from the sonde with the
receiver handle perpendicular to the ADP. The receiver should pick up a clear signal.
Note: The perpendicular position of the receiver to the ADP is opposite of that used when tracing
cables or pipes.
2.4 To locate the path of a nonmetallic conduit or pipe, use a rodding tool and insert the ADP about
3 feet (1 m) into the conduit or pipe. Locate the ADP and mark the spot. Push the ADP about two
paces farther into the pipe. Relocate the ADP and mark the spot.  Repeat this process until the path
is fully mapped. To find the exact surface location follow these steps: Note: When in doubt about the remaining
battery life, use a fresh battery.  If
possible, always start a new job with a
new battery.
Battery Replacement 
						

							60
2.5 With the receiver handle perpendicular to
the path, locate the ADP position by
moving along the suspected path until a
peak or maximum is found. Adjust the
receiver gain as needed.
2.6 At the peak, rotate the receiver
horizontally to maximize the signal.
This ensures that the receiver handle is
perpendicular to the path.
2.7 Next, move the receiver across the path
searching for a peak.  Mark the spot
where the peak along the conduit path
coincides with the peak across the path.