3M CablePipe Locating Techniques Manual

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B Power Frequencies
5.4 An energized cable carrying AC power produces a 50 or 60 Hz signal. Although these are
relatively low frequencies, they can still couple into other conductors buried nearby. You can
detect the conductor because of the signal, but identification is impossible. The signal could be
coming from a power cable, a nearby pipe, or concrete reinforcing bars. However, the knowledge
that these conductors exist is useful.
5.5 Most energized power cables are easy to detect but sometimes power cables are designed to
minimize the strength of radiated signals by twisting the wires so that the ÔgoÕ and ÔreturnÕ current
fields cancel each other.  These cables are difficult to detect. All Dynatel Receivers detect the 9
th
harmonic of the 50 or 60 Hz frequency (450 or 540 Hz).  The 9th harmonic works especially well
with three-phase cables. The fundamental frequency normally cancels in a three-phase installation
but the 9
th harmonic reinforces, generating a stronger signal to trace. Some Receivers detect the 5th
harmonic as well as the 9th. The 9th harmonic is best for most passive power frequency locating,
but if the signal is weak or intermittent, the 5th harmonic may be able to help. These harmonics are
displayed as low (5th) or high (9th).  For 60 Hz the selection on the display would read L60 or H60
and for 50 Hz it would read L50 or H50.
5.6 Some Receivers have a selection for the second harmonic of the 50 or 60 Hz frequency (100 or
120 Hz). This passive power frequency is useful for tracing a conductor carrying a rectified AC
signal. Such signals are used in impressed cathodic protection systems for pipe. The display will
read either 100 or 120 when this frequency is selected.
C Radio Signals
5.7 Low frequency radio signals from local broadcast transmitters will cause currents to flow in
buried conductors.  These signals are then reradiated from the conductor and can be detected by
the Receiver.  The Receiver frequency should be set to the LF selection (available on E version
Receivers only).
D Cable TV Signals
5.8 The second harmonic of the NTSC television horizontal scan frequency is detectable by the
Receiver at 31.5 kHz. This frequency is coupled onto the cable by the yoke coils of an operating
television receiver. The signal is strongest near the TV receiver so it is useful in finding CATV
drops. The display of the locator Receiver reads 31 kHz when this frequency is selected.
E Other Transmitted Signal Sources
5.9 Some long distance copper or fiber optic cable systems have limited access and may have
permanently installed transmitters at strategic locations for use in tracing the cable. If they can
transmit 577 Hz, 512 Hz or 560 Hz, the signal can be detected by the Receiver. 
						

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6. Choosing Trace Modes
6.1 In most cases, you would choose PEAK mode, but for fast or difficult tracing other modes can be
handy. The following is a brief description of each trace mode:
 PEAK: In this mode, the Receiver speaker volume increases to a maximum as the
antenna crosses the cable.  It diminishes as the antenna moves away from the cable
path.  Simultaneously, the bar graph fills from both sides toward the middle as the
peakÕ zone is crossed then opens as the midpoint is passed.  The numeric strength
indicator also increases to a maximum.  Peak mode is useful when tracing changes in
cable direction because speaker volume falls off rapidly if the antenna handle is not in
line with the cable path.  In such a case, a sharp turn or bend in the path is indicated.
PEAK with EXPANDER:  The expander is used with peak mode to sharpen or
enhance the audio response. The effect is to only allow audio response directly over
the cable. If the antenna is moved rapidly, the receiver appears to beep as the antenna
crosses the cable path.  The expander is useful when you wish to rapidly trace a long
straight stretch of buried cable and also to precisely trace a turn or change in
direction.
NULL: In this mode, the signal is a minimum directly over the cable and is maximum
on either side of the cable. The speaker volume and numeric display signal strength
correspond to the signal being received. Some receivers have two bar graph response
modes. Refer to your Operators Manual for more information.
DIFFERENTIAL:  In this mode, the Receiver provides an indication of the relative
position of the cable to the Receiver by displaying right or left arrows (the arrow
points toward the cable). The bar graph increases to a maximum as the Receiver
antenna is moved directly over the cable path. Speaker response is a high warbling
tone to the right of the cable path, a low warbling tone to the left, and a solid tone
directly over the cable.
SPECIAL PEAK:  This mode will increase the signal sensitivity of the Receiver when
the signal is too weak for normal tracing. Use special attention when using this mode
because it is more susceptible to congestion than the normal peak mode.
Note: While tracing cables, keep the Receiver handle in line with the suspected cable path. 
						

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7. One Touch Gain Adjust
7.1ItÕs SimpleÉ No GuessingÉ No Trial & Error... No multiple presses neededÉ When using Peak
or Null Tracing modes, just press the Receiver Gain-key ÒonceÓ to automatically adjust the gain
and set the bar graph reference point.
7.2Always adjust the Receiver gain only when you are over the target cable and in either Peak or
Null mode. If too much signal is indicated by a completely closed bar graph, press the Gain Adjust
key once to automatically adjust the gain and set the bar graph reference point. When the bar
graph is completely open, it indicates a weak signal. Press the Gain Adjust key once and the gain
will be adjusted and a new bar graph reference point will be set.
7.3As you trace cables away from the transmitter, the signal becomes weaker and it is necessary to
readjust the gain.  Press the Gain Adjust key once and recheck the signal before continuing. 
						

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8. Locating Techniques
A. Sweeping
8.1 Sweeping an area allows you to locate all buried
cable in the area.  Use the Induction method to apply
the Transmitter signal.  Use the highest frequency
available so that all cables in the area carry signal.
Walk in a grid pattern over the area as shown, and
cover the area from two directions. Walk the grid
again using the Power mode.  Stop the sweep when
there is a response.  Locate the position of the cable
then trace it until you are out of the area, marking the
path. After tracing the cable, resume the sweep.
B. Positioning
8.2 Positioning is a technique used to quickly find the trace path of a buried cable.  The technique can
save time when the signal is lost while tracing.  Use this technique rather than starting over.
8.3 Place the Receiver in the differential mode.  Place the antenna on the ground and rotate the
Receiver around the antenna as if it were a pivot. Watch the left-right arrows on the display. There
is a point where a small counterclockwise rotation lights the right-arrow and a small clockwise
rotation lights the left arrow.  At this point, note the direction of a line through the Receiver
handle.  Turn the Receiver 90 degrees from this line (right or left makes no difference).  One of
the direction arrows is visible. Side step in the direction of the arrow until the Receiver indicates
that the cable has been crossed.
C. Tracing
8.4 To get the most accurate results when tracing a cable, signal should be isolated to the individual
cable.  This means using either the direct-connect or Dyna-Coupler methods of applying signal.  If
surface access is not possible, then use the induction method. Trace the cable at a slow walk while
moving the Receiver in a side-to-side motion. Periodically mark the path.
8.5 As tracing proceeds, remember that the most powerful signal is near the Transmitter.  As the
Receiver gets farther away from the Transmitter the signal strength drops off. It is necessary to
readjust the gain periodically, to be sure there is adequate signal for the Receiver to operate. Press
the GAIN ADJUST key when the bar graph is no longer visible (too little signal) or when the bar
graph is closed (too much signal). 
						

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D. Identifying a Cable by Depth-Current Measurement
8.6 When you apply signal to a cable (using the direct-connect or Dyna-Coupler methods) the signal
can travel on all the cables which share ground with your target cable. This can cause trouble with
the locate, as a shallow cable with a weak signal can give as good a response as a deep cable with
a strong signal.
8.7 In the illustration below, transmitter signal was applied to cable B and a strong signal current
travels its length.  Cable A shares the same ground as cable B and now carries the same signal,
but the signal current is greatly reduced. Since cable A is shallow (about one foot), it gives a
strong signal response even though the deeper cable B carries more signal current. To identify
which response comes from cable B, find the strongest response over each cable and press the
depth key. During depth measurements, the auxiliary numeric indicator indicates the strength of
the signal current in the cable. The cable with the most current is the target cable. DonÕt forget to
also check the depth readout. Most CATV cables are buried one foot or less.  Telephone cables are
buried at three feet. Power cables and gas pipes are at four feet.
Receiver Displays in
Peak Mode Over
Each Cable
Receiver Displays in
Depth Mode Over
Each Cable
Higher Current Indicated
Transmitter Signal
Applied to this Cable
Cable A at 1 Foot
Cable B at 4 Feet 
						

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8.8 Imagine this problem: You know you have two cables with the same signal and just when you
think youÕve got the target cable identified, the two cables cross. Again, you can use the depth-
current measurement to identify the target cable. When cables A and B cross, they change depth.
This is not unusual and is often the case.  Since the ReceiverÕs signal response varies with depth, it
may be difficult to identify the cables using signal level alone.  However, the signal current in the
cables will not change, and you can compare the bar graph reading in depth mode to identify the
cables.
Cable A at 1 Foot
Cable B at 1 Foot
Cable A at 4 Feet
Cable B at 4 Feet
E Tracing Currents
8.9 Some Transmitters and Receivers will display cable current. These current indications can be used
to select a trace frequency, identify the correct cable, or troubleshoot the set up.
8.10 When using the direct connect method, if the Transmitter output current number in the display
reads LO, or is a number less than 50, it indicates that the tracing signal is too weak.  A number
higher than 70 represents a strong tracing signal.  Maximize the current number by changing the
frequency.  Every ten units on the current display represents a factor of two in current magnitude
in the cable.
8.11 When the Receiver is used to find the cable near the Transmitter connection point, the current
number displayed in the Receiver should correspond (within ±5 points) to the Transmitter number.
You must be over the correct cable, and the signal must not be split between two or more cables.
8.12 When the Transmitter signal is applied using the Dyna-Coupler, the Transmitter display indicates
the current in the coupler and not in the cable. In order to get a measure of the cable current, point
the Receiver at the exposed cable about two feet from the Dyna-Coupler. This will be the current
number that should be used to identify the correct cable. In general, when the Transmitter is set up
to apply more current on the target cable than any other cable, the target cable can be easily
identified because the Receiver current reading will be highest.
8.13 Since some of the signal in a cable bleeds into the earth, it is expected that the Receiver current
indication along the cable will decrease gradually as you move away from the Transmitter. This
effect is more pronounced at higher frequencies. 
						

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9. External DC Power and 5 Watt Output
A External DC Power
9.1 Dynatel Transmitters with option A can be operated from an external 12VDC source as well as its
internal batteries. A cigarette lighter adapter-cable is supplied to connect the DC power from a
vehicleÕs battery to the Transmitter Õs external power connector located next to the output
connector. This lets you save the internal batteries by using an external power source or continue
operating when the internal batteries are discharged. The internal batteries do not recharge when
an external DC voltage is applied to the Transmitters external power connector.
B 5 Watt output
9.2 Dynatel Transmitters with option A are capable of 5-watt output as well as the normal 3-watt
output. An external DC source is required for 5-watt output. Use the higher output when a very
long trace is required. The higher output should also be used on any continuously grounded cables
such as lead shielded cables or non-jacketed concentric neutral cables. 
						

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Section 2 Telephone Cable Locating Techniques
1. Introduction
1.1 Read Section One of this manual to learn more general information about each of the following
signal application methods. The following paragraphs provide specific instruction on applying
signal for telephone cable locating.
2. Applying Signal to Telephone Cable: Induction Method
2.1 The Induction method broadcasts signal into an area. No access to the cable is necessary. Use this
only when there are no other buried conductors present, or when locating all conductive buried
services in a general area.
2.2 Place the Transmitter on the ground over the cable to be located. The Transmitter hinge should be
in line with the cable path, as shown above. Be certain that the Transmitter is directly over the
cable to be located.
2.3 Turn the Transmitter on and choose 33 kHz signal or higher.
2.4 Use the Receiver to test the signal level by placing the Receiver 50 feet away from the Transmitter
on the ground near the cable. Move the Transmitter back and forth across the path. Listen for the
strongest signal from the Receiver. If the Receiver has trouble picking up the cable path, return to
the Transmitter and switch to a higher frequency. If the higher Transmitter frequency will not give
satisfactory Receiver response, then boost the Transmitter output as instructed in the unitÕs
Operating Instructions. You can also increase the response by placing the Transmitter on the
located position over the cable (50 feet away from the Transmitter Õs previous position). 
						

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3. Applying Signal to Telephone Cable: Direct Connect Method
3.1 The direct-connect method requires access to the cable shield.  Disconnect the cable at the near-
end where the Transmitter is connected. Do not disconnect at the far-end since this supplies a far-
end ground.
3.2 Connect the red lead of the Transmitter to the cable shield and the black lead to the ground rod.
Warning! Potential for electrical shock exists when handling connecting cables while the Transmitter is in the
Fault or Tone modes.  Turn the transmitter off before handling connecting cables.
3.3 Place the ground rod as far away from the
cable path as possible (90 degrees from the
suspected cable path).  Never ground to
water pipe or other services in the area.
The returning signal on these services may
mislead the trace.
3.4 Remove the ground bonding at the
near-end. The far-end should have a good
ground. Turn the Transmitter on and
choose the 577 Hz frequency to get greater
signal distance down the cable.