cast-iron water and gas pipes can be readily identified and even classified as to type by the conventional spacing of
joints. Power mains 50/60 Hz service drops can also be identified by a burbling sound that peaks when the receiver is
directly over the power line, as long as it is actively carrying electrical power. As you become more familiar with the
MAC-51Bx System, you will find that switching between MAG, HI and LO modes becomes an invaluable tracing aid
when underground cutter is encountered.
Isolators and Signals Path Continuity (HI Mode)
The tracer current must travel in a closed loop. When it leaves the line being traced, it loops back, one way or another,
to the beginning of the line. If the current cannot complete its loop the locating system will not operate. You should be
aware of this system requirement when tracing lines that have electrical isolators installed.
Electrical isolators are sometimes placed in a gas line at the meter to provide an electrically open circuit which stops the
flow of galvanic current and reduces corrosion. If you are inductively exciting this type of line by placing the transmitter
close to the meter, you must place a shorting wire on the pipe to bypass the isolator. This allows the tracing current to
return to the pipe through the earth ground of the building. An alternate method is to move the transmitter down line a
few yards away from the building to a point where the gas pipe riser provides a current return path, or to place the
inductive signal clamp on the line below the isolator.
Figure 5-7 Shorting Wire in Place to Bypass an Isolator
Isolators and Inductive Excitation (HI Mode)
Electrical isolation sometimes occurs inadvertently on phone cables entering a pedestal because the cable’s shield is not
grounded. In most jurisdictions, grounding the shield inside the pedestal is not required unless the cable shares a
trench with power cables. If there is no ground wire, it is recommended that a wire and clips, as shown in Figure 5-8, be
connected from the cable shield to the pedestal before inductively coupling the transmitter’s signals to the target cable.
This will greatly improve the strength of the induced tracing signal.
Schonstedt Instrument Company
5-5
SHORTING
WIRE