UFC 3-570-06
JANUARY 31 2003
coatings, the potential of a protected structure decreases. An increase in potential
normally indicates current pickup. If the potential of a protected pipeline increases
when crossing a foreign pipeline, interference is likely on that structure. Potential
survey of that structure while interrupting the source of current will indicate the presence
and magnitude of that interference.
Figure 7-38. Normal and Abnormal Potentials of Protected and Foreign Pipelines
Po ten tial S urve y of Pip elin es for I nter fere nce
A node Bed
For eign P i peline
L ocation
Cr ossing
N orm a l
A bn orm a l
Pot entia l
Pot entia l
Surv ey
Surv ey
-1
-.9
P rotecte d
-.8
Pi p elin e
-.7
A bn orm a l
P oten ti als
Pot entia l
-.8
Surv ey
P ickup A rea
-.6
F orei gn
-.4
N orm a l
Pi p elin e
-.2
Pot entia l
P oten ti als
Surv ey
Di scharge A rea
7-14.1.6 A cathodic protection rectifier output contains a waveform, which results in a
pulsating DC signal. This signal can be located using a pipe locator capable of following
a 120 cycles per second signal (such as the Pipe Horn, Model 200 FDAC, in rectifier
mode). Presence of this signal indicates presence of interference on the foreign
structure. Combining this method with a current interrupter on suspected sources of the
interference will quickly locate the source of the interference. Signal strength can
sometimes be used to indicate the direction of the discharge and pickup areas.
Normally, signal strength will increase in the direction of the discharge point. This may
not occur with multiple discharge areas.
7-14.1.7 Another method of interference testing involves using an experimental
drainage bond between structures. Proper drainage can be determined by trial-and-
error, using a variable resistor to alter drainage current. Required drainage current can
also be computed from test data. When the same owner maintains the protected
structure and the foreign structure, this method may be easily used. The source of
interference is cycled on and off at specific intervals and the bond resistance is adjusted
until no potential shift occurs on the foreign structure. When site conditions warrant, the
bond may be sized to apply partial or full protection to the foreign structure. If the
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