UFC 3-570-06
JANUARY 31 2003
direction of current flow and magnitude. Current will flow in the direction of the
discharge point. The magnitude will increase as you near the discharge point. The
direction will reverse upon passing a discharge point. The reversal with the highest
magnitude is the "control" or "critical" point. The discharge point is usually near the
structure, which is causing the interference. The pickup area is usually near the anode
system, which is causing the interference. Locating the pickup area is important in
locating the source of the interference, since it shows the direction that the current is
coming from.
Figure 7-37. Cell-To-Cell Polarity on Foreign Structure
Po siti ve (+ ) R ead ing V oltm et er
+ -
1
2
R e fe r e nc e
R e fe r e nc e
E l e ct r od e
E l ect r od e
C ur r e nt F lo w in th e E a r th
D ir e c t io n of A no di c A re a on P i pe line
7-14.1.4 Measurement of the current flow (IR drop) on a foreign pipeline can be
accomplished if other methods are inconclusive. Current flow (IR drop) readings show
the relationship between pickup and discharge areas on a foreign structure. By
measuring the current flow on a foreign structure, the direction of the discharge point
and pickup areas can be determined. Magnitude indicates seriousness, and increases
as the distance to the discharge point decreases. These measurements can be easily
accomplished if IR drop test stations already exist or if the pipeline is accessible (comes
aboveground or passes through pits). Current is measured at an IR drop test station by
the procedures detailed in paragraph 7-13. Current is measured where access to the
pipeline is possible using a clamp-on milliammeter (such as the Swain CP AmpClip).
The correct size clamp must be used for the pipeline under test. Note: typical clamp-on
meters do not work in this application. It must be capable of measuring DC amps, with
a range capable of accurate measurement of under 30 milliamps.
7-14.1.5 Abnormal or unusual current distribution on a protected structure indicates
possible interference on a foreign structure. Normally, the potential of a protected
structure decreases slightly as distance from the anodes increases. At defects in
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