JANUARY 31 2003
Figure 7-19. Testing for a Shorted Casing with Power Supply
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Testing a Casing Without Cathodic Protection on the Carrier Pipeline.
Install a temporary local cathodic protection system to apply current to the carrier
pipeline (paragraph 7-10). Always install the temporary anodes on the opposite side of
the crossing from the side where the potential measurements are taken. Take a
potential measurement of the carrier pipeline and the casing by changing only the
structure connection without moving the copper/copper sulfate reference electrode.
If the two potential measurements are significantly different (over 10 mV), the
casing is not shorted to the pipeline. With sufficient current applied to the carrier
pipeline, it should have a potential of approximately -0.85 volts DC and the casing
should be between approximately -0.35 and -0.65 volts DC.
If the two potential measurements are not significantly different (under 10
mV), the casing may be shorted to the pipeline and additional testing is required.
Increase the amount of current applied to the carrier pipeline (by turning up power
supply or adding additional temporary anodes), then repeat potential measurement of
the carrier pipeline and the casing. If the potential of the casing remains approximately
the same or changes in a positive direction (less negative) when the potential of the
carrier pipe changes in a negative direction, the insulation is good. If both the carrier
pipeline and the casing potential measurements change more negative as current is
increased, the carrier pipeline is shorted to the casing.
TESTING FOR A SHORT BETWEEN TWO STRUCTURES. Shorts
between two structures can adversely affect the operation of cathodic protection