UFC 3-570-06
JANUARY 31 2003
Figure 7-18 Testing for a Shorted Casing
V oltm et er
+ -
1
2
R efere nc e
E le ct r od e
7-8.1.1
If the two potential measurements are significantly different (over 10 mV), the
casing is not shorted to the pipeline. Under normal conditions, the carrier pipeline
should be at a potential more negative than -0.85 volts DC, and the casing should be
between approximately -0.35 and -0.65 volts DC (a difference of between 200 to 500
mV).
7-8.1.2
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. Install
a temporary local cathodic protection system to increase the current to the carrier
pipeline (paragraph 7-10); or, if possible, merely increase the current level of the
existing system. Note that the temporary system must be installed on the opposite side
of the railway or road crossing from the location of the potential testing. 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.
7-29
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