UFC 3-570-06
JANUARY 31 2003
Figure 7-20. Testing For A Short Between Two Structures
V oltm et er
T est St at io n or
T est St at io n or
M eta llic C on ta ct
2
M eta llic C on ta ct
+ -
1
R eference
E lect rod e
S tru c tu re W ith C P
S tru c tu re W ith o u t C P
7-9.1.1
If the two potential measurements are significantly different (over 10 mV), the
two structures are not metallically shorted together. Under normal conditions, the
structure with cathodic protection should be at a potential more negative than -0.85 volts
DC and the steel structure without cathodic protection should be between approximately
-0.35 volts DC and -0.65 volts DC (a difference of between 200 to 500 mV). If the other
structure is copper or steel in concrete under normal conditions, it should have a
potential between approximately -0.20 volts DC and -0.30 volts DC.
7-9.1.2
If the two potential measurements are not significantly different (under 10
mV), the two structures may be shorted, 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 should be installed where the current should
distribute to just one structure. Repeat the potential measurement of both structures. If
the potential of the unprotected structure remains approximately the same or changes in
a positive direction (less negative), when the potential of the protected structure
changes in a negative direction, they are not shorted. If both potential measurements
change more negative as current is increased, the two structures are shorted together.
7-32
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