In the long line method the test current is applied and the current
flowing through the pipe is determined at the two IR drop test points. The
structure-to-electrolyte potential at the two test points is measured with the
current on. The test current is interrupted and the change in current at the
two IR drop test points (>I1 and >I2) as well as the change in potential at
the two potential test points (>E1 and >E2) are measured. As in the short
line method, if >E1/>E2 is greater than 1.6, >E2 should be multiplied by the
correction factor found in Table 6. If the ratio is greater than 3, the test
section should be shortened. The coating conductance (F/ft) is then
calculated using the following formula:
[(>I1 - >I2) x 106]/(>E
x length (ft)
Continuity Testing. For existing structures, the continuity of the
structure must be evaluated in order to design an effective cathodic
protection system for the structure. If the structure is not continuous then
either continuity must be achieved by the installation of continuity bonds or
the structure must be protected using independent cathodic protection systems
for each electrically isolated section of the structure. Continuity may be
evaluated using several methods.
Method 1. The simplest method used to locate insulating or high
resistance joints is by making structure-to-electrolyte potential measurements
over the protected structure. As shown in Figure 15, all potential
measurements past the insulating joint are actually average potentials of a
larger and larger section of the pipe section to which the meter is connected.
This method is subject to the influence of other structures in the area and
variations in environment along the structure and often is inconclusive.
Method 2. A more reliable method of continuity testing is
illustrated in Figure 16. In this method a current is applied to the
structure and the potential drop along the structure is measured. As shown in
Figure 17, high resistance joints are easily located using this method.
Method 3. A third method of continuity testing involves the use of
the pipe locator signals vary as shown in Figure 17 if a low frequency (audio
frequency) is used.
Insulation Testing. Insulating joints are occasionally used to
isolate sections of structures so that the protection of each section can be
Buried Structures. For buried structures direct measurement of the
insulation resistance of a joint is difficult to determine because the
conductivity of the soil effectively bypasses the joint. Buried insulating
flanges should always be equipped with test stations for testing of the joint.
Connections for testing insulating joints are shown in Figure 18.