Interpreting Interrupted Potentials at a Test Station
A 6-in. coated steel pipeline has a test station at a road crossing. With the rectifier on, the pipe-to-soil potential is -1.18 V CSE. A current interrupter is used; the instant-off reading is -0.93 V, and the native potential measured before energizing the system was -0.58 V. The soil is 2,500 ohm-cm. A grounded well casing is 15 ft away. The rectifier output is 14 A at 38 V. The operator proposes raising the rectifier output to bring the on-potential to -1.25 V because another test station reads -0.86 V on.
1. Calculate the IR drop and polarization at the first test station, and determine whether the -0.85 V and 100 mV criteria are met.
Read the worked answer
IR drop = E(on) - E(instant-off) = -1.18 V - (-0.93 V) = 0.25 V.
Polarization = E(instant-off) - E(native) = -0.93 V - (-0.58 V) = 0.35 V, or 350 mV.
The instant-off potential of -0.93 V is more negative than -0.85 V, and the 350 mV polarization exceeds the 100 mV criterion. At this test station, protection is adequate. The on-potential of -1.18 V overstates protection because it includes 0.25 V of IR drop.
2. Advise whether raising the rectifier output is justified, and explain what should be done about the -0.86 V on-potential reading at the other test station.
Read the worked answer
Raising the rectifier output is not justified by the first station; it already meets both criteria. Increasing output there would add IR drop and could overprotect the coating without improving true polarization.
The -0.86 V reading at the other station is an on-potential, so it cannot be judged against -0.85 V or the 100 mV criterion. An instant-off survey at that location is needed. If interruption is impractical, a coupon or permanently installed reference electrode can provide an IR-free reading.
Because a grounded well casing is nearby, raising current also increases the risk of stray-current interference on the casing. Check casing-to-soil potentials and bond current before any output increase.
3. List the field checks you would perform before changing rectifier taps, and state the decision rule you would apply.
Read the worked answer
Perform an interrupted close-interval survey or coupon survey at the -0.86 V location; measure native and instant-off potentials; check the well casing potential and any bond current; verify the rectifier output with a clamp-on ammeter and shunt; and inspect test-station connections for contact resistance.
Decision rule: increase output only if an IR-free potential at the questionable location fails both the -0.85 V criterion and the 100 mV polarization criterion, and only after confirming that the increase does not create casing interference or overprotection elsewhere. If the instant-off reading already meets the criteria, correct the measurement or connection problem instead of raising current.