Technical Practice Guide

Current Interruption: Rectifier Interruption, Synchronization, and Instant-Off Timing

Current interruption is used to temporarily cycle cathodic protection current sources OFF and ON so instant-off potentials can be measured before significant depolarization occurs.

Overview

Current interruption is a field method used to temporarily change the current condition so instant-off potentials can be measured.

The measured value is still a structure-to-electrolyte voltage between a structure connection and a reference electrode. Interruption helps define the current condition at the moment of measurement.

ON potentials are measured while CP current is applied. ON potentials may include IR drop.

Instant-off potentials are measured immediately after current interruption. The instant-off value is intended to better represent the polarized potential of the structure.

Current interruption is used in many interrupted surveys, close-interval surveys, and criterion evaluations because it can help separate polarized potential from current-related voltage effects.

The value of the data depends on knowing what current sources were ON, what sources were interrupted, whether the OFF cycle was captured at the right time, and whether other current influence remained.

Core concept:

Current interruption is not the final interpretation. It is the measurement control that makes instant-off interpretation possible.

Technical Basis

When CP current flows through soil or another electrolyte, voltage drop can occur between the structure and the reference electrode.

An ON potential includes the structure’s polarized condition plus voltage effects from current flow through the electrolyte.

  • When CP current is interrupted, current-related voltage drop is reduced.
  • The potential measured immediately after interruption is the instant-off potential.
  • If the reading is delayed, the structure may begin to depolarize and shift electro-positive.
  • The longer the current remains off, the less the reading represents the instant-off condition and the more it may represent partial depolarization.
  • If multiple current sources influence the structure, interrupting only one source may not remove all relevant current effects.
  • Synchronized interruption helps place all influencing current sources into the OFF cycle at the same time.

Poor synchronization, incomplete interruption, missed interruption cycles, delayed readings, multiple current sources, foreign-current effects, or dynamic interference can produce questionable data.

Interruption can reduce IR-drop influence from interrupted sources, but it should not be described as removing every possible current influence in every field condition.

When Current Interruption Is Used

  • Instant-off potential measurements
  • Interrupted pipe-to-soil potential surveys
  • Close-interval surveys
  • Annual CP surveys
  • Rectifier influence evaluation
  • Depolarization testing
  • Troubleshooting questionable ON potentials
  • Evaluating the −850 mVCSE polarized potential criterion
  • Investigating current distribution from multiple rectifiers
  • Verifying survey data where IR drop may be significant

Equipment Typically Used

Equipment Purpose
Current interrupter Cycles CP current source ON and OFF at a controlled interval.
Rectifier access Allows interrupter installation or output control where appropriate.
High-impedance voltmeter or data logger Measures ON and instant-off structure-to-electrolyte potentials.
Copper-copper sulfate reference electrode Provides a stable reference potential for soil measurements.
GPS-synchronized interrupters, where required Coordinate OFF cycles across multiple current sources.
Field log or survey software Documents timing, current source status, readings, and abnormal conditions.
RMU or rectifier monitor, where installed May help confirm output status, polling, and interruption behavior.
Safety note:

Do not install or adjust equipment inside energized rectifiers unless qualified and authorized. Follow site safety requirements and applicable electrical safety procedures.

General Field Method

  1. Identify all CP current sources that may influence the structure being tested.
  2. Confirm which rectifiers, bonds, galvanic systems, or foreign current sources must be considered for the survey objective.
  3. Confirm site access and applicable electrical safety requirements.
  4. Install or configure interrupters on applicable current sources where appropriate and safe.
  5. Select the interruption cycle required by the survey method or specification.
  6. Synchronize interrupters when multiple current sources affect the same structure.
  7. Verify that each current source is cycling ON and OFF as intended.
  8. Measure the ON potential during the ON cycle.
  9. Measure the instant-off potential immediately after the OFF cycle begins.
  10. Document the cycle timing, current sources interrupted, current sources not interrupted, and any abnormal conditions.
  11. After testing, remove or disable interruption equipment as required.
  12. Confirm CP current sources are restored to normal operation.

Exact procedures vary by owner specification, survey objective, rectifier design, and site safety requirements.

Valid Data Conditions

  • Correct identification of influencing CP current sources
  • Known current condition for each measurement
  • Correct interrupter installation or configuration
  • Synchronized interruption where multiple current sources influence the structure
  • Verified ON/OFF cycling before relying on readings
  • Instant-off measurement captured immediately after interruption
  • Stable reference electrode contact
  • Correct structure contact
  • Documentation of cycle timing and interruption method
  • Documentation of current sources that remained energized
  • Documentation of reference electrode location, contact condition, and any unstable readings
  • Confirmation that CP current sources were restored after testing

If the wrong current source is interrupted, the instant-off data may be invalid.

If current sources are not synchronized, the reading may include remaining current effects.

If the reading is captured late, it may not represent the instant-off condition.

If foreign current sources remain energized, IR drop or interference effects may remain.

A single abnormal OFF value does not prove the cause. Review the pattern of readings, current-source status, survey timing, nearby DC systems, and field notes before deciding whether the issue is poor interruption, IR drop, stray current, contact error, or a true protection concern.

Common Errors and Misinterpretations

Error Why It Matters
Interrupting only one rectifier when multiple rectifiers influence the structure Remaining current sources may continue to create voltage gradients and IR drop.
Using unsynchronized interrupters The OFF cycle may not occur at the same time for all influencing current sources.
Recording the instant-off value too late The structure may have already begun depolarizing.
Interrupting the wrong current source The measured value may not represent the intended CP system condition.
Assuming interruption eliminates all IR drop Foreign current sources, bonds, telluric effects, or remaining currents may still influence readings.
Failing to document the interruption cycle Later reviewers cannot verify what condition the readings represent.
Not confirming that the rectifier resumed operation after testing The CP system may be unintentionally left out of service.
Calling a delayed OFF reading an instant-off value The value may represent partial depolarization rather than the instant-off condition.

Interpretation

Current interruption supports interpretation but is not itself the criterion conclusion.

  • A valid instant-off potential depends on correct interruption and timing.
  • The instant-off potential may be compared to the −850 mVCSE polarized potential criterion where applicable.
  • Current interruption can reduce IR drop effects, but it does not guarantee all current influence has been removed.
  • When interruption is incomplete or unsynchronized, results should be considered questionable.
  • Interruption records should identify which current sources were interrupted and which were not.
  • A valid interrupted survey must connect readings to current source status.
  • One interrupted reading should be interpreted with the surrounding survey pattern, field conditions, and measurement notes.
Observation General Interpretation
All influencing rectifiers synchronized and verified cycling Instant-off data may be suitable for polarized potential evaluation.
Only one of several influencing rectifiers interrupted Instant-off readings may still include current effects from other sources.
OFF reading captured late Value may represent partial depolarization rather than instant-off potential.
Interruption cycle not documented Data condition is uncertain and harder to defend.
Rectifier not restored after testing CP system may be unintentionally left out of service.

Worked Example

A pipeline is influenced by two rectifiers. During an interrupted survey, the technician records:

Condition Observation
Rectifier R-1 Interrupted on a 4-second ON / 1-second OFF cycle
Rectifier R-2 Left operating continuously
Test point ON potential −1,060 mVCSE
Recorded OFF potential −895 mVCSE

The recorded OFF potential is more electro-negative than −850 mVCSE.

However, only R-1 was interrupted.

R-2 remained operating and may still have contributed current effects and IR drop.

The recorded OFF value should be considered questionable as a true instant-off potential unless R-2 was proven not to influence the test location.

Correct conclusion:

The correct conclusion is not simply that the criterion was satisfied. The correct conclusion is that the interrupted data may be invalid or incomplete because not all influencing current sources were interrupted.

Practice Questions

Question 1

Why is CP current interrupted during an instant-off survey?

  1. To increase pipeline resistance
  2. To reduce current-related voltage effects so instant-off potential can be measured
  3. To permanently depolarize the structure
  4. To determine coating thickness

Answer: B

Question 2

Why must multiple interrupters be synchronized when several current sources influence the structure?

  1. To reduce soil resistivity
  2. To increase rectifier current output
  3. To improve coating adhesion
  4. To ensure OFF cycles occur simultaneously for influencing current sources

Answer: D

Question 3

What can happen if the OFF reading is captured too late?

  1. The structure may begin depolarizing before the reading is captured
  2. The ON cycle becomes longer automatically
  3. The reference electrode polarity reverses
  4. The rectifier output permanently changes

Answer: A

Question 4

What should be documented during interrupted testing?

  1. Only the final criterion conclusion
  2. Only the rectifier serial number
  3. Cycle timing, interrupted sources, and abnormal conditions
  4. Only the ON potential values

Answer: C

Question 5

What is the correct interpretation when one rectifier is interrupted but another influencing rectifier remains ON?

  1. The OFF value automatically satisfies all criteria
  2. The interrupted data may still include current effects from the remaining source
  3. The ON potential becomes invalid permanently
  4. The reference electrode must be defective

Answer: B

Related Pages

Tank Bottom CP Systems Ring anodes, grid anodes, reference cells, liners, and current distribution. AST CP Testing Tank-bottom reference cells, rectifier interruption, criteria, and interpretation. UST CP Testing Direct-connect systems, coupons, criteria, and UST CP interpretation. CP Coupons Coupon measurements, instant-off readings, depolarization, and interpretation. ACVG Survey Applied AC signal coating defect detection and interpretation. DCVG Survey Coating defect detection, voltage gradients, and field interpretation. Current Requirement Testing Temporary current application, current distribution, and CP design interpretation. Galvanic Anode Testing Anode output, current measurement, anode life, and interpretation. Interference Testing Foreign current influence, bonds, stray current, and interpretation. Casing Testing Carrier pipe isolation, casing shorts, and electrolytic contact interpretation. Electrical Isolation Testing Isolation joints, continuity checks, and CP current distribution. Instant-Off Potential Interrupted structure-to-electrolyte potential measurements. Close-Interval Surveys Detailed interrupted pipe-to-soil potential profiles. Pipe-to-Soil Potential Surveys Field potential survey methods and interpretation. Depolarization Testing 100 mV polarization criterion evaluation. Reference Electrode Use Reference electrode placement and measurement quality. Rectifier Inspection Rectifier output evaluation and interruption considerations.