Technical Practice Guide

Current Interruption

Current interruption temporarily cycles applicable cathodic protection current sources OFF and ON so measurements can be made under a controlled current condition. It is especially important when measuring instant-off potentials and when multiple current sources influence the same structure.

What Is Current Interruption?

Current interruption is a field method used to temporarily change the operating condition of applicable cathodic protection current sources. The sources are cycled between ON and OFF conditions according to a selected interruption schedule.

The purpose is not simply to turn a rectifier OFF. The purpose is to establish a known current condition at the time a structure-to-electrolyte potential is measured.

During the ON portion of the cycle, cathodic protection current is flowing from the applicable sources. When the OFF portion begins, the reduction in that current allows an instant-off potential to be captured before substantial depolarization occurs.

Core concept:

Current interruption is the measurement control. The instant-off potential is the measurement produced under that controlled condition. Interpretation comes after the measurement has been shown to be valid and representative.

Why Interrupt CP Current?

Cathodic protection current flows through soil, water, concrete, or another electrolyte before reaching the protected structure. Because the electrolyte has resistance, current flow can produce voltage gradients between the structure and the reference electrode.

An ON structure-to-electrolyte potential can therefore be influenced by current-related voltage drop in the measurement circuit. The magnitude of that influence depends on the field conditions, current distribution, reference-electrode location, and other current sources.

Interrupting the applicable influencing CP current reduces voltage-drop effects associated with those interrupted sources. The potential measured immediately after the OFF cycle begins can then better represent the polarized condition of the structure before substantial depolarization occurs.

1. Current ON

Applicable CP current is flowing. The measured potential may include current-related voltage-drop effects.

2. Current Interrupted

The applicable current sources enter the OFF condition and their current-related voltage-drop contribution is reduced.

3. Instant-Off Captured

The potential is recorded immediately after interruption, before substantial depolarization occurs.

Current interruption should not be described as guaranteeing that every possible current influence has disappeared. Other CP systems, foreign DC sources, bonds, dynamic interference, or other field conditions may still affect the measurement.

What Actually Gets Interrupted?

The survey objective determines which current sources need to be considered. For an instant-off measurement, the technician must identify the CP current sources that significantly influence the structure at the measurement location.

On an impressed-current system, this commonly means one or more rectifiers. A long pipeline, interconnected facility, or complex CP system may be influenced by several rectifiers at the same location.

Other current paths may also need to be understood. Bonds, nearby CP systems, galvanic systems, and foreign structures can affect current distribution even when they are not devices that can simply be placed on an interrupter.

Do not assume one rectifier equals one isolated area of influence.

If another rectifier significantly influences the test location and remains operating, the recorded OFF potential may still contain current-related voltage-drop effects from that source.

Why Does Synchronization Matter?

When several interruptible current sources influence the same structure, their OFF periods need to overlap at the time the instant-off measurement is captured.

Synchronization coordinates the interrupters so the applicable sources enter the intended ON and OFF conditions together. GPS-synchronized interrupters are commonly used when current sources are separated by distance.

Consider two rectifiers that both influence the same pipeline test point. If R-1 enters its OFF cycle while R-2 is still ON, the technician has not created the intended all-applicable-sources-OFF condition at that location.

Interruption Condition What It Means
R-1 OFF and R-2 OFF together The applicable rectifiers are in the intended OFF condition at the same time.
R-1 OFF while R-2 remains ON R-2 may continue to influence the measurement.
R-1 and R-2 both cycle, but their OFF windows do not overlap The interrupters are not providing the intended synchronized OFF condition.
Interrupters appear synchronized, but one rectifier does not actually stop output The timing may be correct while the field current condition is still wrong.
Synchronization must be verified, not merely programmed.

Matching interrupter settings are useful, but the technician still needs confidence that the applicable current sources are actually cycling as intended.

When Is Current Interruption Used?

  • instant-off potential measurements
  • interrupted pipe-to-soil potential surveys
  • close-interval surveys
  • annual or periodic CP surveys when interrupted data is required
  • polarized-potential criterion evaluation
  • depolarization testing
  • rectifier influence and current-distribution investigations
  • troubleshooting questionable ON or OFF potentials
  • field investigations where current-related voltage drop may affect interpretation

The required interruption method and cycle depend on the survey objective, owner requirements, system configuration, and field conditions.

How Is Current Interruption Performed?

  1. Identify the structure and the survey objective.
  2. Identify the CP current sources that may significantly influence the structure at the measurement locations.
  3. Determine which sources can and should be interrupted for the survey objective.
  4. Confirm site access, equipment requirements, and applicable electrical safety requirements.
  5. Install or configure interrupters on applicable interruptible current sources where appropriate and safe.
  6. Select the required ON/OFF cycle.
  7. Synchronize the interrupters when multiple applicable sources must enter the OFF condition together.
  8. Verify that each intended source is actually cycling ON and OFF as expected.
  9. Measure the ON potential during the ON portion of the cycle.
  10. Capture the instant-off potential immediately after the applicable CP current is interrupted.
  11. Document the interruption cycle, sources interrupted, sources not interrupted, and abnormal field conditions.
  12. After testing, remove or disable the interruption equipment as required.
  13. Confirm that CP current sources have returned to their intended normal operating condition.
Electrical safety:

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

Exact procedures vary with rectifier design, owner requirements, survey method, and site conditions.

What Makes Interrupted Data Valid?

A meter displaying an OFF value does not by itself prove that the reading represents the intended interruption condition.

Check Why It Matters
Influencing sources identified A missed current source may continue to affect the measurement.
Correct sources selected for interruption Interrupting a source that does not control the relevant current condition does not establish the intended survey condition.
Interrupters synchronized when required The applicable sources need to reach the intended OFF condition together.
Actual cycling verified Correct programming does not prove that the current source actually stopped output.
Instant-off captured immediately A delayed reading may include depolarization.
Suitable reference electrode and good electrolyte contact Reference-electrode problems can produce unstable or inaccurate potential data.
Correct structure connection A measurement on the wrong structure does not represent the intended asset.
Current condition documented Later interpretation depends on knowing which sources were ON, OFF, or not controlled.
Normal operation restored A CP source unintentionally left OFF can leave the structure without its intended protection.

Common Problems in the Field

Problem Why It Matters
Only one of several influencing rectifiers is interrupted Remaining sources may continue to produce current-related voltage-drop effects.
Interrupters are not synchronized The applicable sources may not be OFF at the same time.
The OFF reading is captured too late The structure may already be depolarizing.
The wrong rectifier is interrupted The measurement may not represent the intended CP-system condition.
An interrupter is programmed correctly but the rectifier does not actually interrupt The expected OFF condition does not exist even though the timing settings appear correct.
Other current influence is ignored Foreign DC sources, bonds, other CP systems, or dynamic interference may still affect the measurement.
The interruption cycle is not documented Later reviewers cannot confidently determine what current condition the data represents.
Normal rectifier operation is not confirmed after testing A CP source may be unintentionally left out of service.

A single abnormal OFF value does not identify its own cause. Review the pattern of measurements, interruption status, timing, nearby current sources, reference-electrode condition, structure connection, and field notes before drawing a conclusion.

How Are Interrupted Measurements Interpreted?

Current interruption establishes a measurement condition. It does not, by itself, establish whether the structure satisfies a cathodic protection criterion.

Once the interruption condition has been verified, the resulting instant-off potential can be evaluated according to the applicable survey objective or criterion.

  • A valid instant-off potential depends on correct interruption and timing.
  • An instant-off potential may be compared with an applicable polarized-potential criterion.
  • Interrupted data should identify which current sources were interrupted and which remained operating or uncontrolled.
  • Incomplete or unsynchronized interruption makes the resulting OFF data questionable for the intended instant-off evaluation.
  • One reading should be interpreted with the surrounding survey pattern, field conditions, and measurement notes.
Keep the sequence straight:

First establish the intended interruption condition. Then capture the instant-off potential. Then determine whether that valid and representative measurement supports the applicable evaluation.

Worked Field Example

A pipeline test point is significantly influenced by two impressed-current rectifiers. During an interrupted survey, the technician records the following:

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

1. What does the recorded OFF value appear to show?

The recorded OFF potential is −895 mVCSE. Numerically, that value is more electro-negative than −850 mVCSE.

2. Was the intended current condition established?

No. R-1 was interrupted, but R-2 remained operating even though it significantly influences the test point.

3. Why does R-2 matter?

Current from R-2 may continue to produce voltage gradients and current-related voltage-drop effects at the measurement location. The recorded OFF value therefore cannot automatically be treated as representative of the intended all-applicable-sources-OFF condition.

4. What is the correct conclusion?

The −895 mVCSE value should be treated as questionable for the intended instant-off evaluation unless R-2 is shown not to materially influence the test location under the survey conditions.

Do not let the number outrun the measurement condition.

A potential that appears to satisfy a numerical comparison is not enough if the interruption condition needed to support that comparison was not established.

Check Your Understanding

Question 1

What is the primary purpose of current interruption during an instant-off survey?

  1. To permanently shut down the CP system
  2. To establish a controlled current condition so an instant-off potential can be measured
  3. To increase the structure's electrical resistance
  4. To determine coating thickness

Question 2

Why may several rectifiers need to be synchronized during an interrupted survey?

  1. To make their OFF periods overlap when they influence the same structure
  2. To increase soil resistivity
  3. To make every rectifier produce the same current
  4. To change the reference-electrode potential

Question 3

Two interrupters have matching programmed ON/OFF times. What additional verification is important?

  1. Confirm that the applicable current sources actually cycle as intended
  2. Confirm that both rectifiers have identical nameplate ratings
  3. Confirm that the soil resistivity is identical at both rectifiers
  4. Confirm that both reference electrodes are connected to the rectifiers

Question 4

What is the main concern if an OFF potential is recorded too long after interruption begins?

  1. The rectifier automatically increases output
  2. The structure may begin depolarizing before the reading is captured
  3. The reference electrode changes polarity
  4. The ON potential becomes the native potential

Question 5

R-1 and R-2 both significantly influence a test point. R-1 is OFF when the potential is recorded, but R-2 remains ON. What is the best interpretation?

  1. The value is automatically a valid instant-off potential
  2. R-2 may continue to influence the measurement, so the OFF value is questionable for the intended interruption condition
  3. The reference electrode must be defective
  4. The pipeline has completely depolarized

Question 6

Which information is most important to document with interrupted survey data?

  1. Only the final criterion conclusion
  2. Only the rectifier manufacturer
  3. The interruption cycle, sources interrupted, sources not interrupted, and abnormal conditions
  4. Only the most electro-negative potential

Question 7

After an interrupted survey is complete, what is an essential final field check?

  1. Confirm the CP current sources have returned to their intended normal operating condition
  2. Leave all interrupters cycling for the next survey
  3. Disconnect the structure from the CP system
  4. Replace every reference electrode used during the survey

Continue Learning

Instant-Off Potential Learn how the immediate OFF reading is captured and interpreted after current interruption. IR Drop Understand why current flowing through the electrolyte can affect measured structure-to-electrolyte potential. Close-Interval Survey See how synchronized interrupted potentials are used to build detailed pipeline potential profiles.