Technical Practice Guide

Instant-Off Potential

An instant-off potential is measured immediately after cathodic protection current is interrupted. The measurement helps evaluate the polarized condition of a protected structure while reducing voltage-drop error associated with CP current flowing through the electrolyte.

What Is an Instant-Off Potential?

An instant-off potential is a structure-to-electrolyte potential measured immediately after cathodic protection current is interrupted, before substantial depolarization occurs.

The meter is still measuring the voltage difference between the structure connection and the reference electrode. What changes is the current condition during the measurement.

The important word is instant. An instant-off potential is not simply any potential measured while a CP source is OFF. It is the potential captured immediately after interruption.

Illustrative graph showing an ON potential of minus 1100 millivolts CSE, an immediate change to minus 950 millivolts CSE when CP current is interrupted, and depolarization toward minus 690 millivolts CSE over time.
ON, instant-off, and depolarized potentials represent different measurement conditions. The values and depolarization response shown are illustrative.

Why Isn't the ON Potential Enough?

Cathodic protection current flows through soil, water, concrete, or another electrolyte to reach the protected structure. Because the electrolyte has resistance, current flowing through it produces voltage drop.

A structure-to-electrolyte potential measured while CP current is flowing can therefore include current-related voltage drop between the structure and reference electrode. This voltage drop is commonly called IR drop.

As a result, an ON potential may appear more electro-negative than the polarized condition of the structure itself.

Interrupting the applicable CP current reduces current-related voltage-drop error in the measurement. The potential captured immediately after interruption is used to better represent the polarized structure condition before substantial depolarization occurs.

Side-by-side illustration comparing a pipeline potential measurement while CP current is flowing with the same measurement immediately after interruption. The meter connections remain unchanged while the current condition changes.
The meter connections remain the same. The applicable CP current is interrupted so the instant-off reading can be captured with current-related voltage-drop error reduced.

Learn more about IR Drop →

ON, Instant-Off, and Depolarized Potentials

These measurements represent different conditions and should not be treated as interchangeable.

ON

Condition: CP current is flowing.

The reading may include significant current-related voltage drop.

Instant-Off

Condition: Measured immediately after applicable CP current is interrupted.

The reading is intended to represent polarized potential with current-related IR drop reduced.

Depolarized

Condition: Measured after polarization has been allowed to decay.

The reading can be used with the polarized potential to evaluate polarization change.

Do not confuse instant-off with depolarized potential.

Instant-off is captured immediately after interruption. A depolarized potential is measured after polarization has been allowed to decay.

When Is Instant-Off Testing Used?

Instant-off measurements are used when the polarized condition of a cathodically protected structure needs to be evaluated while reducing the influence of current-related voltage drop.

Routine Surveys

  • annual CP surveys
  • close-interval surveys
  • interrupted structure-to-electrolyte surveys
  • tank-to-soil potential surveys

System Evaluation

  • system commissioning
  • criterion evaluation
  • verification following CP-system adjustment

Troubleshooting

  • questionable or unexpected potentials
  • unexpected system response
  • interference investigations

Instant-off measurements can be especially important on impressed-current systems because these systems can produce significant voltage gradients in the electrolyte.

How Is an Instant-Off Potential Measured?

The basic measurement requires a high-impedance voltmeter, a suitable reference electrode, test leads, access to the structure, and a means of interrupting the applicable CP current source or sources.

For a typical pipeline structure-to-electrolyte measurement, connect the red meter lead (POSITIVE) to the structure and the black meter lead (NEGATIVE) to the reference electrode.

Pipeline instant-off field measurement setup showing the red voltmeter lead connected to the pipeline through a test station and the black lead connected to a copper copper sulfate reference electrode at the soil surface.
Typical instant-off field setup. The measurement connections remain in place while the applicable CP current is interrupted and the instant-off potential is captured.

Before Interruption

  1. Confirm the structure being tested and the correct structure connection.
  2. Confirm that the reference electrode is suitable, in good condition, and making good electrolyte contact.
  3. Identify the CP current source or sources that influence the structure.
  4. Measure and record the ON potential with CP current applied.

During Interruption

  1. Interrupt the applicable CP current source or sources.
  2. Capture the instant-off potential immediately after interruption.
Timing matters.

Waiting too long after interruption allows depolarization to begin. The resulting value may no longer represent the intended instant-off condition.

After Interruption

  1. Restore CP current.
  2. Confirm that the interrupted current source or sources returned to normal operation.
  3. Record relevant field conditions and anything that could affect interpretation.
Interruption survey versus depolarization survey:

During an interruption survey, the ON/OFF cycle repeats and the instant-off reading is captured during each OFF cycle. During a depolarization survey, the CP current remains OFF after the instant-off reading so the change in polarization can be measured over time.

What Makes an Instant-Off Reading Valid?

Obtaining a number immediately after a rectifier turns OFF does not automatically make that number representative instant-off data.

Check Why It Matters
Correct structure connection Measuring another structure can invalidate the result.
Suitable reference electrode with good electrolyte contact Poor electrode condition or contact can produce unstable or inaccurate measurements.
Applicable influencing current sources interrupted Current remaining from another source may continue to produce voltage-drop effects.
Interrupters synchronized when necessary Multiple sources need to reach the intended interruption condition at the appropriate time.
Reading captured immediately Waiting allows the structure to begin depolarizing.
Meter and test leads functioning properly Measurement-equipment problems can affect the recorded potential.
Field conditions documented Unusual or questionable data needs measurement context for proper interpretation.
Field scenario:

Suppose three rectifiers influence the same pipeline. Two rectifiers are synchronized and interrupted, but the third remains operating. The technician may still see an immediate potential change when the two rectifiers switch OFF, but current from the third rectifier continues to influence the structure.

The resulting reading should not automatically be assumed to represent the intended instant-off condition because an influencing CP current source remains operating.

How Is an Instant-Off Potential Interpreted?

Instant-off potentials are commonly used when evaluating the −850 mVCSE polarized-potential criterion.

Instant-Off Potential General Interpretation
More electro-negative than −850 mVCSE May satisfy the −850 mVCSE polarized-potential criterion when the reading is valid and representative.
Less electro-negative than −850 mVCSE Does not satisfy the −850 mVCSE polarized-potential comparison being evaluated.
Questionable or unstable Additional testing or verification may be required before drawing a conclusion.

A value that does not meet the −850 mVCSE polarized-potential criterion does not automatically establish inadequate CP under every applicable evaluation method. Cathodic polarization may also be evaluated using an applicable polarization criterion.

The conditions and limitations associated with polarization criteria belong in the dedicated 100 mV Polarization Criterion and Depolarization Testing lessons.

Worked Field Example

A technician records the following potentials at a pipeline test station:

Measurement Potential
Native −530 mVCSE
ON −1,100 mVCSE
Instant-off −950 mVCSE
Depolarized −690 mVCSE

1. What does the ON potential tell us?

The ON potential is −1,100 mVCSE. This is a very electro-negative measured potential, but CP current is flowing. The reading may therefore include current-related voltage drop.

The ON value alone should not be treated as proof that the polarized potential satisfies the −850 mVCSE criterion.

2. What does the instant-off potential tell us?

Immediately after interruption, the potential is −950 mVCSE.

Because −950 mVCSE is more electro-negative than −850 mVCSE, the test location satisfies the −850 mVCSE polarized-potential comparison being demonstrated, assuming the measurement is valid and representative.

3. What does the depolarized potential tell us?

After polarization has been allowed to decay, the measured potential is −690 mVCSE.

The magnitude of the potential change between the polarized and depolarized conditions is:

|−950 mV − (−690 mV)| = 260 mV

The calculation preserves the signs of the measured potentials and reports 260 mV as the magnitude of the potential change.

4. What can we conclude?

The instant-off value in this example satisfies the −850 mVCSE polarized-potential comparison being demonstrated. The 260 mV potential change also exceeds 100 mV; however, application of the 100 mV polarization criterion has additional conditions and limitations that are covered in the dedicated 100 mV Polarization Criterion and Depolarization Testing lessons.

Check Your Understanding

Question 1

Why is an instant-off potential generally preferred over an ON potential when evaluating polarized potential?

  1. Because the instant-off value is always more electro-negative
  2. Because interrupting applicable CP current reduces current-related voltage-drop error
  3. Because an instant-off measurement does not require a reference electrode
  4. Because instant-off measures coating thickness

Question 2

A pipeline has an ON potential of −1,050 mVCSE and an instant-off potential of −810 mVCSE. What is the best interpretation?

  1. The pipeline satisfies the −850 mVCSE polarized-potential criterion at that location
  2. The instant-off value does not satisfy the −850 mVCSE polarized-potential criterion at that location
  3. The ON potential proves adequate CP
  4. The reference electrode has definitely failed

Question 3

A test point has an instant-off potential of −825 mVCSE and a depolarized potential of −690 mVCSE. What is the magnitude of the potential change?

  1. 85 mV
  2. 100 mV
  3. 135 mV
  4. 1,515 mV

Question 4

Which condition can make an instant-off potential questionable?

  1. Correct test-lead connection
  2. Good reference-electrode contact
  3. Unsynchronized current interrupters where synchronization is required
  4. Proper current restoration after testing

Question 5

A structure does not satisfy the −850 mVCSE polarized-potential criterion but demonstrates 125 mV of cathodic polarization. What should the learner understand?

  1. The structure automatically has no CP
  2. An applicable polarization criterion may provide another method of evaluation, subject to its conditions and limitations
  3. The ON potential should replace all other readings
  4. The depolarized potential should be ignored

Question 6

Three rectifiers significantly influence a pipeline. Two are interrupted together, but the third remains operating. Why should the resulting instant-off reading be treated cautiously?

  1. The meter cannot measure while any rectifier is operating
  2. The third rectifier may continue to produce current-related voltage-drop effects in the measurement
  3. The pipeline immediately becomes depolarized
  4. The reference electrode changes polarity

Question 7

A technician waits after CP current is interrupted before recording the OFF potential. What is the main concern?

  1. The reading may include the effects of depolarization and no longer represent the intended instant-off condition
  2. The meter automatically changes from millivolts to amperes
  3. The reference electrode becomes an anode
  4. The ON potential becomes more accurate

Continue Learning

IR Drop Understand why current flowing through the electrolyte affects measured structure-to-electrolyte potential. Current Interruption Learn how CP sources are interrupted and why synchronization matters. Depolarization Testing Continue beyond instant-off and learn how polarization change is evaluated.