Technical Practice Guide

Interference Testing: Foreign Current, Stray Current, Bonds, and Interpretation

Interference testing evaluates whether external current sources or nearby metallic structures are changing the potential, current flow, or corrosion risk of the structure being evaluated.

Overview

Interference testing is used to evaluate whether current from one system is affecting another structure.

CP systems do not exist in isolation. Nearby pipelines, tanks, casings, grounding systems, rail systems, rectifiers, bonds, and foreign structures can affect measured potentials.

Interference can make potentials more electro-negative or more electro-positive. Current pickup areas may appear more protected, while current discharge areas may experience increased corrosion risk.

Interference testing helps determine whether abnormal potentials are caused by inadequate CP, foreign current influence, bonds, isolation problems, or current distribution effects.

Interference testing does not replace CP criterion evaluation. It helps explain why measured potentials behave the way they do.

Core concept:

Interference testing asks whether the structure’s potential is being changed by current that did not originate from, or is not controlled by, the intended CP system.

Technical Basis

Stray current corrosion occurs when current leaves a metallic structure and enters the electrolyte at a discharge area.

  • Where current enters a structure, the potential may shift more electro-negative.
  • Where current leaves a structure, the potential may shift more electro-positive.
  • Current discharge areas are often the corrosion concern.
  • An impressed current rectifier can influence nearby foreign structures.
  • A foreign rectifier can influence the structure being tested.
  • Bonds may be installed to control current flow between structures.
  • Resistance bonds can limit or control current transfer.
  • Structure-to-structure voltage can help identify driving force between metallic systems.
  • Current interruption can help determine whether a rectifier is influencing a structure.
  • Dynamic stray current may vary with time and may require time-based monitoring.

Interference interpretation usually requires comparing multiple readings including affected structure potentials, foreign structure potentials, structure-to-structure voltage, bond current, rectifier timing, and historical data.

When Interference Testing Is Used

  • Investigating abnormal pipe-to-soil potentials
  • Evaluating suspected foreign CP system influence
  • Testing the effect of a rectifier on nearby structures
  • Investigating electro-positive potential shifts
  • Evaluating current pickup and current discharge areas
  • Checking bond performance and bond current direction
  • Assessing resistance bond settings
  • Evaluating structures near crossings, parallel pipelines, and congested corridors
  • Investigating interference near casings, isolation devices, and grounding systems
  • Supporting close-interval survey anomaly review
  • Evaluating dynamic stray current where potentials vary over time

Equipment Typically Used

EquipmentPurpose
High-impedance voltmeterMeasures structure-to-electrolyte potentials and structure-to-structure voltages.
Copper-copper sulfate reference electrodeProvides a stable reference potential for soil measurements.
Current interrupterCycles rectifiers ON and OFF to evaluate influence.
Data loggerRecords time-based potential changes for dynamic interference.
Shunt or bond current measurement deviceHelps determine current magnitude and direction through a bond.
Test station or bond box accessProvides access to affected structure, foreign structure, and bond leads.
Rectifier output records or RMU dataSupports correlation between output changes and potential shifts.
Facility drawings, alignment sheets, or CP mapsHelp identify nearby structures, crossings, bonds, and current sources.
Caution:

Do not disconnect bonds, alter resistance bonds, or interrupt foreign rectifiers unless qualified, authorized, and coordinated with the affected parties.

General Field Method

  1. Identify the affected structure, suspected interfering structure, rectifier, bond, crossing, or current source.
  2. Review drawings, bond records, rectifier records, test station labels, and prior survey data where available.
  3. Determine what current source or connection is being evaluated.
  4. Measure baseline structure-to-electrolyte potentials on the affected structure.
  5. Measure foreign structure potentials where access is available and appropriate.
  6. Measure structure-to-structure voltage where appropriate.
  7. Measure bond current or shunt millivolts where a bond or resistance bond is present.
  8. Interrupt or vary the suspected influencing current source only where authorized and coordinated.
  9. Observe potential shifts on the affected structure and foreign structure during ON/OFF or changed-output conditions.
  10. Document current direction, current source status, timing, bond configuration, and field conditions.
  11. Use time-based logging where potentials vary dynamically.
  12. Compare results to nearby survey data, historical data, and route features.

Exact methods vary depending on whether the suspected interference is steady-state DC, dynamic DC, foreign CP influence, bond-related, or another condition.

Valid Data Conditions

  • Correct identification of the affected and suspected interfering structures
  • Known current source status during each measurement
  • Stable reference electrode contact
  • Correct test lead identification
  • Documented rectifier output or current source condition
  • Coordinated interruption or adjustment where required
  • Known bond configuration and bond current direction where applicable
  • Structure-to-structure voltage measured with clear polarity
  • Time-based logging when potentials fluctuate dynamically
  • Awareness of crossings, casings, isolation devices, grounding systems, and parallel structures
  • Verification of unusual shifts before reaching a conclusion

Interference conclusions require timing correlation. A single measurement rarely proves interference by itself.

Common Errors and Misinterpretations

ErrorWhy It Matters
Assuming every electro-positive shift is caused by interferenceReference electrode contact, soil changes, timing, or system operation can also affect readings.
Assuming more electro-negative potential always means improved protectionIt may represent current pickup from a foreign source rather than controlled CP performance.
Ignoring current direction through a bondBond current direction helps distinguish pickup and discharge behavior.
Interrupting only the local rectifier when a foreign rectifier is suspectedThe actual influencing current source may remain energized.
Changing bond settings without coordinationCan create new interference conditions or affect another structure’s protection.
Using one reading to prove interferenceInterference interpretation usually requires multiple correlated measurements.
Ignoring dynamic variationShort-duration readings may miss time-dependent stray current effects.
Failing to document polarityStructure-to-structure voltage and bond current direction can be misinterpreted.

Interpretation

  • A more electro-negative shift may indicate current pickup.
  • A more electro-positive shift may indicate current discharge or loss of protective current.
  • Current discharge areas are typically more concerning for corrosion risk.
  • If an affected structure potential changes when a foreign rectifier is interrupted, the foreign rectifier may be influencing the structure.
  • If bond current direction indicates current leaving the affected structure, possible discharge should be evaluated.
  • If potentials vary over time without deliberate interruption, dynamic stray current may be present.
  • Interference mitigation must consider both structures, not only the structure being tested.
ObservationGeneral Interpretation
Affected structure shifts electro-positive when foreign rectifier is ONMay indicate current discharge or adverse interference.
Affected structure shifts electro-negative when foreign rectifier is ONMay indicate current pickup or cathodic influence from the foreign source.
Potential shift correlates with rectifier interruption timingSupports the conclusion that the current source influences the structure.
Bond current direction shows current leaving the affected structureMay indicate discharge through the bond or toward another structure.
Potentials fluctuate without deliberate interruptionMay indicate dynamic stray current or changing operating conditions.

Worked Example

A pipeline crosses near a foreign impressed current CP system. During coordinated testing, the following readings are recorded:

ConditionAffected Pipeline PotentialForeign Rectifier Status
Baseline−910 mVCSEON
Foreign rectifier interrupted−780 mVCSEOFF
Foreign rectifier restored−905 mVCSEON

The affected pipeline shifts more electro-negative when the foreign rectifier is ON. The potential shifts electro-positive when the foreign rectifier is interrupted.

This suggests the affected pipeline may be picking up current from the foreign CP system at that location.

The more electro-negative value does not automatically mean the condition is acceptable. Current pickup at one location can be associated with current discharge elsewhere.

Additional testing should evaluate nearby discharge locations, structure-to-structure voltage, bond current, CIS data, and the foreign structure’s protection.

Practice Questions

Question 1

What does interference testing evaluate?

  1. Only coating resistance
  2. Only soil resistivity
  3. Whether foreign current sources or nearby structures are affecting a structure
  4. Only rectifier output voltage

Answer: C

Question 2

Why can a more electro-negative potential from a foreign source still require investigation?

  1. Because current pickup at one location may be associated with current discharge elsewhere
  2. Because electro-negative shifts automatically fail the −850 mVCSE criterion
  3. Because electro-negative shifts prove the reference electrode is damaged
  4. Because electro-negative shifts always indicate AC interference

Answer: A

Question 3

What does timing correlation with a foreign rectifier help show?

  1. Only coating thickness
  2. Only soil moisture
  3. That the foreign structure is isolated
  4. Whether the current source is influencing the affected structure

Answer: D

Question 4

Why is bond current direction important?

  1. It permanently determines coating condition
  2. It helps identify current pickup and discharge behavior
  3. It eliminates the need for interruption testing
  4. It proves the structure satisfies the 100 mV polarization criterion

Answer: B

Question 5

Why is one isolated potential reading usually not enough to prove interference?

  1. Because all interference is AC-related
  2. Because one reading always includes IR drop
  3. Because interference interpretation usually requires multiple correlated measurements
  4. Because interference cannot affect pipe-to-soil potentials

Answer: C

Related Pages

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. Soil Resistivity Testing Wenner four-pin method, data quality, and CP interpretation. Electrical Isolation Testing Isolation failures, shorts, and current distribution. Casing Testing Carrier pipe isolation and electrolytic contact interpretation. Close-Interval Surveys Potential profiles and localized anomaly review. Current Interruption Rectifier interruption and synchronized timing. Rectifier Inspection Rectifier output evaluation and system influence. Pipe-to-Soil Potential Surveys Structure-to-electrolyte potential measurements. Reference Electrode Use Measurement quality and reference electrode placement.