Category Archives: AC Interference

AC Interference – Basic Theory | Video Training Course

What is the impact of AC interference on pipelines?

This 16-minute video training course reviews the 3 basic effects of AC interference on pipelines, including:

  1. Fault conditions
  2. AC induced corrosion
  3. Safety and the 15 volts AC threshold

The summary below includes video timeline indicators so you can easily find your topic of interest in the video.

What is AC Interference?

(0:25) AC interference is an interaction that occurs between high voltage power lines and pipelines in a common utility corridor.

1. Fault Condition Interaction Modes

In the video, our AC mitigation expert Ted Huck explains fault currents and two modes of interaction with pipelines, conductive coupling and stress voltage.

Conductive Coupling

(1:09) Conductive Coupling is a relatively rare occurrence when there is a fault condition along the power transmission line and a large amount of electricity is dumped to the earth. The collocated pipeline is subject to this discharge of electricity through arcing, defined as the flow of current through the soil. Although rare, conductive coupling can burn a hole through the pipeline and cause a catastrophic failure.

Determining the Safe Distance from Tower to Pipeline for Arcing

This chart show how you can determine the safe distance from an electrical transmission tower to a pipeline and the potential for arcing to occur to aid in mitigating AC interference.
(2:08) Arcing depends on soil resistivity and voltage.

Arc Length

This chart shows how far arcing can occur through the soil based on system voltage.
(2:49) We can predict how far arcing can occur through the soil by measuring soil resistivity and system voltage.

(2:16) In this segment, our AC mitigation expert describes a real customer case scenario where arcing caused catastrophic failure of a gas pipeline.

(4:01) Another issue that can occur with conductive coupling is a voltage rise radiating out from the location where the electricity is dumped to the earth. Newer pipeline coatings cannot handle excessive voltage stress.

Stress Voltage

This chart shows maximum AC interference stress voltage for various types of pipeline coatings.
(4:41) Exceeding maximum voltage stress can damage the pipeline coating.

Electromagnetic Induction

(5:44) Electromagnetic Induction is a steady state occurrence where current flowing through the line creates an induced current flowing in the opposite direction along the parallel pipeline. If the pipeline is close enough to the power transmission line, and runs parallel to it for some length, it will be in the electromagnetic field that exists around the AC transmission system. Being in that electromagnetic field, it will inductively pick up current throughout the longitudinal electrical field.

Longitudinal Electrical Field (LEF)

This illustration shows how you can measure the longitudinal electrical field (LEF)
(6:56)  The LEF field can be measured by running a copper cable along the pipeline and measuring the current.

2. AC Induced Corrosion

(7:26) AC induced corrosion occurs when alternating current is picked up by the pipeline that cannot effectively dissipate back to the earth. Well coated pipelines have very few places for the current to exit the pipeline and are at risk for significant, rapid AC corrosion. Older coating systems have many defects, or natural grounding points enabling AC on the pipeline to naturally dissipate, so AC corrosion is a relatively new concern. With newer coatings, AC current continues to build until it finds a small coating holiday (typically 1-3 cm2) to exit the pipeline, risking catastrophic failure.

This image shows a closeup of AC interference induced corrosion, which appears as round craters in the pipeline coating.
(9:00)  AC induced corrosion on a pipeline appears as round craters in the pipeline coating.

How likely is AC corrosion to occur?

This chart shows how likely it is for AC induced corrosion to occur for various current densities.
(9:43)  We can determine the likelihood of AC induced corrosion based on the current density.

Current Density Formula

This formula shows how to calculate current density for a given holiday size; our example is based on a holiday surface area of 1cm².
(10:05)  This formula shows how to calculate current density for a given holiday size; our example is based on a holiday surface area of 1cm².

In our example, 4.4 volts AC is all it takes to cause AC corrosion. With older pipeline coatings that threshold is in the 15 volts AC range.

100 A/m² Threshold – When will AC corrosion occur?

This chart shows holiday size and AC voltage required to exceed the 100 A/m² AC corrosion threshold at varying soil resistivity.
(11:37)  This chart shows holiday size and AC voltage required to exceed the 100 A/m² AC corrosion threshold at varying soil resistivity.

Refer to NACE Report 35110, AC Corrosion State-of-the-Art: Corrosion Rate, Mechanism, and Mitigation Requirements for additional information about the 100 A/m2 threshold.

In Europe, refer to standard BS EN 15280:2013, evaluation of AC corrosion likelihood of buried pipelines applicable to cathodically protected pipelines.

The Relationship Between AC Induced Corrosion and Cathodic Protection


  • No cathodic protection – high likelihood of AC corrosion
  • Excessive CP current, or over polarization may increase AC corrosion
This chart shows how cathodic protection affects the likelihood of AC corrosion.
(12:35)  Properly applied cathodic protection reduces induced AC corrosion risk.

3. Safety

(12:50) Pipelines have above ground appurtenances such as valve stems and test stations that are subject to the AC currents picked up by the pipeline. These can pose a serious safety risk to workers, including shock or death. These risks are referred to as step and touch potential.

Touch Potential is defined as current flowing from touching an electrified device, through the body and down to the earth.

This image illustrates step and touch potential.
(13:38)  Step and touch potential can cause serious safety risks, including injury or death. A maximum of 15 volts is the industry standard threshold for safety.

Step Potential can occur even without the worker touching the electrified device. In this case, current can flow up one foot, through the body and back down to the earth through the other foot, potentially causing serious injury or worse.

Refer to NACE SP0177-2014 (formerly RP0177), Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems,  Paragraph for additional information on the 15 volt safety criteria.


There are 3 basic effects of AC interference on pipelines, including the maximum 15 volt safety threshold—how much voltage can accumulate on the pipeline before it becomes a safety hazard to a person touching the pipeline? If there is more than 15 volts AC, we must do something to drop that voltage. Then there are rare but potentially catastrophic fault conditions, or the dumping of current to the earth. Finally there is AC induced corrosion, a result of the interaction of the electromagnetic field generated by current flowing through the lines and how it reacts with the pipeline. Pipeline operators must be prepared to mitigate these risks.

Have questions or need a quote to mitigate the risks of AC interference? Contact us at the link below.


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AC Modeling – The Basics

AC Modeling Overview

Pipeline Right-of-Way
AC Modeling enables pipeline operators to evaluate and plan for mitigating AC corrosion.

There continues to be much greater awareness by pipeline owners and regulators of the adverse interactions (AC Interference) that can occur between buried pipelines and above ground high voltage AC transmission systems that share some parallelism in a common right of way. When AC Interference conditions exist, it is important that the potential impact is evaluated and when necessary mitigated. For many applications, the most cost-effective approach to assess and mitigate the impact of AC Interference is to use a complicated computer AC modeling program.

The term AC Modeling really covers multiple modeling evaluations as an AC corridor can often be quite complex. They may include multiple HVAC transmission systems and multiple pipelines in a common corridor or multiple shared right of ways along a long length of pipeline. Each may require its own AC modeling. In addition, the modeling looks at several different risks assessing how the pipeline is affected by steady state AC induced current, the impact of fault current along the pipeline and an evaluation of the impact of a fault current on above ground appurtenances to assure safe operation in accordance with IEEE std. 80 step and touch potential criteria.

Thus, it is very important for any successful AC modeling effort that the modeling software be of an extremely high quality and capable of properly handling the complex interactions of these various networks. The engineer or technician developing the model must also have sufficient experience and expertise to properly configure and operate the model, and evaluate the results.

AC modeling involves four key phases:

  1. Data Collection
  2. Creating the Model
  3. Establishing criteria
  4. Evaluating mitigation strategies

Data Collection

The data collection is critical to a successful modeling effort (the old adage garbage in = garbage out is quite applicable for these projects). The data requirements can be broadly broken out into three categories:

  1. The characteristics of the AC Transmission Line(s)

    • Physical geometry data on the tower including GPS location, height, # of AC circuits, tower configuration, height of each conductor, lowest point of each conductor, separation distance between conductors, shielding wire type and location, location of any phase transpositions, etc…
    • Electrical data on the Transmission Line(s), including peak and average AC Load (in each direction), fault current max and duration.
  2. The characteristics of the Pipeline(s)

    • GPS location, depth of cover, coating type, coating resistance, pipeline diameter, pipeline wall thickness, location of all above ground appurtenances, location of all CP test stations and bonds to foreign structures.
  3. The characteristics of the Environment

    • Detailed soil data at multiple depths along the length of the pipeline, location of any crossings, presence/location of any foreign CP Stations or other interference conditions.

Collecting all the appropriate data often requires some field studies and working with both the pipeline owner(s) and the transmission line operator to get the required data. In some cases, the modeler cannot get all the required information and must make an educated guess the accuracy of which can affect the quality of the results.

Creating the Model

AC Modeling
AC modeling software enables input of pipeline, transmission and environmental characteristics

Once all the data is collected, the modeler creates the model space detailing all the pipelines and HVAC systems and providing the requisite parameters associated with each of these elements. There are several commercially available AC modeling software packages that each have their own format for inputting the pipeline, transmission and environmental characteristics. Once the model has been built, it can take hours, days and in some cases weeks of processing time to run simulations and for the model to provide the results of the simulation.

Evaluating the Model Results Against Established Criteria

The results of the initial model run need to be evaluated against the criteria that is established by the pipeline owner. In the absence of specific guidance from the owner, MATCOR’s default criteria are:

  • No more than 20 A/m2 AC current density for mitigating AC corrosion during steady state induced AC current
  • 3000 volts maximum coating stress during fault conditions for newer FBE type coated pipelines in accordance with NACE standard SP0177-2014
  • 15 VAC for step and touch potentials at above ground appurtenances

For any given application, one or more of these criteria may be exceeded along the model’s area of analysis.

Adding AC Mitigation and Reevaluating the Modeling Results

Once the initial unmitigated results have been evaluated against the criteria that has been established, the modeler then adds, based on their experience with these systems, a mitigation scheme to the model with grounding at selected locations. This is often an iterative project where the model is run and the results evaluated and then if necessary additional mitigation can be added or excess mitigation can be removed and the model rerun again in search of an “optimized” modeling solution that addresses all of the threats and results in meeting the requisite criteria.

Final Report

Once the AC modeling effort has developed a solution, the modeler develops a final report. Typical components of a final report include an introduction detailing the scope of the study, graphical illustrations of the pipeline(s) and transmission line(s) overlaid on to a satellite image, description of the modeling software used, detailed graphs/charts showing the results of the modeling, detailed drawings and bill of materials for the mitigation solution being recommended and appendixes with the underlaying data.


AC Interference issues can be quite complex and modeling often offers the only valid way to assess and mitigate the risks from AC faults and steady state induced currents. When considering AC modeling it is important to look at the model being used and the modeler performing the evaluation.

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Questions about AC interference, modeling or mitigation? Please contact us at the link below. Our experts are happy to help.

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