Daphne C. D’Zurko, Director, RD&D, NYSEARCH, Northeast Gas Association, New York, New York

Anton Kacicnik, Program Manager, Enbridge Gas Distribution Inc., Toronto, Ontario

A new device called the Stray Current Mapper (SCM) has shown promise in its ability to help gas utility personnel assess and mitigate stray current interference on their pipelines.

Stray currents are sometimes present on gas mains and transmission pipelines as a result of interference by external source(s) such as direct current (DC) transit systems, anode beds from other protected pipes, and industrial plants. Stray currents can adversely affect pipelines by overriding cathodic protection (CP) currents, resulting in corrosion, resistance shorts, or coating damage.

Responding to the need for more accurate, time saving interference assessment methods expressed by corrosion personnel, in 1997 a consortium of natural gas transmission and distribution companies began developing the SCM tool. Members of the international consortium included Northeast Gas Association (formerly New York Gas Group, or NYGAS), Southern California Gas, Pacific Gas & Electric, Gas Technology Institute (GTI), GasUnie (Holland), SNAM (Italy), and Radiodetection (UK). These companies and organizations were involved in design, field tests, and feedback sessions during development.

The SCM proved its value in the field tests, demonstrating an ability to assess and resolve stray current interference on underground gas transmission or distribution pipelines. SCM is commercially available through Radiodetection, Bristol, UK. That company also manufactures another current-measuring device known as the Pipeline Current Mapper (PCM).

Tool definition

The Stray Current Mapper is a non-intrusive tool that can be used anywhere along a gas pipe to determine the presence of stray currents, and the location of current pickup and discharge points. While the system components have changed over the design life cycle, the end product has met this initial objective.

As shown in Figure 1, the base product consists of a sensor bar (powered by alkaline batteries) and a ruggedized portable computer (not sold as part of the system) for on-site analysis. Ancillary components, necessary for situations in which there are multiple pipes or rectifiers, include a smart interrupter (Figure 2), and for congested areas, a smart probe (Figure 1).

The smart interrupter is used to introduce synchronized signals (on-off patterns using current from the rectifier) to the CP system, to distinguish between multiple current sources. Several smart interrupters, each having their own unique signature, may be used together to enable assessment of the magnitude and direction of stray currents from multiple foreign CP supplies and multiple pipelines. When in congested urban environments, the smart probe can be placed directly over the pipe to measure low current magnitudes, thus significantly reducing magnetic interference from other pipelines or passing vehicular traffic.

Stray current assessment

To assess stray currents being transferred onto or from a gas distribution or transmission system, the magnitude and direction of pipe-soil currents must be measured. The SCM is a portable tool that uses a number of sensitive magnetometers, which have a wide dynamic range and can separate currents in the milliamp range from other large current sources such as the earth’s magnetic field. In addition to measurement of current, the sensor bar magnetometers locate pipe in both the horizontal and vertical planes.

Unlike the SCM, traditional techniques to assess and resolve stray currents require that the instrumentation be physically connected to the pipe at test stations. This may be time consuming, and may limit the extent/level of information that can be acquired.

Stray currents can be divided into two different types, static and dynamic. Each has its own characteristics and requirements for accurate measurements. Static currents arise from multiple sources of CP and do not change over time. Dynamic currents typically come from DC rail transit systems and must be measured over time.

The SCM is equipped to measure both types of stray current, and has an on-board data logger to take continuous measurements over time. Static stray currents can come from interfering CP systems, foreign anode ground beds, interference bonds with foreign pipes, and shorted insulators between two pipes.

Compared to dynamic stray current sources, static strays from foreign CP rectifier systems are less complicated, due to the constant output level of the interfering source.

Advance preparation is required prior to the initial SCM assessment. This includes trying to understand the characteristics of the problem site – its history, the line location, and other nearby facilities (rectifiers, pipes, current carrying structures). Initially, SCM surveys can be done at large intervals to begin a wider area search, and begin to narrow down the sources of stray current pickup and discharge.

Tool operation

Simple to operate, the SCM sensor bar is placed above the target pipe, and the SCM is connected to the laptop. The software program activates the SCM sensor bar to measure current on the pipe.

The SCM sensor bar transmits information, via a two-way communications link, in real-time to the laptop computer. A bold black arrow indicates whether the current is traveling in the same direction as the red arrow on the sensor bar, or in the opposite direction. The magnitude of the current is also displayed. By measuring the current magnitude, subsequent measurements are used to confirm stray current on the pipeline and pick-up or discharge points. Although most static interference mapping is done in real-time, the information can still be logged. It may be beneficial to use a smart probe to filter out unwanted traffic interference in urban areas, the effects of close-by parallel magnetically interfering pipelines, or low current magnitudes.

When stray current interference is dynamic, such as from an electrified rail system, one SCM sensor bar is placed above the subject pipeline, and the second bar is used to take measurements along the pipeline at appropriate intervals. Each sensor bar is capable of logging and storing up to 36 hours of data on an 8Mb smart media card. The data is downloaded to the laptop PC for analysis to reveal the profile of the dynamic stray current over the monitored section of pipeline, and location(s) of stray current pick up and discharge points. A smart interrupter is not used when taking dynamic measurements.

Case studies

DC current operated transit systems have been the greatest source of difficulty for one project sponsor: Enbridge Gas Distribution, Toronto, Canada. Dynamic stray currents from these transit systems affect nearly 50% of the Enbridge gas distribution system in the greater Toronto area.

Stray current interference situations generated by foreign CP rectifier sources are relatively non-complicated (compared to transit sources) due to the fairly constant output level of the interfering source. Stray current interference caused by DC transit systems, on the other hand, is inherently more complicated to ascertain and resolve due the ever-changing magnitudes of the interfering current source.

A common operating potential for light transit systems such as streetcars is 600 volts, with the rails serving as the current return portion of the circuit. The streetcar system load current, which can reach thousands of amperes in peak periods, is supposed to return to the streetcar substation via the rails, which are connected to the negative bus at the substation. If the rails are completely insulated from the ground, a return to source via the earth is unlikely and, therefore, adjacent utilities are not adversely affected. However, due to design and construction flaws or, more commonly, the deterioration or lack of maintenance of rail insulating materials or devices, leakage to ground of some portion of the total transit system’s DC load is common.

Pipelines in the area constitute a good return path for a portion of the stray earth current. Such a pipeline will carry the current to a location in the vicinity of the DC substation, where the current will flow from the pipeline to earth and return to the negative bus of the substation. At such areas forced pipeline corrosion will result if corrective measures are not taken. If there are insulation joints in the pipeline, there may be enough driving voltage to force current to bypass the joint, and to corrode the pipe on the side where the current leaves the pipe.

Where the pipeline is picking up stray current it is also receiving cathodic protection. In severe cases, as found out during SCM testing in Toronto, the pipe may be many volts negative to adjacent earth in this area and, at the same time, many volts positive to earth in the current discharge area near the DC substation. In spite of site complexities, the SCM tool can be used to systematically, accurately and quickly evaluate and resolve dynamic stray current interference.

Stray current discharge

Determining the exact location of stray current discharge is the most challenging part of any stray current troubleshooting work. SCM fieldwork in Toronto showed that stray current interference could be experienced even on pipelines that are not close to a DC substation. If, for example, a pipeline crosses a railway system, stray current may be picked up at the point of crossing as trains pass. This stray current will flow in both directions from the crossing and discharge at remote locations (Figure 3).

If a pipeline parallels a rail line, but does not approach a DC substation closely, the pipeline will pick up current as streetcars pass the parallel section. This current may be discharged from the ends of the parallel section. In finding its way back to the DC substation, stray current may jump from pipeline to pipeline, at crossings, in order to follow the most direct or least resistant path. Corrosion damage at such crossings can be severe.

For example, the Enbridge Gas Distribution corrosion prevention department suspected coating damage and stray current interference on 2.5-km (7,500-ft), NPS 12 (12-in.) high-pressure main on Wilson Avenue in Toronto. The department’s previous efforts to fully resolve interference at the site were not successful, and mitigation solutions were deemed too costly. In earlier tests, the department had assessed the condition of the coating on the main using the PCM tool, and had identified an area of considerable coating degradation just west of Avenue Road.

As part of the project, the extent of stray current interference was determined using the SCM. Figure 3 depicts the manner in which the measurements were taken along the pipeline. The first of the two sensor bars, called the reference bar, was placed at the most easterly end of the pipe and left in position to log data continuously. Meanwhile, the second bar was used to take stray current readings at various locations along the pipeline to determine the amount of current discharge, as compared to the reference bar readings.

Results (Figure 4) show the magnitude of stray current coming from the east end of the line remained fairly consistent at most points on the pipe, but dropped off rapidly near the end of the line (near the insulator) – precisely where the earlier assessment had identified poor coating conditions. It became apparent that literally all of the stray current was leaving the company’s pipeline at this point and jumping onto another structure in the area, causing premature deterioration. Using SCM technology, the complete distribution pattern of stray current interference on Wilson Avenue site was readily obtained and mapped, providing for a more suitable mitigation plan.

Since the measurements were made non-intrusively (Figure 5), the site assessment was completed quickly. There was no need to establish numerous contact points along the pipeline at locations of primary interest or concern, as is required by most of the traditional stray current investigation methods.

Given the site environment, pipeline history, rectifier locations, and the extent of stray current interference, installation of an insulator was deemed to be the most appropriate solution and approval was obtained for implementation.

An insulator has since been installed at the Yonge Street crossing, increasing line circuit resistance and eliminating damaging stray current interference on this line. The insulator’s effectiveness in controlling interference was verified with the SCM on a subsequent site visit. Pipe-to-soil readings along the pipeline show that the line is adequately protected throughout.

Static stray current interference

Static stray current situations occur when some of the current used by a foreign CP protected system is forced onto a gas pipe network. The extent of static stray current interference can be highly variable, and depends upon:

1. Driving voltage and current output of the foreign rectifier

2. Distance between the foreign ground bed rectifier

3. Distance between the foreign ground bed and the affected gas piping

4. Distance between the foreign structure and the affected pipe at pipeline crossings

5. How well each facility is coated

6. The overall resistance between the systems.

Even small amounts of stray current, if not detected and properly mitigated, can quickly result in substantial damage to the affected distribution system.

Figure 6 depicts the most basic case of static stray current interference at a pipeline crossing. At current pickup locations, a beneficial effect occurs as the affected pipeline is made more cathodic to the surrounding soil. However, the interfering current seeks to return to its source; and where it jumps from the affected pipe to the interfering structure, the current discharge area is made anodic and active corrosion will occur.

Such a situation can effectively be resolved using the SCM, by interrupting the interfering rectifier with a signal having a specific signature (smart interrupter), and by measuring interrupted current magnitude and direction along the affected pipeline. Using such an approach, the affected system can be completely mapped, including areas of current pick-up and discharge. This ensures the involved parties can quickly reach an appropriate and mutually acceptable mitigation solution.

In case of a multi-rectified pipeline system (Figure 7), the tool can again be used effectively to determine and map distribution of interfering and CP currents. Using the device along with multi-signature smart interrupters, the contribution of current magnitude and direction from each rectifier can be measured and displayed in real time.

Based on this approach, current attenuation from each rectifier and current overlap (or lack of it) between any two rectifiers can also be determined.

The Enbridge Gas Distribution (EGD) corrosion prevention department had long been trying to resolve an interference situation at a pipeline crossing in Barrie, Ontario, between the company’s pipeline and two parallel transmission pipelines operated by TransCanada PipeLines (TCPL). From previous CP potential surveys, it had been determined that TCPL was affecting the EGD pipeline. However, due to lack of high quality data to substantiate the claims, mitigation actions were not taken.

In light of persistent interference problems at this site and countless hours already spent troubleshooting the problem, EGD corrosion prevention personnel decided to use the SCM to assess the site and resolve the problem. It is interesting to note that this field example very much resembled a “text-book” example shown in Figure 6. Furthermore, this assignment required EGD corrosion staff to work together with their counterparts at TCPL, since a smart interrupter had to be (temporarily) installed at the TCPL rectifier.

Next, the crew, including EGD and TCPL staff, took SCM measurements along the affected EGD pipe (Figure 8). As expected, the interference current gradually increased through the current pickup area to approximately 450 milliamperes at the crossing. However, on the other side of the crossings the measurements showed that only about 70 milliamperes of current was flowing back to the TCPL rectifier, which contradicted the crew’s expectation and the theory of the “text-book” example. This finding, nevertheless, pointed to a likely problem further east from the crossing. The crew then followed the current flow, which led them to a short in the gate station. It was not SCM equipment malfunction, but it was the short that was drawing more current than was returning to the rectifier’s anode bed. In addition to fully quantifying the extent of interference at the crossing, the crew also determined the existence of a fault at another location.

The short at the gate station has since been cleared, reference cells were installed (at the depth of each pipeline) at the crossing to enable accurate periodic pipe-to-soil measurements at this location, and a “hot” anode bed has been installed in proximity of the crossing to divert the current flow into the bed, and away from the current discharge location at the crossing. Subsequent visits to the site verified the implemented solutions are effective in controlling stray current interference.


Based on a paper presented at the 2002 AGA Operations Conference in Chicago, Illinois, May 12-14, 2002.