Native Potential Surveys and DC Leakage

Dairyland technical experts are occasionally asked how decoupler leakage current may affect potential readings of pipelines with no CP current applied. This article is intended to help answer questions related to the effects of DC leakage current through decouplers and how native or depolarized potential surveys may be impacted.

DC Leakage Current 

Dairyland decouplers have a high resistance to DC current flow.  There are situations where some DC current flow, known as DC leakage current, may be witnessed especially during native or depolarized potential pipeline surveys. For example, there may be a situation where a brand-new pipeline is being checked for its baseline potential voltage (i.e. native potential) to establish limits for use in the 100mV shift criterion from NACE SP0169. In this case, there may be some influence on the pipeline from the grounding wire connected through a decoupler for AC mitigation. This will be further explained in the paragraphs below.  

This leakage current is typically in the single to double digit milliampere range. Graphs are presented in the Technical Literature for the products, and an example is shown here for the PCR decoupler: 

These graphs reveal that as the DC voltage difference across the two decoupler terminals increases, there will be an increase in leakage current. Similarly, as temperatures increase, there will be an increase in leakage current. For example, in the graph shown above, a PCR decoupler with 2 V-DC across its terminals at 20 deg C (68 deg F) will have approximately 0.3 mA of DC leakage current. Note that the 65 deg C (149 deg F) is a graph to illustrate the upper temperature limits of the PCR.  

This small amount of DC leakage can sometimes be noticed during normal operations. For example, at a site with active AC mitigation, the amount of AC amperage flowing through the decoupler is often measured. Note that small amounts of DC amperage may also be measured through the decoupler. A DC ammeter with a high resolution would typically be needed to see the DC amperage flow values and they will normally be less than 2 milliamps, with acceptable values reaching up to 20 milliamps.

 

Native Potential Surveys 

Native potential surveys are used to get the open circuit potential for a metal placed into a soil using a reference electrode, such as a copper-copper sulfate reference electrode (CSE). The native potential is often captured after the pipeline is installed and allowed to acclimate to the surrounding soil conditions. There is no cathodic protection applied before the survey takes place. Another method to gather this baseline metal potential is to perform a depolarized survey. This means that cathodic protection was applied to the buried metal system, but is subsequently disconnected and the pipe is allowed to depolarize and return to it’s native state.   

However, during  the native potential survey the pipeline may be influenced if it remains connected to a decoupler with a grounding wire or another CP circuit on the positive side of the decoupler. Leakage current may impact expected pipe-to-soil readings of the pipeline with no cathodic protection applied, due to the small amount of DC leakage through the decoupler. The type of connected system can have varying effects on the native potential values on the pipeline as described below:   

1. Copper Grounding Systems: With a copper grounding system, the native potential of the copper is naturally at a less negative value on the galvanic series scale compared to a steel pipeline. Note that there is a very small differential in voltage potential between copper and steel, so the leakage current is very low and likely will not be noticed on the pipeline native potential surveys. Therefore, even if the decoupler remains connected between the pipe and the copper ground, there will be minimal impact to the values read during a native potential survey.  
   
2. Zinc Grounding Systems: A zinc grounding system presents a different scenario to a native potential survey as compared to copper. Since zinc is more anodic to the steel, there can be elevated pipe potentials found on a steel pipeline due to DC leakage current through the decoupler if it remains connected to the zinc grounding. This can be especially pronounced with new pipeline installations since the coating will be in pristine condition, resulting in extremely low amounts of current from the zinc to elevate the pipe potentials.
3. Other CP Systems: The cathodic protection levels of a decoupled CP system can similarly affect the native potentials of the surveyed pipeline. The CP system will influence the native potential readings similar to how the zinc grounding system would behave, by increasing the potentials of the surveyed pipeline.  

As a best practice, during native potential surveys, Dairyland recommends disconnecting all decouplers between the pipeline and the grounding circuit, and at locations where CP systems are isolated, such as isolation flanges. This ensures that the native potential survey will not be influenced by decoupled grounding or CP systems. 

 

Leakage Current Values for Various Decouplers 

For reference, the following table is provided to clarify the DC leakage current for various decoupler models and fault ratings. Note that this table covers the SSD, PCR, PCRH and PCRX decouplers only. For other Dairyland products, please consult Tech Support. 

Product Name	Threshold	Fault Rating	Leakage Current Amperes
SSD	-2/+2	1.2 kA	<5mA
		2 kA	<5mA
		3.7 kA	<5mA
		5 kA	<5mA
SSD	-3/+1	1.2 kA	<5mA
		2 kA	<5mA
		3.7 kA	<5mA
		5 kA	<5mA
PCR, PCRH	-2/+2	3.7 kA	<5mA
		5 kA	<5mA
		10 kA	<10mA
		15 kA	<20mA
PCR, PCRH	-3/+1	3.7 kA	<5mA
		5 kA	<5mA
		10 kA	<5mA
		15 kA	<20mA
PCRX	-4.5/+3.5	5.0 kA	<5mA
		10 kA	<10mA
		15 kA	<20mA
PCRX	-5.5/+2.5	5.0 kA	<5mA
		10 kA	<10mA
		15 kA	<20mA