EXPERT TIP #13: TESTING PIPELINE CP SYSTEMS FOR BROKEN TEST WIRES
Corrosion technicians conduct routine operational status checks on Cathodic Protection (CP) Systems to ensure that the system is providing adequate levels of protection. Many common systems can include multiple 2-wire, magnesium anode test stations (1 wire for the anode and one wire for measuring Pipe-to-Soil (P/S) potentials) with simple 2-wire P/S test stations between the anode test stations. Normal potential readings should be between -0.950 and -1.150 volts with respect to a CuSO4 reference electrode along the pipeline.
Checking Test Equipment and Test Stations
When an unusually low reading occurs at one of the test stations, the first step is to make sure that the test equipment and the test station are not the source of the issue. The following are the first checks to make:
- Verify that you removed the plastic protective cap from the reference cell. (It happens.)
- Verify that you have adequately wetted the soil to obtain proper reference electrode contact-to-earth. Note: Digging down 2 or 3 inches and then wetting the soil can make a difference.
- Check to ensure that your meter test leads are not broken. Put your meter on the “Ohms” function and short the test leads together. The meter should read less than 1 ohm and be stable if the leads are good.
- Ensure that the connection point(s) within the test station are clean, tight and free from corrosion.
Indicators of a Defective or Broken Test Wire
Once you are confident that the test equipment and test station are serviceable, the next step is to find the
indicators of a defective or broken test wire. These include:
- Lower than the expected potential and likely unstable or drifting readings.
This is the most common situation. If a pipeline test lead wire is broken, you are, in effect, reading the open circuit potential (OCP) of the end of the broken wire with respect to your reference electrode.
- In addition to low readings, a higher than expected potential can be an indicator of a broken test wire. This is rare but can be found in 1-wire magnesium anode test station. The high reading exists because the test wire to the pipeline is broken. Because the test wire is still connected to the magnesium anode through the test station, you are now reading OCP of the magnesium anode. In this case, you need to disconnect the anode and perform an “applied current” test, detailed below, to confirm the test wire is indeed broken.
- Zero current output from a magnesium anode (maybe). This can occur even with serviceable test wires if the pipeline is the exact same potential as the anode.
TEST METHODS
- In a 2-wire test station, the quickest way to test conductor continuity is to measure the voltage between the
test wires. With two good test wires, the voltage should be zero.
- In an anode test station, the anode should be disconnected before measuring the voltage between the test wires. If there is a voltage differential or if the reading is not stable, it is likely that one of the test wires is defective. If there was any measurable anode current it would be safe to conclude that the anode wire is in good condition.
- In some instances, where only one test wire exists, a modified version of the above test may be possible. Extend a separate test wire between the suspect test wire and a contact point that is electrically continuous with the pipeline, i.e., an above ground valve. Measure the voltage differential between the two points of contact. If the test wire is good, the voltage should be “zero” or within a few mV of “zero”. If a large or variable voltage differential exists, this confirms that a test wire is broken.
- Another option is to use a simple Ohmmeter test. Set your meter function switch to “Ohms” on the lowest scale. Connect the test leads between the test lead to be tested and a contact point that is electrically continuous with the pipeline. Read and record the value in ohms. A good test wire could read less than 1 ohm to as much as 20 ohms but should be stable. This method will always include the resistance of the wire on your test reel. Therefore, it will be necessary to measure the resistance of the test reel wire and subtract that value from the total measured resistance.
- Using an “applied current” to the suspect test wire from a temporary ground anode can be a very effective test. Use a 9-volt battery and a small probe rod (resistivity pin, screwdriver, etc.).
- Drive the probe rod a few inches into the ground. Do not be concerned with the location as this is not critical. Set up the meter and reference electrode to measure the pipe-to-soil potential on the suspect wire. Place the reference electrode about six inches from the probe rod. Record the reading value from the meter.
- Connect the probe rod to the positive of the battery. Briefly (1 to 2 seconds), connect the negative of the battery to the suspect test wire. During the application of the current, regardless of the condition of the test wire, you will observe a significant change in the potential of the wire and may read a potential as high as 8 volts. This high potential is due to IR drop because the reference electrode is close to probe rod. As you disconnect the current, watch the potential change. If the potential immediately returns to the original reading, this is indicative of a good test wire. If the potential slowly changes or is unstable, you can be certain that it is a broken test lead.
Why this test works:
- If the test wire is good, the amount of current generated from the small probe rod (likely less than 1 mA) will have little effect on a large, buried structure.
- If the test wire is broken, there is only a very small surface area contacting the earth. Therefore, the small amount of current will have a very large effect on the potential of the exposed end of the broken wire and quickly polarize it.
The time and effort required to test a suspected broken test wire can be minimized if all test stations are specified to have a minimum of two test wires connected to the protected structure.