# Pipeline Safety Interpretation PI-72-0121

- **operation:** document
- **citation:** PI-72-0121
- **title:** Pipeline Safety Interpretation PI-72-0121
- **source type:** guidance
- **agency:** Pipeline and Hazardous Materials Safety Administration
- **status:** guidance
- **official:** true
- **published on:** 1972-08-01
- **effective on:** Not available
- **summary:** PI-72-0121 concerning 192.457.
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- **source url:** https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/legacy/interpretations/Interpretation%20Files/Pipeline/1972/gAB72_08_01_AB_72_8_192.457_nlmx.pdf
**body:**

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PI-72-0121
August 1, 1972
INTERPRETATIONS OF REGULATIONS
Relates to 49 CFR, §192.457
OPS has been requested to provide further clarification of an Interpretation on the two-electrode
surface potential survey method for locating areas of active corrosion, which appeared in the
Advisory Bulletin 72-8, August 1972.
OPS Interpretation:..."Your letter suggests that the "leap-frogging' technique is no longer in use
and that, therefore, it may not be appropriate to refer to it. However, the interpretation quoted in
the Bulletin was in response to a specific question, and, as indicated 'leap-frogging' is not useful
for Dresser-coupled pipelines with insulated joints. On the other hand, the 'leap-frogging' surface
potential survey method is being successfully used for locating areas of active corrosion on
electrically continuous pipelines. As your letter recognizes, the purpose of 'leap-frogging' is to
compensate for electrode potential differences. While we recognize that there is equipment
available today which makes it unnecessary to use 'leap-frogging,' the leap-frogging technique
remains available for use on electrically continuous pipelines.
"Your letter also states that the two-electrode surface potential survey method is the most
satisfactory approach for providing Dresser-coupled pipelines with adequate cathodic protection.
To the extent that you are referring to the use of two electrodes in continuous contact with the
earth, we agree. The proper technique provides for a continuous voltage difference measured
between one electrode placed over the pipeline and the other electrode located five feet from the
pipeline and perpendicular to the pipeline. The survey is normally conducted on both sides of the
pipeline to identify existing galvanic anodes."
Joseph C. Caldwell
Director
Office of Pipeline Safety
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