Over the past decade, pipeline owners and operators have become increasingly conscious of alternating current present on their pipelines. This induced AC is a result of the fact that a majority of cross-country, underground pipelines either share common rights-of-way with overhead electric transmission systems or intersect them such that conditions are met for the induction to take place. In general, whenever physical conditions satisfy the boundary conditions for Faraday’s Law, an AC voltage will be induced onto an underground pipeline due to its conductive properties and the presence of the electromagnetic field surrounding the transmission cables.

Recent advancements in materials science have introduced protective coating systems with very high dielectric properties for corrosion prevention. This advancement further increases the probability that AC will be induced onto underground pipelines by satisfying the conditions of Gauss’ Law. Therefore, pipelines that are coated with high-quality protective coating systems and that are located in close proximity to overhead electric cables are very much prone to induced AC interference.

Induced AC interference is a concern from both the pipeline integrity and safety standpoints. There has been much discussion on the topic of AC corrosion in recent years, and the results are somewhat inconclusive. Further research and testing will be needed to ascertain exactly how AC voltage affects cathodically-protected structures. However, the results from a safety standpoint are far better understood.

Operating personnel can be exposed to electrical shock whenever the induced AC exceeds either the “touch” potential (the potential difference which exists between hand and foot) or the “step” potential (the potential difference which exists between both feet while walking or standing). Guidelines exist for both conditions.

It is possible to predict the levels of induced AC based upon knowledge of a number of variables (e.g., coating resistance, soil resistivity, etc.) using a variety of mathematical models. Once the voltage levels are approximated, one can devise mitigation measures to reduce the AC potentials to acceptable levels.

AC interference can manifest itself as inductive, ohmic or capacitive interference and all these modes need to be considered, evaluated and dealt with in the detailed design. Capacitive and ohmic interference in particular can cause problems during construction of the pipeline.
As every situation is unique, Isinyithi Cathodic Protection works in conjunction with a specialist electrical engineer to ensure that our clients receive the most cost-effective and practical AC Mitigation design.

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