The long-term trend in the United States and many other countries is shared land use for pipelines and other utilities. This movement has increased the number of overhead high voltage transmission lines parallel to and sharing a corridor with an underground pipeline. This combination has increased the need for grounding of the pipeline to dissipate interference.028

In these systems, A.C. voltages are transmitted to the pipeline by conductive or inductive interference. Magnetic induction acts along the pipeline or pipeline segment that is approximately parallel to the power line and can cause significant pipeline potentials even at relatively large separation distances. Consideration must be given to safety of personnel and the public who may come into contact with above ground portions of the pipeline such as valves and test stations. These exposed structures can be a potential shock hazard when touched while the soil is at a significantly different potential.

8-17 015For inductive interference, gradient control wires provide additional grounding for the pipeline and decrease the induced pipe potential rise. At the same time, they raise local ground potentials, thus sharply reducing touch potentials and coating stress voltages. For conductive interference, gradient control wires dampen the soil potential rise close to the pipe while raising pipe potentials, thus providing reduced touch voltages and decreasing coating stress voltages.

Importance must also be placed on reducing the potentials at valve sites, metering stations, pig launchers and receivers and other accessible installations for worker safety. Gradient control grids or grounding mats raise local ground potentials in the same way gradient control wires do. These grounding mats can be made in several forms. Spiral and rectangular designs are generally standard for grounding mats.Dairyland_Electrical_Logo_with icon

Reference should be made to NACE Standard SP0177 – Standard Practice on Mitigation of Alternating Current & Lightning Effects on Metallic Structures and Corrosion Control Systems. Also, ANSI/IEEE Standard 80 specifies safety design criteria for determining maximum acceptable touch and step voltages during fault conditions.

Precision Electric offers consultation, procurement and installation on a wide variety of AC Mitigation systems and equipment.