A brief description of high voltage (HV) alternating current (AC) interference on buried coated pipelines is provided. Computer simulation for AC interference will be discussed. Relative ranking for: Separation distance, HVAC phase current, soil resistivity, co-location length and crossing angle will be provided and discussed. The case history portion of the paper includes an investigation that illustrated the problems inherent in AC mitigation with zinc ribbon in corrosive soils.
There are two main reasons to be aware of induced AC on a pipeline: (1) Personnel safety and (2) Pipeline damage from AC interference.
Concerning personnel safety, AC interference on pipelines can cause harmful shock to pipeline technicians. The shock experienced will vary with the pipe-to-soil AC voltage at the time of contact, and the resistance between the body part in contact with metal and ground. Resistance to electrical current varies in the human body.1 Externally, the resistance of dry skin is between 100,000 and 600,00 ohms, whereas the resistance of wet skin is 1,000 ohms. Internally, from hand to foot, the resistance is 400 to 600 ohms. From ear to ear, the resistance is 100 ohms. Resistance of the body to pipe and body to ground is influenced by the resistance of personal protective equipment (i.e. gloves and boots), and condition of the ground at the technician’s feet (i.e. moisture content, resistivity, presence of dielectric barrier or presence of equipotential mat).
60-Hertz alternating current can cause harm at relatively low current levels.1 At 1 milliamp or less there is no sensation. Painful shock begins at 8 milliamps, and 50 milliamps causes severe muscular contractions and breathing difficulties. Ventricular fibrillation can occur above 50 milliamps, and above 100 milliamps defibrillation may be required to restore normal heartbeat. Over 200 milliamps, severe burns and severe muscular contraction accompany ventricular defibrillation. In case of AC Interference, as current is induced on the pipeline, or in the event of a fault condition, a voltage gradient can occur around the above ground structures such that touching or even standing near these structures could result in electrical shock. The industry standard for AC mitigation on pipelines is to maintain voltage gradient (step-touch potential) below15 VAC.