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Cathodically protected buried pipelines are often fitted with protective electrical earthing when constructed near power lines. The earthing electrodes are typically decoupled from the pipeline by polarisation cells or solid state electronic devices, referred to as surge diverters. Field experience has demonstrated that buried, well-coated pipelines near alternating current power lines often show unusual results when instant-OFF potential measurements are attempted. Their instant-off potentials can appear to remain similar to the ON potentials, rather than exhibiting the ohmic drop or 'IR drop' seen on other pipelines. The electrical capacitance of the decoupling devices is a likely cause of this phenomenon. This paper reviews and examines the effects of common decoupling devices on pipeline cathodic protection systems, using an existing Australian gas pipeline as a case study. Techniques to accurately assess cathodic protection status on pipelines with earthing and decoupling devices are discussed.