Stray current interference protective measures

Fig. 15-8 Synchronous current, voltage and potential recording with stray current interference from dc railways (a) Without protective measures, (b) direct stray current drainage to the rails, (c) rectified stray current drainage to the rails, (d) forced stray current drainage with uncontrolled protection rectifier, (e) forced stray current drainage with galvanostatically controlled protection rectifier (constant current), (f) forced stray current drainage with potentiostatically controlled protection rectifier (constant potential), (g) forced stray current drainage with potentiostatically controlled protection rectifier and superimposed constant current.

One technique that has heen applied for many years uses two sets of measurements where one of the electrodes is positioned over the pipeline and the second electrode is placed several meters to the side. The data from this approach provide a comparison of the local potential to the more distant potential made to the side of the pipeline, as illustrated in Fig. 4. Regions where the "Over Pipe potential are more positive than the 25 ft From Pipe potential are cathodic to other sections of the pipe Une. Potential effects of stray currents, galvanic currents, md cathodic protection interference are identified from this information [2i]. 

Limitations due to convection. At longer times the buildup of density gradients and stray vibrations will cause convective disruption of the diffusion layer, and usually result in currents larger than those predicted by the Cottrell equation. The time for the onset of convective interference depends on the orientation of the electrode, the existence of a protective mantle around the electrode, and other factors (1, 2). In water and other fluid solvents, diffusion-based measurements for times longer than 300 s are difficult, and even measurements longer than 20 s may show some convective effects. 

---