Stray currents are formed in the earth’s external crust as a result of natural phenomena and human activity. Stray currents are defined as currents flowing into electrolytic environments (soil, water) from electric circuits inadequately insulated to environment. These currents change over time and space. Natural sources (stray earth current) related to solar activity and atmosphere, they are commonly known as telluric currents or geocurrents. Some main artificial sources formed stray currents such as DC electric tractions (tram, railway, metro), DC industrial devices, high-voltage direct current (HVDC), cathode protection systems, etc…
Stray current running through the metal structure placed in the electrolytic environment, they cause metal corrosion (often referred to as electrolytic corrosion). The DC stray current cause stronger degree of metal corrosion, whereas AC stray current is very small degree of metal corrosion. Stray current can be observed by measuring the voltage on the soil surface. So, the survey to detect the existence of stray currents in the project area, understanding the distribution from which provide measures of their harmful reduction is a necessary work.
To discover the stray current, voltage measurements were performed on two mutually perpendicular directions (orthogonal setting), along and cross with the survey profile with electrode spcacing >= 30 m (see figure). The direction of electrode couples may be change to 450 so that electrodes have good contact with ground. Potential difference is measured every 10 seconds by two voltmeters on the same time, during about 10 minutes at each point. The results are recorded in a special field-log table.
At each measurement point, calculate the maximum fluctuations of the measured potential difference in each electrode couple. Select a highest value of two maximum fluctuations at each measurement point, set up a data collection of maximum fluctuations of measured potential difference.
Make map of the maximum fluctuation of the potential difference for the above data collection. Areas with high fluctuation of the potential difference register the existence of stray current. Fieldwork, processing and interpretation of stray currentsurvey are carried out in accordance with Vietnamese standard 9417:2012 or Russian ГОСТ 9.602-2005.