Measurement of touch voltages on 25-kV underground distribution systems

Underground distribution lines in Canada are installed in duct banks with manholes placed at regular intervals for easy installation of the line hardware and ready access for maintenance purposes. During work in these manholes, touch voltages may occur on the line under repair as a result of a fault on another line. Tests were performed on a 25 kV system to measure the level of such voltages and to validate a computer model. The maximum touch voltage measured was 220 V. Over 6000 system configurations were examined in a computer study which showed that the touch voltage appearing at the work site can reach as much as 1500 V in some cases, although operating experience shows that the probability of such high voltages is very slight