Specially designed rectifiers capable of operating in off-potential mode have been operating
successfully for more than two years in the Greater Copenhagen Natural Gas1 distribution system. The
rectifier system is based on a conventional transformer rectifier operating in potential mode and a PLC
controlled off-potential measurement and feed-back procedure. The IR drop is fed to the rectifier feed
back loop hereby offsetting the conventional potential control by this (IR) amount.
The parameters resulting from the above described system include on-potential, off-potential,
rectifier current, IR-drop and the average coating resistance of the pipeline. Benefits include:
• The off-potential facility has proved to effectively provide a reproducible measure of the average
coating resistance enabling the operator to conduct instant investigations on cause of coating
damages and to pursue any third party that might have caused coating or pipeline damage.
• The remote controlled T/R system automatically produces measurements in the silent hours
(where no DC traction systems are active) as required in stray current areas thereby saving the
operator for inconvenient and costly working hours.
Keywords: cathodic protection, remote monitoring, coating resistance, off-potential, third party damage