Current leakage measurement
For cathodically protected pipeline entering into complex terminal areas it might be challenging to avoid undesirable connections to secondary structures and grounding systems. Current leakage measurements effectively detect lack of isolation.
Lack of isolation can effectively be detected by pipeline current mapper technique.
A pipeline subject to cathodic protection, is ideally electrically isolated from other nearby conductive structures, in order to be the only structure to receive the protective current transferred through the ground from ICCP anodes. In complex systems however, full isolation may be difficult to achieve, causing conductive elements nearby, other than the intended target, to pick up and transfer part of the protective current to common ground.
Current leakages may reduce the protective current distribution and hence, the effect of the ICCP system. The significance of a leak depends on the size and whether there is spare capacity in the ICCP plant to compensate for the loss.
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Measurement technique
A pipeline current mapper (PCM) system can identify short circuits caused by contact to other structures and detect current leakage. This measurement technique is based on measuring the current flow in a single conductor when the cathodic protection system is switched off. The detection works by connecting the PCM transmitter negative terminal to the isolated structure (e.g. a pipeline) and the positive terminal to an anode. When the PCM starts transmitting, a coded current is sent through the anode to the ground. All conducting structures in the range of the anode will have the ability to pick up the current, but only if this structure is in electric contact with the PCM negative terminal (see the principle sketch below).
The portable PCM receiver (detector) is used to locate the coded current. Once a coded current is located, the portable PCM receiver can tell the approximate size and the direction of the current. During the inspection, first the proportion of the current picked up by the pipe is established, and then the surrounding area is surveyed to map the extent of leak currents being transferred by other conductors.
This measurement technique does not quantify the isolation resistance value but can effectively verify isolation joints and find undesirable connections to other metal structures.
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