Lack of isolation leads to stray current

Current flow between steel structures as a result of unwanted interconnections, will reduce the control of any installed cathodic protection system and may lead to corrosion.

Basically, a buried or submerged steel structure should be mechanical and electrical isolated from other structures in the same electrolyte (soil, water, etc.). However, during installation there may be a lot of challenges in order to obtain sufficient electrical isolation, such as:

• Electrical grounding of pumps, valves, etc.
• Electrical grounding of measurement instruments/meters
• Supports
• Thermal isolation
• Isolating joints/flanges

Electrical grounding

In order to avoid voltage difference, and hence reduce the risk for sparks, all electrical systems (pumps, valves, motors, instruments, etc.) should be connected to the same earth potential. However, if a steel structure as a buried tank, a buried or submerges pipeline is connected to a copper grounding system, high risk of corrosion attacks on the structure may occur. Hence, a sufficient isolation between the steel structure and the copper grounding system must be achieved. The equipment housing, where the grounding is connected should either be isolated from the steel structure or the grounding connected on the copper grounding side of the isolation.

Supports

When installing supports for e.g. a pipeline, special car must be taken in order to make sure all supports are isolated, either from the steel structure or from the copper grounding system.

Thermal isolation

A connection between e.g. a pipeline and its thermal isolation cover will normally not increase the risk of corrosion, but will reduce the control of the cathodic protection and make line current measurement more difficult and unreliable. Hence, thermal isolation for pipelines should be installed in such way that there are sufficient isolation between the pipeline and the thermal isolation cover.

Isolation joints

Isolation joints shall be installed to isolate buried or submerged steel structures from other structures and copper grounding systems. Sufficient quality of each isolation joints should be verified prior to installation, and tested periodically. Insufficient isolation may result in serious corrosion attacks.