ELECTRICAL EARTHING OF CATHODIC PROTECTION
The object under cathodic protection should be electrically isolated from common or plant earthing systems to avoid a loss of current.
For safety reasons however it may be necessary to install a safety earthing, which should be covered by the design. The earthing system shall be compatible with the cathodic protection system by installing polarisation cells or diode circuits, suitable specified and rated for the purpose, in the earthing circuit or by the installation of separate earthing electrodes, made of zinc or galvanised steel, buried in low resistivity backfill and not in direct electrical continuity with other earthing systems.
If pipeline earthing is to be installed to mitigate the effect of AC induced voltages on the pipeline, this should be done at the locations where the anticipated or measured voltages to the ground are highest and where the pipeline is exposed and can be touched by personnel.
POLARIZATION CELLS OF CATHODIC PROTECTION
How Polarization Cells Works?
Polarization cells have pairs of pure nickel plates immersed in KOH electrolyte. The surfaces of these plates become polarize when
separated metallic structures are connected to opposite terminals. The polarizing film blocks de, to contain cathodic protection current, while freely passing ac. Personnel are safely protected from shock hazards during ac faults.
Insulating fittings and equipment are protected during electrical disturbances. Polarization cells are indispensable to secure safe electrical isolation.
When specified, polarisation cells shall be supplied complete with steel enclosures. The polarisation cell should be placed on a reasonably level, stable, well-vented and protected location. The polarisation cells shall be assembled in accordance with Manufacturer’s written instructions. Safety and personal protection shall be applied.
The cables shall be sized and the cable connections shall be made in accordance with Manufacturer’s written instructions.
TEST COUPONS OF CATHODIC PROTECTION
Test coupons may be installed to monitor the effectiveness of the cathodic protection. Test coupons shall be sized in accordance with the design of project. Test coupons shall always be installed in combination with a potential monitoring station. Two separate cables shall be attached to the test coupon and the cables terminated in the test post.
The distance between the test coupon and the object shall conform with the design and a record of installation details shall be kept.