Which of the following is NOT a standard element in cathodic protection systems?

In cathodic protection systems, the main goal is to prevent corrosion on metal surfaces, particularly those that are buried or submerged, such as underground storage tanks. Key elements of these systems include corrosion-resistant coatings, which protect the surface of the metal; sacrificial anodes made of materials like zinc or magnesium, which corrode instead of the structure being protected; and electronically isolating fittings that help ensure that the electrical current is directed appropriately to provide protection.

Refrigeration units, on the other hand, do not play any role in cathodic protection systems. They are not involved in the process of preventing corrosion and are unrelated to the mechanisms that protect metal structures from corrosive environments. This distinction highlights that while the other options are crucial components of cathodic protection, refrigeration units do not contribute to this purpose at all.