When the CP current is interrupted, the difference in potential with current applied and the instant off potential is called:

The difference in potential observed when the cathodic protection (CP) current is interrupted is known as the IR drop. This phenomenon occurs because the potential difference measured while current is applied is affected by the resistance of the electrolyte and the anode, causing a voltage drop that is proportional to the current. When the current is turned off, the instantaneous potential measured reflects the true coating potential, not influenced by the IR drop caused by the current passing through the system.

In cathodic protection systems, it's important to distinguish between the potential readings obtained under current flow conditions and those obtained when the system is off. The IR drop assists in understanding the effectiveness of the system in protecting against corrosion by showing how much of the applied voltage is lost across the resistive elements in the system. Recognizing this helps in evaluating the true protection level experienced by the structure being protected.