Test results are presented for a buried and cathodically protected structure coupled to a copper ground. When the 100 mV criterion is applied on a structure polarized by foreign uninterruptible current sources, the structure potential decays to its polarized potential instead to its corrosion potential making the use of the 100 mV criterion
questionable. Use of coupons eliminates this problem because
the coupon potential after interruption always decays to its corrosion potential.
Keywords: cathodic protection, 100 mV criterion, coupons, CP probes, protection criteria, potential monitoring, voltage drop, IR drop, potential decay.