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Distribution transformer secondary faults are shown to be potentially difficult for a primary interrupting device to clear. Evaluation test programs, therefore, should include this type of fault-interrupting test. Secondary fault recovery voltage data presented is derived from synthetic techniques, field interrupting tests and laboratory interrupting tests. TRV frequency is confirmed to be inversely proportional to the transformer voltage rating, directly proportional to the one-half power of kVA rating, and dependent upon transformer and system grounding connections. The trend toward lower transformer impedances may create problems for interrupting devices since TRV frequencies and the resultant rates of rise of recovery voltage are higher. Laboratory circuits using typical distribution transformers can be set up to be equivalent to field conditions and are satisfactory for exploring the adequacy of interrupting devices for clearing transformer secondary faults.