The paper develops an expert system to assist protection engineers in preparing design specifications for protection current transformers used in differential protection of power transformers based on the rule-based expert system methodology. The relationships of the design quantities and the design process are expressed as dependency trees. The dependency trees and the constraints on the design quantities are then expressed in the form of the if-then rules. These rules are held in the knowledge base and are processed by the control structure of the system, in which an iterative forward chaining paradigm is employed. The transparency of the reasoning process of the developed system is enhanced by including in the system an explanation subsystem. The system is implemented using Prolog. An application of the expert system is presented, demonstrating the suitability of the design selection knowledge representation method and the inference process developed together with the capability of the explanation subsystem.