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The current rules and practices for setting transformer current differential protection using electromechanical relays are discussed. These rules are compared with the ones, which are currently available on microprocessor-based technology. Major differences in relay setting approaches are outlined. With the electromechanical technology, selection of proper current transformer (CT) ratios and settings for a small size transformer connected to a high fault duty bus has been and continues to be a challenge as the system grows. A sample of such case is analyzed through examples using both electromechanical and microprocessor based technologies and some guidelines are suggested for each case.