One that supports the development of technology is the amount of electrical energy availability that must be fulfilled properly. the more increasing number of people in an area, the more increasing demand of electricity need. Therefore the transformer used must have a protective system from overload. Over Current Relay (OCR) is an overcurrent safety relay that will work if there is more current installed in high voltage networks, medium voltage, and also on the transformer safety. The method is used by using ETAP simulation and calculation using with formula. OCR is a relay that will work when the current passing through the relay, this study will analyze the constraints that exist in the Over Current Relay (OCR) system in the 150 KV Medari substation Sleman. By using ETAP software as the main test material.