Presently, most of the normal distribution system configuration is a radial type. When a fault occurs in the radial system, one or more loads experience an outage for a few minutes until the fault is cleared. But if the distribution system is configured with a loop type, then the power can be supplied to all customers without any outage. So, for improving reliability of power supply, the research of the loop distribution system has been performed recently. Previously, At the KEPCO PT Center (PT: Power Testing), several tests like single fault, communication failure, CB open failure, etc, were performed to verify performances of the coordination control algorithm for loop distribution. However, there are some limitations in this field test for overall its functions. For example, a few tests such as a three-phase fault, line to line fault and the Closed-loop Control Device (CCD) internal fault are hard to fulfill in field test. Therefore, it should be tested by Real Time Digital Simulator (RTDS). The RTDS is a fully digital power system simulator in real-time and hence can make an environment similar to physical power system. In this paper, the validity of the coordination algorithm for a loop distribution system is verified with RTDS which is connected with the protection relays called CCD.