Influences of a fault current limiter (FCL) on an isolated operation of a customer system with a synchronous generator are discussed for a fault occurrence. The customer system is supposed to be interconnected to a medium voltage feeder of the utility system through the FCL to reduce the fault current from the customer generator. When the fault occurs at the feeder, the customer system has to be disconnected from the utility system to keep the power supply in the customer system. If the separation of the customer system from the utility one is too late (in other words, if the continuous interconnecting time period after the fault occurrence is too long), the customer generator could not make the shift to the isolated operation mode because of the inadequate balance between the generator and loads. The allowable interconnecting time period to make the successful shift to the isolated operation mode is simulated by using PSCAD/EMTDC. It was found that a resistive type fault current limiter (R-type FCL) does not improve the allowable interconnecting time period since the consumption power in the R-type FCL causes the generator to decelerate. However, it was shown that the allowable interconnecting time period increases owing to the application of inductive type fault current limiter (L-type FCL). This is because the active power of load is nearly equal to the generator output. The influence of reverse power flow out of customer system on the isolated operation was also clarified.