A fused system can be designed with full selectivity from the smallest branch circuit to the main electrical service equipment, and over the entire range of possible fault currents. This means the protective device immediately on the line side of any fault isolates (that is, "selects") that particular circuit without actuating any other protective devices in the system.
Data is available from fuse manufacturers showing the minimum ratios required between fuses in series of the same and different classes to provide such a selective system. This information is helpful when planning to add a feeder with a number of other feeders and branch circuits in a radial system. Working back toward the source, each fuse is chosen (in turn) so that its current rating is adequate to meet the prescribed ratio with the nearest fuse "below" it. This prevents needless blowing of fuses and will result in a properly coordinated system. While this is straightforward, it sometimes calls for oversizing a main feeder or service entrance when diversity has been included in the preliminary sizing. If this problem arises, it may be necessary to decide whether the fuse coordination is essential, or whether a circuit breaker with time delays should be used.
