ANSWERS:

Select an appropriate fuse size for the mystery fuse near the recloser symbol that best coordinates with the downstream 100T fuse as well as the upstream feeder relays. Full coordination with both is not possible so you'll have to make some compromises.

Initially, we would find the upstream fuse by selectivity at maximum fault current at downstream fuse location. That is:

For I = 5712 A, the minimum melting time of the upstream fuse should be greater than the total clearing time of the downstream fuse plus the coordination time interval. On the other hand, the minimum melting time of the fuse curve will be multiplied by 0.75 in order to consider that the fuse can be previously pre-heat by load conditions.

The total clearing time of the downstream fuse (100T) at 5712 A is 0.04 seconds, approximately.

The coordination time interval will be 0.1 seconds.

Therefore, the minimum melting time of the upstream fuse at 5712 A should be greater than 0.14 seconds divided by 0.75. The value 0.14 seconds divided by 0.75 is 0.19 seconds. The highest fuse rating is 200A and this fuse does not fulfill this requirement.

The minimum melting time of 200T fuse at 5712 A is 0.115 seconds. If this value is multiplied by 0.75, in order to consider that the fuse can be previously pre-heat by load conditions, the result is 0.086 second. This value is greater than the total clearing time of the downstream fuse (100T) at 5712 A (0.04 seconds), but the difference between both values is only 0.046 seconds (which is lower than the required coordination time interval of 0.1 seconds). However, the use of 200T fuse is the most selective option for the upstream fuse (that is, the “mystery fuse”).

The coordination of this fuse with upstream relays is discussed in the next parts.

Design a three-phase balanced power system where the load is connec...