Is it true that a downstream 30A fuse will always blow before an upstream 100A fuse?

The correct understanding revolves around the relationship between upstream and downstream fuses in an electrical system. A downstream 30A fuse does not always blow before a 100A fuse under all conditions; this can depend on various factors including the type of fault and the characteristics of the electrical load.

In normal circumstances, if there is an overload or fault situation, the downstream fuse is designed to protect the wiring or equipment connected downstream. However, if the fault current exceeds the rating of the upstream fuse, it may blow first, regardless of the downstream fuse’s rating. The time it takes for a fuse to blow depends not only on the magnitude of the fault current but also on the specific characteristics of the fuses themselves.

In cases of short circuits, while it's often expected for the downstream fuse to blow first due to the quicker response to overload, this isn't guaranteed. If a fault causes a very high current that exceeds the maximum capacity of the upstream fuse, it can blow before the downstream fuse.

Thus, a downstream 30A fuse will not always blow before an upstream 100A fuse; instead, it depends on the specific circuit conditions, including the type of load and fault conditions.