Fault current isn't a fixed number for a system — it drops the farther a fault is from the source, because more impedance sits between them. Protective devices are coordinated around exactly this falloff.
Power-system analysis evaluates how a system behaves under both normal conditions (power flow, voltage regulation) and abnormal conditions (short-circuit faults, stability events). Fault current analysis, in particular, determines the maximum current a fault can draw at any given point in the system — a number that can be tens or hundreds of times normal load current, and one that protective devices, conductor ratings, and equipment short-circuit ratings all have to be selected around.
Every piece of equipment between the source and a fault point — transformers, cables, bus duct — adds impedance to the fault current path. Since fault current is roughly source voltage divided by total impedance to the fault, more impedance (more distance, more transformers) means lower available fault current at that point. This is why a fault right at a service entrance can draw far more current than the same type of fault at the end of a long branch circuit.
Every breaker and fuse must have an interrupting rating equal to or greater than the maximum available fault current at its location — undersized interrupting ratings can literally cause a protective device to fail catastrophically instead of clearing the fault safely. This is why a short-circuit study calculating available fault current at every bus is a required part of any serious electrical distribution design, not an optional analysis.
Coordination means selecting and setting protective devices so the one closest to a fault trips first, isolating only the smallest necessary portion of the system, while devices further upstream stay closed to keep the rest of the system running. This depends directly on understanding how fault current — and therefore trip time — varies by location, which is exactly what fault current analysis provides the data for.
Because there is less impedance (fewer transformers, less cable) between the low-impedance utility source and that point — less impedance in the path allows more current to flow during a short circuit, following the same relationship as Ohm's Law applied to the fault path.
The breaker can fail to safely interrupt the fault — potentially exploding, welding contacts closed, or otherwise failing catastrophically instead of clearing the circuit, which is a serious safety hazard. This is exactly why a short-circuit study and correctly rated equipment are essential, not optional, parts of electrical system design.
Not necessarily for system operation — a robust, low-impedance source with high fault current actually gives better voltage regulation and motor-starting performance under normal conditions. The tradeoff is that all downstream equipment must be rated to withstand and interrupt that higher available fault current safely.
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