Short-Circuit Fault Current (Simplified)
Short-Circuit Fault Current (Simplified)
When a short circuit occurs, the current that flows is limited only by the impedance between the source and the fault — not by any load — so it can be many times the normal operating current. Estimating this available fault current from Ohm’s law is the starting point for selecting circuit breakers and fuses that can safely interrupt it.Under-rated protective devices can literally explode when asked to interrupt a fault current beyond their rating, so every panel and breaker in a system must have an interrupting rating that exceeds the calculated available fault current at its location.
The fault current is I_sc = V/Z, the system voltage divided by the total impedance to the fault. where V is the system voltage and Z is the total impedance from the source to the point of the fault.
Inputs
V_sys := 400 V=400 V
Z_sys := 0.05 Ohm=0.05 Ω
Ohm’s law applied to the fault loop gives the current that flows once the load is replaced by a direct short.
I_sc := V_sys/Z_sys=8000 A
Results
A fault current of 8000 A is a substantial but realistic value close to a low-impedance source such as a distribution transformer secondary; fault current drops off quickly further downstream as conductor impedance adds up. Every breaker installed between the source and this fault point must have an interrupting rating at or above this value, or it risks failing catastrophically rather than clearing the fault.
Assumptions & limitations
- Purely resistive/simplified impedance (X/R ratio and reactance not modeled separately).
- Bolted (zero-impedance) fault assumed at the fault point.
- Source impedance treated as constant, not affected by fault current magnitude.