DC Battery Bank Short-Circuit Calculator
Calculate prospective DC short-circuit currents, internal cell impedances, loop time constants, and protective device AIC ratings per IEEE 946 and IEEE 1375.
Battery String Ratings
DC Feeder Cable to Panel
DC Short-Circuit Fault Results
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Frequently Asked Questions
Why are DC short circuits more dangerous to interrupt than AC short circuits?
AC currents pass through a natural zero-voltage and zero-current crossing 100 or 120 times every second, allowing circuit breakers to quench the electric arc as current crosses zero. DC currents have no natural zero-crossing; the protective device must forcibly generate an arc voltage higher than system voltage to drive current to zero, causing severe contact erosion and sustained plasma arcing.
What role does the battery bank time constant (L/R) play?
The time constant (τ = L/R) dictates how quickly the short-circuit current reaches its peak value. A typical station battery system has an L/R of 15 to 30 milliseconds. Circuit breaker test standards (e.g. IEEE C37.14) require testing at specific time constants because high inductance stores magnetic energy that prolongs arcing during interruption.
Why does float voltage rather than nominal voltage determine fault current?
Station batteries in continuous service are maintained at a continuous float charge (e.g., 2.25 V/cell for VRLA or 135 V on a 125 V nominal bank). Because the battery and charger are both at float potential when a fault strikes, the initial driving potential is the float voltage.