How Do You Calculate Phase-to-Phase Short Circuit?

By Dwight Chestnut, eHow Contributor

 
 
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Short circuit current calculations are valuable in power distribution systems.

A short circuit is a fault that occurs when an electrical short occurs in an electrical system. Short circuit means the resistance goes to zero and current increases exponentially. To visualize it mathematically, current is voltage divided by resistance where, if resistance is zero or is very small, it's equivalent to dividing by zero or a small number and current increases dramatically. For this reason, to calculate the short circuit current, you assume the core resistance is zero and the only resistance left is the natural resistance associated with the electrical network such as the resistance associated with cables and busbars.

Instructions

1. • 1
     Find the phase-to-phase voltage, or "Vphase." Phase voltage is the voltage carried on all three phase conductors. Refer to system specifications. As an example, assume 208 volts.
   • 2
     Find the total phase impedance associated with the three-phase network. The network includes all downstream components such as cable, circuit-breaker and busbars.

     Use the formula: Zt = sqrt(Rt² + Xt²) where Zt is the total impedance, Rt is the total equivalent resistance and Xt is the total equivalent reactance of the network. To determine Rt and Xt, you need to model all components in the network using a computer-based circuit analysis program similar to the Simulation Program with Integrated Circuit Emphasis. As an calculation example, assume Rt is 0.008 ohms and Xt is 0.002 ohms.

     Zt = sqrt (0.008² + 0.002²) = sqrt (0.000064 + 0.000004) = sqrt (0.000068) = 0.00824 ohms
   • 3
     Calculate the short circuit current using the formula: Isc = Vphase/Zt. Continuing with the example:

     Isc = 208/0.00824 = 25,242 amps. Note: Short circuit currents are normally huge.