The following calculator conservatively estimates the maximum expected magnetic forces between current carrying conductors (bus bars) in 3-phase switchgear and power capacitor banks during fault and non-fault conditions.
The calculator uses equations referenced in C37.32-1996 – American national Standard for High Voltage Switches, Bus Supports, and Accessories – Schedules of Preferred Ratings, Construction Guidelines, and Specifications.
Calculator-1
Known variables: Short-Circuit Current (RMS asymmetrical, RMS symmetrical, or peak amperes), phase spacing (feet or meters).
Formulas and Variables
Electromagnetic Force Formula
Where:
F = Force on the current carrying conductor (pounds/foot)
M = Multiplying factor
I = Short-circuit current
S = Space between centerlines of conductors (inches)
Note: The electromagnetic forces exerted between two current-carrying conductors is a function of the current, its decrement rate, the shape and arrangement of conductors, and the natural frequencies of the complete assembly, including mounting structure, insulators, and conductors. It is not possible to cover each and every case with one simple equation, even if some approximations are made, because of the large number of variables involved including the wide range of constants for support structures.
The force calculated by this calculator is that produced by the maximum peak current. In most cases, the calculated force is higher than that which actually occurs due to inertia and flexibility of the system, and this fact tends to compensate for the neglect of resonant forces.
