7. The estimated test modes of electrical systems

7.2. Calculation of the maximum and minimum expected short-circuit current

The maximum expected short-circuit current - a current to the line terminals of the device to protect against short circuits, which can be calculated when the known parameters of the power supply and the electrical parameters of the supply side to the place of installation of the protection device.

The minimum expected short-circuit current - a current corresponding to a short circuit in the farthest point of the protected circuit, short-circuit between phase and neutral or, if the neutral is not distributed between the phases. The installation of the power from multiple sources is considered only one source having a maximum internal impedance.

In the absence of sufficiently specific information, the following simplifying assumptions can be made for the calculation of the minimum short-circuit currents:

it is assumed that the wiring resistance is increased by 50% relative to its value at 20 0 C due to the heat conductor short-circuit current;

if the impedance of the circuit from the power source is not known, it is assumed that the power supply voltage is reduced to 80% of rated voltage.

The calculation of the minimum short-circuit current is produced by the following formulas.

For three-phase circuits with undistributed neutral (fault between the phases):

I = 0,8U / (1,5 r 2L / S)

where I - the expected short-circuit current, A; U - phase to phase supply voltage, V; r - the resistivity of the cable core, Ohm mm 2 / m, at 20 0 C; L - protected cable length, m; S - the cross-sectional area of the cable core, 2 mm.

For three-phase circuits with neutral distributed with reduced or undiminished cross section (fault between phase and neutral):

I = 0,8U 0/ (1,5 r (1 + m) L / S)

where U 0 - nominal supply voltage between phase and neutral, B; m - the ratio between the resistance of the neutral conductor and phase conductor resistance (or ratio between the cross-sectional area of ​​the phase conductor and the cross-sectional area of ​​the neutral conductor, if they are made of the same material - copper or aluminum).

Remarks:

  1. r is 0.018 to 0.027 for copper and aluminum.
  2. For conductors having a cross sectional area greater than 95 mm2 reactance must be considered.
  3. Factor 1.5 takes into account the increase in wire resistance due to the temperature rise.

Calculated short-circuit currents are used to determine the required breaking capacity of the protection device during short circuit. At the minimum short-circuit current is selected current instantaneous tripping of the circuit breaker, which should not be less than the calculated minimum short-circuit current. Operating conditions may, however, require the selection of protection by breaking capacity under short-circuit device, for example, if the device is located on the main electrical input.