Medium Voltage Design Guide by Schneider Electric

Updated: Jul 7

Prefabricated metal-enclosed switchgear

In order to design a medium-voltage cubicle, you need to know the following basic magnitudes:

  • Voltage

  • Current

  • Frequency

  • Short-circuit power.

Warning: The content of this article is cited from the Medium Voltage Design Guide by Schneider Electric

The voltage, the rated current and the rated frequency are often known or can easily be defined, but how can we calculate the short-circuit power or current at a given point in an installation? Knowing the short-circuit power of the network allows us to choose the various parts of a switchboard which must withstand significant temperature rises and electrodynamic constraints.

!!! Knowing the voltage (kV) will allow us to define the dielectric withstand of the components (E.g.: circuit breakers, insulators, CT)

Disconnection, control and protection of electrical networks is achieved by using switchgear. Metal enclosed switchgear is sub-divided into three types: metal-clad, compartmented and block.

Short-circuit power

The short-circuit power depends directly on the network configuration and the impedance of its components: lines, cables, transformers, motors… through which the short-circuit current passes. It is the maximum power that the network can provide to an installation during a fault, expressed in MVA or in kA rms for a given operating voltage.

Possible sources of short circuit

The customer generally imposes the value of short-circuit power on us because we rarely have the information required to calculate it. Determination of the short-circuit power requires analysis of the power flows feeding the short-circuit in the worst possible case.

Possible sources are:

  1. Network incomer via power transformers.

  2. Generator incomer.

  3. Power feedback due to rotary sets (motors, etc); or via MV/LV transformation


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