Medium Voltage Switchgear | Source: Schneider Electric

All electrical installations have to be protected against short-circuits, without exception, whenever there is an electrical discontinuity; which more generally corresponds to a change in conductor cross-section.

The short-circuit current shall be calculated at each stage in the installation for the various configurations that are possible within the network, in order to determine the characteristics of the equipment that has to withstand or break this fault current.

In order to choose the right switchgear (circuit breakers or fuses) and set the protection functions, three short-circuit values must be known:

1. Minimal short circuit current
2. RMS value of maximal short circuit current
3. Peak value of the maximum short-circuit current

Minimal short - circuit current (Isc)
This corresponds to a short-circuit at one end of the protected link fault at the end of a feeder and not just behind the breaking device. Its value allows us to choose the setting of thresholds for overcurrent protection relays and fuses; especially when the length of the cables is high and/or when the source is relatively impedant (generator, UPS).

• Isc min = kA rms
• Example: 25 kA rms

 Short-circuit currents

RMS Value of Maximal Short-Circuit Current (Ith)

This corresponds to a short-circuit in the immediate vicinity of the downstream terminals of the switching device (see figure above). It is defined in kA for 1 or 3 second(s) and is used to define the thermal withstand of the equipment.

• Ith = kA rms 1s or 3s
• Example: 25 kA rms 1s

Peak value of the maximum short-circuit current (Idyn)

It determines the breaking capacity and closing capacity of circuit breakers and switches, as well as the electrodynamic withstand of busbars and switchgear.

 Peak value of the maximum short circuit current

• Idyn = kA peak
• Example: 2.5 x 25 kA = 62.5 kA peak for a DC time constant of 45 ms and a rated frequency of 50 Hz as per IEC 62271-100

Generally, Idyn is equal to:

• 2.5 • Isc at 50 Hz (IEC) and 45 ms DC time-constant or,
• 2.6 • Isc at 60 Hz (IEC) and 45 ms DC time-constant or,
• 2.7 • Isc (IEC) for higher DC time-constants

The IEC uses the following values: 8 - 12.5 - 16 - 20 - 25 - 31.5 - 40 - 50 kA rms. These are generally used in the specifications.

A specification may give one value in kA rms and one value in MVA as:

• Isc = 19 kA rms or 350 MVA at 10 kV.
If we calculate the equivalent current at 350 MVA we find,

• Isc = 350 / (1.73 x 10) =  20.2 kA rms

The difference depends on how we round up the value and on local habits. The value 19 kA rms is probably the most realistic.

Reference:
Title: Medium Voltage Technical Guide | Basic of MV Cubicle Design
Publisher: Schneider Electric, 2009
Page: 15