Calculations according to IEC 60909
elec calc™ calculates short-circuit currents according to the recommendations of standard IEC 60909. In accordance with the recommendations of this standard, the symmetrical component method is used. This method is the most accurate, then leading to an optimized sizing compared to the other conservative simplified methods.
The short-circuit currents are recalculated automatically every time a change likely to alter them is made on the one-line diagram. Depending on their location and the earthing system, the following short-circuit currents can be calculated:
- Maximum, minimum and peak 3-phase short-circuit current Ik3
- Maximum, minimum and peak 2-phase short-circuit current Ik2
- Maximum, minimum and peak phase/neutral short-circuit current Ik1
- Maximum, minimum and peak phase/earth short-circuit current If
Specific case: in LV with an IT earthing system, the line-to-ground short-circuit currents calculated and displayed are those in case of two separate insulation faults. The first fault current is calculated to determine the touch voltage.
The short-circuit currents are used in the following functions:
- Short-circuit protection: the tripping or melting threshold of the protection is determined in relation to the minimum short-circuit current through this protection, i.e. the place where it is the weakest in the area of influence of the protection.
- Breaking capacity: the aim is to verify that protection devices are able to cut the short-circuit current likely to appear in the area of influence of the protection without deteriorating (symetrical short-circuit current within the meaning of the standard IEC 60909).
- Making capacity: the aim is to verify that the breaking devices are able to close on the peak short-circuit current likely to occur when the device closes without deteriorating.
- Thermal stress: The aim is to check that the wiring systems are able to withstand the let-through energy during the protection tripping time when a short-circuit occurs.
- Indirect contacts: the aim is to check that the operating time of the protections remains below a certain value, depending on the touch voltage. This touch voltage is the voltage which can appear between the exposed conductive part of a device and the earth in the event of a line-to-ground fault. It is therefore calculated based on the line-to-ground short-circuit current If.
- Dynamic withstand: the aim is to check that certain components (busbar trunking systems, current transformers, etc.) have sufficient dynamic withstand to resist the peak short-circuit currents likely to pass through them.
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