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Short Circuit Current


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My customer says that his LV network short circuit (SC) current is 65KA and peak SC is 137 KA. Please tell me how does it affect the motor designer. E.g is my customer expecting 65 KA to flow through the motor windings (stator/rotor) for certian duration e.g 65KA for 0.25 sec.? Please clarify.
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The 65 kA SC curren (Isc) let us know the net impedance (impedance at the connection point) (Z=Uf/Isc). The 137 kA peak SC current let us know the R/X relationship of Z.


This values let you know what are the minimum ratings of the first protection panel devices and cable sections between this panel and the connection point.


Inside the installation, you must coordinate protection and cable with your loads. The characteristics of loads are very important. Asynchronous motors usually have a starting current 6-8 times higher than the nominal current, but the time that this current is demanded by the motor depends on the inertia and the mechanical load of the motor. More mechanical load implies more starting time, so more energy is dissipated by the conductors (you must increase the mm2). Furthermore, if the sum of all impedances are high (including the net impedance), the voltage at the motor pins will be low while starting, this implies less torque and more starting time. The starting time must be less than the specified value by the manufacturer, because of the temperature of the windings.


Maybe I've explain a lot of things in one, but those are the main factors that affects the design of the installation between the supplier and the load.

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