Rule of thumb for quick estimation
If we consider a branch circuit going away from the point of supply and follow it radially along all the connections and branches, we shall find that further away we are from the transformer, lower is the value of the maximum possible short-circuit current.
Example of Isc at the end of feeder
A prospective short-circuit current of 50 kA at the secondary terminals of a transformer at 400 V, will be limited to about 10 kA at the end of a connecting lead with a length of 10 m and cross-section of 10 mm2. If the same feeder has a cross-section of 25 mm2, the length of the wire is to be 25 m for reducing the current down to 10 kA.
Specimen example //
The expected short-circuit current at the end of the feeder is expressed by the relation :
Where:
- IK” –Â Short-circuit current [kA]
- UNTrafo –Â Rated voltage of the supply transformer on the low-tension side [V]
- ZTrafo –Â Impedance of the transformer
- ZConducting – Impedance of the conducting lead
However, short-circuit at the point 5 is according to the above mentioned formula:
Reference: Basics of circuit breakers – Rockwell
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Edvard Csanyi
Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.
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Why is the I_shortcircuit for transformer different when calculated using kVA and different while using this relation you mention?
For example, Xfmr_FLA=kVA/(1.73 x kVsec) = 630/(1.73×0.4) = 909A then Xfmr_I_shortcirc=Xfmr_FLA/0.06=15.15kA
While I_sc_xfmr = kV_sec/(1.73 x Zxfmr) = 0.4/(1.73 x (3.1^2+13.5^2)^0.5) = 16.694kA
Why are these two giving mismatch? what am I missing?
Good Example but I dont understand some values. How are they calculated Uk , all R/mohm , X/mohm and all cable length and cross section best regards.
could you help me pls
best regards
Yes I’m an Electrical Engineer
Nice article. The procedure is similar to IEC 60909 method, only the correction factor in case of the transformer and voltage is not considered. Nevertheless, it is a good way of estimating the short circuit current.
Those are very instructive articles, they help me a lot Thank you
La simpleza hace hermoso los desarrollos matemáticos. Calculo con Melshort de Mitsubishi dio el mismo resultado