## 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.

**an impedance**which helps to reduce the 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 mm ^{2}**. If the same feeder has a cross-section of

**25 mm**, the length of the wire is to be

^{2}**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:**

– Short-circuit current [kA]*I*_{K”}– Rated voltage of the supply transformer on the low-tension side [V]*U*_{NTrafo}– Impedance of the transformer*Z*_{Trafo}

**Z**– Impedance of the conducting lead_{Conducting}

**However, short-circuit at the point 5 is according to the above mentioned formula:**

**rather conservative and includes a factor of safety**as other sources of current reduction like the arc voltage, the contact resistance and the internal resistances of the different devices in the path of the short-circuit were not considered.

**Reference:** Basics of circuit breakers – Rockwell

Andy

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?

Mustafa

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

T.N.Ramesh

Yes I’m an Electrical Engineer

Girish

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.

Vicente Martinez

Those are very instructive articles, they help me a lot Thank you

LALO EL HIPPIE

La simpleza hace hermoso los desarrollos matemáticos. Calculo con Melshort de Mitsubishi dio el mismo resultado