ElectroYou

This is regarding cable sizing and protective device selection in a switchboard. Kindly help me out to understand the basics in the attached schedule which is made in a Italian software https://www.exel.it/ (Progetta Integra) and that is the reason I'm here.

1) What is Id?
2) What is Igt end of line?
3) What is I2t start of line? Why is it calculated only for Phase conductor? Is it mean the cable impedance is calculated for the first few metres of the cable from the switchboard? Can you give some example? I can easily arrive at the value of K^2S^2 by multiplying XLPE copper, K =143 and the S= Cross section area of the conductor.
4) What is I2t end of line? Why is it calculated for neutral conductor?
5) What does Lmax mean? Is it the maximum length that is permitted for a particular cable size to not to exceed the allowable voltage drop?

How are the values for the above parameters arrived at in the attached schedule? can anybody give inpus

Your inputs are much appreciated!

Hi, the I2t or Joule integral is a measure of the thermal stress or thermal energy let through by the fuse during short circuit interruption. You might have a failure at the end or at the beginning of your cable.

1) Nominal current of differential protection relay : Id = 0,03 A or 30 mA

2) In reference to standard CEI 64-8 (IEC 60364) section 533.3 :
Igt = I (short circuit end of line), in this case phase-neutral Isc

Isc = 0,8 Uo/(1,5 r 2 L/S) = 421,3 A

Uo = 231 V (nominal phase-neutral volt)
r = copper resistivity @ 20°C 0,0172 (Ohm x sq mm/m)
L = simple line length (85 m)
S = copper line cross section (10 sq mm)

..............

Id is the differential current or residual current of RCD (residual current device).

Why is it calculated for neutral conductor?

In case you have a smaller cross-section for the neutral conductor

5) What does Lmax mean?

Maybe the maximal cable lenght, that the fuse can protect

No, I think you were right

Hi Selk
I know very well Progetto INTEGRA, so:

1) What is Id?
It is the RCD current (with Type of RCD)

2) What is Igt end of line?
It is Current fault between Phase and PE

3) What is I2t start of line? Why is it calculated only for Phase conductor? Is it mean the cable impedance is calculated for the first few metres of the cable from the switchboard? Can you give some example? I can easily arrive at the value of K^2S^2 by multiplying XLPE copper, K =143 and the S= Cross section area of the conductor.
i2t is not calculated, but it is read on let trough energy curve, by intersection with correspondant fault current. Usually (but not always), i2t at start of line is the maximum value, and it is compared with K2S2 (calculated).

4) What is I2t end of line? Why is it calculated for neutral conductor?
at the start of line, there is a value of Ik(max), that corresponding a i2t value read on curve; at the end of line, ther is another value of Ik(min) and related i2t, but I think it was a mistake the text "End of line".

5) What does Lmax mean? Is it the maximum length that is permitted for a particular cable size to not to exceed the allowable voltage drop?
Exactly