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# Short circuit studies to IEC60909

## Short circuit studies to IEC60909

(OP)
Could someone please explain what value of resistance should be used in the plant impedance parameters when calculating the DC component of breaking current using the IEC60909 methodology for maximum short circuit current calculations. I understand the temperature should be at 20°C but I do not understand if I should enter the AC resistance or the DC resistance. The resistance needs to be entered for generators, cables and transformers. One of the problems with the software we use is that only cable data has fields for both AC & DC resistance. Transformers and generators have only a single field for resistance without any indication as to it being AC or DC.

### RE: Short circuit studies to IEC60909

I don't think it is very important since total equivalent impedance error is more than the conductor resistance permissible error.
For maximum current you may use the d.c. resistance at 20 or 25 oC. For minimum current you may use the temperature of 70oC for cable and 50oC for overhead lines and for cross section more than 70 mm^2 also skin effect and neglecting proximity effect.
IEC 60909-0 ch. 2.5 Minimum short-circuit currents recommends only d.c. resistance at maximum temperature.
Analyzing the example from IEC 60909-4 ch. 3 Calculation of short-circuit currents in a low-voltage system Un = 400 V Table 3 the cable of 70mm^2 and 240mm^2 copper no skin effect and even no temperature factor is taken into consideration.

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