DC resistance of cable

[urjaengrs]

All manufacturers mention value of DC resistances for their cables. We normally use ac resistance for calculation purpose. What is nescessitiy of mentiong DC resistance then? Is it only for confirmation to limitation of resistances to standard?

Please help me out. In advance, thanks....

 

[Marmite]

The ratio of AC Resistance to DC Resistance per unit length is the skin effect ratio S. This is used for calculating the heat losses in the conductor. Heat generated in the conductor = I squared R x skin effect ratio S.

Regards
Marmite

 

[ScottyUK]

DC resistance is useful for DC applications!


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If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

One good use of DC resistance is when you measure cable length with a precise ohm meter. Done that on signal cables to find out if there were any stubs attached. DC resistance -> length. Capacitance -> length including stubs. If ratio C/R is more than specified in the data sheet, then you have got stubs somewhere along your cable. They produce echos that you do not want. Yes, did this even when I had a TDR.

Not sure, though, if DC and AC differs much in a signal cable. Don't think so.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[7anoter4]

The AC resistance is merely the DC resistance corrected for the rated temperature [usually DC is registered for 20o C and the rated temperature may be 70 for PVC or 90 for EPR or XLPE], and multiplied by the skin factor. Skin effect is the phenomenon where current escapes from conductor center to the periphery –as if the current flows only through the "skin" of the conductor.
Skin factor depends upon conductor overall diameter and frequency.
As Skogsgurra said for control cables of small diameter conductor cables this factor is close to 1.
For large power cable conductor the difference begins to be significant and for cable heat calculation purpose, where one needs to appreciate accurately the power losses,it is important, as Marmite said.
For three core cable or three single phase cable there is another factor to mention: the proximity factor.
The proximity effect is the phenomenon where the current will flow mainly in the opposite side of the conductor vecinity –as if the proximity of the second conductor press the current in the opposite side.
This factor may be significant also for large conductor and elevated frequency.