Answer / rampal.m.

Power supply is 24vdc this can be load wear upto 600 ohm minimum operating voltage 10.5vdc for load 250 ohm (abb tx) so minus the load 600-250 =450 ohm from graph.
Now inst cable copper conductor has resistance per km approx= 25 ohm/km then divided 450/25=18km.
Answer=18km .

Answer / praful

depending upon the cable size and its resistance we can calculate the max. cable length. e.g. if 1 sq.mm copper cable is used then its resist. is 33ohm/km and operating voltage is 24v then its voltage drop for 1km is 17v.so if instrument operating supply is less than the drop voltage i.e (24-17) then we can use upto 1 km otherwise reduce the length as per opr. voltage.

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