7. You are hired by CDOT as an engineering consultant to tell the power company

Question

7. You are hired by CDOT as an engineering consultant to tell the power company where to dig in order to make a repair to a transmission line that is burie insulated metal cables. Each metal cable has a resistance-per-length of 17 CDOT claims th line. The transmission line has access every 13 km where you can make independent nission line consists of two parallel /km. at they are experiencing excessive power loss in the transmission electrical connection to the two wires. (a) What should you expect the current to be if you attach a 120-volt source across the two electrical connections at an access point. (b) When you connect a 120-volt source across the connections at station F, a current of 270 mA flows from the source. When this is done at Station G (the next station 13 km east of Station F), a current of 590 mA flows. You conclude that somewhere along the transmission line between Station F and G, the insulation is damaged, allowing current to flow through the dirt from one cable to the other. Where should CDOT dig up the highway?

Explanation / Answer

a)

equivalent resistance of transmission line up to the access point

Req=2*(17 ohms /km)(13km)=442 ohms

Therefore current at an access point is

I=V/R=120/442

I=0.2715 A =271.5 mA

b)

Let X be distance where the fault occurs from station F ,then the voltage across station F is equal to Voltage across station V

VF=VG

(0.27)*(2*13 ohms/km)*x=0.59*(2*13 ohms/km)*(13 km-x)

0.27x=0.59(13-x)

0.86x=7.60

x=8.92 km =9 km (approx)

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