Lesson plan
Murray loop bridge
test instrument capable of performing Murray and Varley loop tests. The Murray loop bridge is a bridge circuit used for locating faults in underground or underwater cables. It has been used for more than 100 years but is being replaced by the more precise time-domain reflectometer. One end of the faulted cable is connected through a pair of resistors to the voltage source. Also a null detector is connected. The other end of the cable is shorted. The bridge is brought to balance by changing the values of R B1 and R B2 , which is achieved when: \frac{R_x}{R_g + R_y} = \frac{R_{B1}}{R_{B2}} which is equivalent to: R_x = (R_g + R_y) \cdot \frac{R_{B1}}{R_{B2}} The value of resistance R x is proportional the length L x , thus the location of the fault can be calculated: L_x = 2 \cdot L \cdot \frac{R_{B1}}{R_{B1}+R_{B2}} where L is the total length of the cable under test - a value proportional to R g . The method assumes a single fault exists, of low resistance compared with the undamaged cable insulation resistance, and that the cable conductors have uniform resistance per unit length.
Article summary
test instrument capable of performing Murray and Varley loop tests. The Murray loop bridge is a bridge circuit used for locating faults in underground or underwater cables. It has been used for more than 100 years but is being replaced by the more precise time-domain reflectometer. One end of the faulted cable is connected through a pair of resistors to the voltage source. Also a null detector is connected. The other end of the cable is shorted. The bridge is brought to balance by changing the values of R B1 and R B2 , which is achieved when: \frac{R_x}{R_g + R_y} = \frac{R_{B1}}{R_{B2}} which is equivalent to: R_x = (R_g + R_y) \cdot \frac{R_{B1}}{R_{B2}} The value of resistance R x is proportional the length L x , thus the location of the fault can be calculated: L_x = 2 \cdot L \cdot \frac{R_{B1}}{R_{B1}+R_{B2}} where L is the total length of the cable under test - a value proportional to R g . The method assumes a single fault exists, of low resistance compared with the undamaged cable insulation resistance, and that the cable conductors have uniform resistance per unit length.
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
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- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
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