Murray loop bridge

The Murray loop bridge is a <a href="/facts/Bridge_circuit/TjwcbgwT">bridge circuit</a> used for locating faults in underground or underwater <a href="/facts/Electrical_cable/Dhzlz5hn">cables</a>. It has been used for more than 100 years but is being replaced by the more precise <a href="/facts/Time-domain_reflectometer/md9VY2JY">time-domain reflectometer</a>.
One end of the faulted cable is connected through a pair of <a href="/facts/Resistor/JrsGReM6">resistors</a> to the voltage source. Also a <a href="/facts/Null_detector/Y3rFC18f">null detector</a> is connected. The other end of the cable is shorted. The bridge is brought to balance by changing the values of RB1 and RB2, which is achieved when:

 
 
 
 
 
 
 R
 
 x
 
 
 
 
 R
 
 g
 
 
 +
 
 R
 
 y
 
 
 
 
 
 =
 
 
 
 R
 
 B
 1
 
 
 
 R
 
 B
 2
 
 
 
 
 
 
 {\displaystyle {\frac {R_{x}}{R_{g}+R_{y}}}={\frac {R_{B1}}{R_{B2}}}}

which is equivalent to:

 
 
 
 
 R
 
 x
 
 
 =
 (
 
 R
 
 g
 
 
 +
 
 R
 
 y
 
 
 )
 ⋅
 
 
 
 R
 
 B
 1
 
 
 
 R
 
 B
 2
 
 
 
 
 
 
 {\displaystyle R_{x}=(R_{g}+R_{y})\cdot {\frac {R_{B1}}{R_{B2}}}}

The value of resistance Rx is proportional the length Lx, thus the location of the fault can be calculated:

 
 
 
 
 L
 
 x
 
 
 =
 2
 ⋅
 L
 ⋅
 
 
 
 R
 
 B
 1
 
 
 
 
 R
 
 B
 1
 
 
 +
 
 R
 
 B
 2
 
 
 
 
 
 
 
 {\displaystyle 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 Rg.
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.

Murray loop bridge open-in-new

Murray loop bridge