Which test is used for open-circuit (broken conductor) faults?

Murray capacitance loop (capacitance comparison)

When is the Blavier test used?

Earth fault when no sound core is available

What does the Murray loop test need besides the faulty core?

One sound core of the same size running alongside

An earthed sheath of different material

2079 Chaitra · 6 marks

Cable fault location techniques and Murray loop

Short answer

Techniques include Murray loop, Varley loop, capacitance comparison, TDR, Blavier test, and thumper. The Murray loop test for short-circuit faults connects faulty and sound cores at the far end, balances a bridge, and locates the fault at x = (Q/(P+Q))*2L from the test end.

What are the techniques of locating faults in underground cable? Explain Murray loop test.

Underground cable faults are first found to exist with an insulation (megger) test. They are then located by measuring from the cable ends, and finally pinpointed on the route.

Techniques of locating faults

Method	Used for
Murray loop test	Earth and short-circuit faults (resistance bridge)
Varley loop test	Earth and short-circuit faults (bridge with an extra variable resistor)
Murray capacitance loop / capacitance comparison	Open-circuit (broken conductor) faults
Pulse echo (TDR)	Any fault, from the travel time of a reflected pulse
Blavier test	Earth fault when no sound core is available
Thumper (surge generator) and acoustic detection	Pinpointing on the route

Murray loop test (short-circuit or earth fault)

This is a Wheatstone bridge method. It needs one sound core running alongside the faulty core.

   +--------------(G)--------------+
   |                               |
   B ---/\/\------+------/\/\----- A
   |     P        |        Q       |
   |           battery             |
   |              |                |
   |            earth              |
 sound core                   faulty core
   |                               x  <- fault to earth
   |                               |
   +====== loop at far end ========+

At the far end, connect the faulty core to the sound core with a low-resistance link, so the two cores form a loop.
Connect ratio arm $P$ (a variable resistor) to the test end B of the sound core and ratio arm $Q$ to the test end A of the faulty core, and connect the galvanometer across A and B. Connect the battery between the junction of $P$ and $Q$ and earth (for an earth fault). For a short circuit between two cores, connect the battery to the other shorted core instead of earth.
Adjust $P$ and $Q$ until the galvanometer reads zero.

Let $R_x$ be the resistance of the faulty core from the test end to the fault, and $R_L$ the resistance of the whole loop. The arm next to $P$ is the rest of the loop, $R_L - R_x$ , and the arm next to $Q$ is $R_x$ . At balance:

\frac{P}{Q} = \frac{R_L - R_x}{R_x} \;\Rightarrow\; R_x = \frac{Q}{P+Q}\,R_L

If both cores have the same cross-section and the cable length is $L$ , resistance is proportional to length:

x = \frac{Q}{P+Q}\times 2L

Here $x$ is the distance from the test end to the fault. The fault resistance carries only battery current, so it does not affect the balance.

Example

Take a 2 km cable (loop length 4 km) that balances at $P = 3\ \Omega$ and $Q = 1\ \Omega$ :

x = \frac{1}{3+1}\times 4 = 1\ \text{km}

So the fault is 1 km from the test end.

✓ Answer checked by Claude Opus 5.5 (high effort)

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Cable fault location techniques and Murray loop

Techniques of locating faults

Murray loop test (short-circuit or earth fault)

Example

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