Locating faults in underground cables

Faults in underground cables (insulation breakdown to sheath, short between cores, or open conductor) cannot be seen, so they are detected and located electrically in three steps.

1. Identify the faulty core and type of fault

• Use an insulation tester (megger) between each core and earth/sheath and between cores. Very low reading = earth or short-circuit fault.
• Use a continuity test (cores shorted at far end) to find open-circuit faults.

2. Locate the fault (pre-location) – loop tests

Murray loop test (for earth and short-circuit faults): the faulty core and a sound core are joined at the far end to form a loop, which with ratio arms $P$ and $Q$ forms a Wheatstone bridge. The battery is connected from the junction of $P$ and $Q$ to earth, and the galvanometer across the two cable ends at the test end.

     A o-----[Battery]------+  Earth (sheath)
       |                    :
   +---+---+                :  fault resistance
   P       Q                :
   |       |                :
 D o--(G)--o B              :
   |       |                :
   |       +== faulty core =X========+
   |        <------ x ----->         |
   +========= sound core ============+
 Test end                 Far end (cores joined)

At balance:

$$\frac{P}{Q} = \frac{R}{R_x},\qquad x = \frac{Q}{P+Q}\times2l$$

where $x$ = distance of fault from test end and $l$ = cable length (cores of equal area).

Varley loop test: Similar loop, but with fixed ratio arms $P$, $Q$ and a variable resistor $S$ in series with the faulty-core arm; the loop resistance $(R_1+R_x)$ is first measured by a separate balance. Balance gives

$$R_x = \frac{(R_1+R_x)Q - SP}{P+Q}$$

and $x = R_x/r$. It is useful when the loop resistance is not known.

Pulse echo (time domain reflectometry, TDR): A short pulse is sent into the cable; it reflects at the fault. Distance $= v t/2$, where $v$ is the pulse velocity in the cable and $t$ the echo time. Fast and suitable for open and short faults.

3. Pinpoint the exact spot

Use an acoustic thumper (surge generator, listen for the discharge sound) or tracer/audio-frequency methods along the route before excavating.