Wireman · Finding an underground cable fault: troubleshooting for electricians · 2023 NEC ·

Finding an underground cable fault

Megger to decide, loop resistance and TDR to find distance, and when to replace instead of dig.

Before you start

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How it works

A buried cable can fail by losing insulation to the earth (a ground fault), shorting between conductors, or opening (a break). An insulation resistance test from both ends tells you whether it’s bad. A time-domain reflectometer (TDR) sends a pulse down the cable and times the reflection to show distance to an open or a low-resistance short. Faults to earth in direct-buried cable can also be pinpointed from above with an earth-gradient (A-frame) fault locator. Then you decide whether to dig and splice or replace the run.

Step by stepFull flowchart

Every question and answer in this guide. Tap “Step by step” to walk through it.

What does the circuit do?
- Breaker trips instantlyTest: Insulation-test the cableExpect: Hundreds of megohms or more on every pair. Under about 1 MΩ is a fault.
  - Near zero (a hard fault)Test: Find the distance with a TDR
    - Clear reflection at a distanceHow old is the cable, and is this the first fault?
      - Newer cable, first faultDig and splice
      - Old cable or repeated faultsReplace the run
    - No clear reflectionIs the cable direct-buried with the fault to earth?
      - Yes, direct-buriedPinpoint from above with an earth-gradient locator
      - No, it’s in conduitFault in a conduit run
  - Low, but not zeroIs the cable direct-buried with the fault to earth?
    - Yes, direct-buriedPinpoint from above with an earth-gradient locator
    - No, it’s in conduitFault in a conduit run
  - All highThe buried cable is good
- GFCI trips, breaker holdsTest: Insulation-test the cableExpect: Hundreds of megohms or more on every pair. Under about 1 MΩ is a fault.
  - Near zero (a hard fault)Test: Find the distance with a TDR
    - Clear reflection at a distanceHow old is the cable, and is this the first fault?
      - Newer cable, first faultDig and splice
      - Old cable or repeated faultsReplace the run
    - No clear reflectionIs the cable direct-buried with the fault to earth?
      - Yes, direct-buriedPinpoint from above with an earth-gradient locator
      - No, it’s in conduitFault in a conduit run
  - Low, but not zeroIs the cable direct-buried with the fault to earth?
    - Yes, direct-buriedPinpoint from above with an earth-gradient locator
    - No, it’s in conduitFault in a conduit run
  - All highThe buried cable is good
- Dead or low voltage at the far endTest: Measure loop resistanceExpect: Close to the expected value for the length and size.
  - Open (OL)Test: Find the distance with a TDR
    - Clear reflection at a distanceHow old is the cable, and is this the first fault?
      - Newer cable, first faultDig and splice
      - Old cable or repeated faultsReplace the run
    - No clear reflectionIs the cable direct-buried with the fault to earth?
      - Yes, direct-buriedPinpoint from above with an earth-gradient locator
      - No, it’s in conduitFault in a conduit run
  - Much higher than expectedHigh-resistance splice or damaged conductor
  - As expectedThe buried cable is good