Wireman · Ground-fault alarm on an ungrounded or HRG system: troubleshooting for electricians · 2023 NEC ·

Ground-fault alarm on an ungrounded or HRG system

Read the ground detector lamps, confirm the faulted phase and find the feeder before a second fault.

Before you start

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How it works

On an ungrounded system no conductor is tied to earth, so the first phase-to-ground fault doesn’t trip anything: that phase just drops to ground potential and the other two rise to full line-to-line voltage to ground. A high-resistance grounded (HRG) wye works the same way but uses a resistor from neutral to ground to limit fault current to a few amps and hold voltages steady. Both keep running through a first fault, so the code requires ground detection, and the fault must be found before a second fault on another phase becomes a phase-to-phase short through the ground.

Step by stepFull flowchart

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

What kind of ground detection do you have?
- Three indicating lampsWhat are the three lamps doing?
  - One dark, the other two brighterTest: Measure each phase to groundExpect: Roughly equal readings, about 277 V each on a 480 V system.
    - One phase near 0 V, others near line-to-lineWith operations’ approval, open feeders one at a time, closing each before opening the next. Does the indication clear when one is open?
      - Clears with one feeder openTest: Insulation test the faulted circuitExpect: High megohm readings on cable and load.
        Cable reads lowGround fault in the cable
        Cable good, load reads lowGround fault in the equipment
        Everything tests goodIntermittent fault
      - Never clearsFault on the bus, the source or the detector
    - One phase low but not 0 VWith operations’ approval, open feeders one at a time, closing each before opening the next. Does the indication clear when one is open?
      - Clears with one feeder openTest: Insulation test the faulted circuitExpect: High megohm readings on cable and load.
        Cable reads lowGround fault in the cable
        Cable good, load reads lowGround fault in the equipment
        Everything tests goodIntermittent fault
      - Never clearsFault on the bus, the source or the detector
    - All about equalNo ground fault right now
  - One dimmer, not outTest: Measure each phase to groundExpect: Roughly equal readings, about 277 V each on a 480 V system.
    - One phase near 0 V, others near line-to-lineWith operations’ approval, open feeders one at a time, closing each before opening the next. Does the indication clear when one is open?
      - Clears with one feeder openTest: Insulation test the faulted circuitExpect: High megohm readings on cable and load.
        Cable reads lowGround fault in the cable
        Cable good, load reads lowGround fault in the equipment
        Everything tests goodIntermittent fault
      - Never clearsFault on the bus, the source or the detector
    - One phase low but not 0 VWith operations’ approval, open feeders one at a time, closing each before opening the next. Does the indication clear when one is open?
      - Clears with one feeder openTest: Insulation test the faulted circuitExpect: High megohm readings on cable and load.
        Cable reads lowGround fault in the cable
        Cable good, load reads lowGround fault in the equipment
        Everything tests goodIntermittent fault
      - Never clearsFault on the bus, the source or the detector
    - All about equalNo ground fault right now
  - One dark, the others look normalPress the lamp test or swap in a known-good lamp. Does the dark one light?
    - It was a bad lampBurned-out indicating lamp
    - Still dark with a good lampTest: Measure each phase to groundExpect: Roughly equal readings, about 277 V each on a 480 V system.
      - One phase near 0 V, others near line-to-lineWith operations’ approval, open feeders one at a time, closing each before opening the next. Does the indication clear when one is open?
        Clears with one feeder openTest: Insulation test the faulted circuitExpect: High megohm readings on cable and load.
        Cable reads lowGround fault in the cable
        Cable good, load reads lowGround fault in the equipment
        Everything tests goodIntermittent fault
        Never clearsFault on the bus, the source or the detector
      - One phase low but not 0 VWith operations’ approval, open feeders one at a time, closing each before opening the next. Does the indication clear when one is open?
        Clears with one feeder openTest: Insulation test the faulted circuitExpect: High megohm readings on cable and load.
        Cable reads lowGround fault in the cable
        Cable good, load reads lowGround fault in the equipment
        Everything tests goodIntermittent fault
        Never clearsFault on the bus, the source or the detector
      - All about equalNo ground fault right now
  - All three equalNo ground fault right now
- HRG alarm (resistor and relay)Does the HRG system have a pulsing feature?
  - YesTest: Trace the pulsing currentExpect: Pulsing on one feeder only.
    - Traced to a motor or piece of equipmentTest: Insulation test the faulted circuitExpect: High megohm readings on cable and load.
      - Cable reads lowGround fault in the cable
      - Cable good, load reads lowGround fault in the equipment
      - Everything tests goodIntermittent fault
    - Lost the pulse or can’t follow itWith operations’ approval, open feeders one at a time, closing each before opening the next. Does the indication clear when one is open?
      - Clears with one feeder openTest: Insulation test the faulted circuitExpect: High megohm readings on cable and load.
        Cable reads lowGround fault in the cable
        Cable good, load reads lowGround fault in the equipment
        Everything tests goodIntermittent fault
      - Never clearsFault on the bus, the source or the detector
  - NoWith operations’ approval, open feeders one at a time, closing each before opening the next. Does the indication clear when one is open?
    - Clears with one feeder openTest: Insulation test the faulted circuitExpect: High megohm readings on cable and load.
      - Cable reads lowGround fault in the cable
      - Cable good, load reads lowGround fault in the equipment
      - Everything tests goodIntermittent fault
    - Never clearsFault on the bus, the source or the detector