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High Impedance Fault Detection on Rural Electrical Distribution Systems Craig Wester Jakov Vico Mark Adamiak Ashish Kulsrestha GE Digital Energy Multilin Overview Definitions & causes of Hi-Z faults Importance of Hi-Z detection Misconception about Hi-Z faults

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High Impedance Fault Detection on Rural Electrical Distribution Systems

Craig WesterJakov VicoMark AdamiakAshish KulsresthaGE Digital Energy Multilin



  • Definitions & causes of Hi-Z faults
  • Importance of Hi-Z detection
  • Misconception about Hi-Z faults
  • Characteristics of High Impedance (Hi-Z) Faults
  • Fault currents on various surfaces
  • Research and development at Texas A&M University
  • Hi-Z & Arc detection using Microprocessor-Based Technology
  • Implementation & response strategies
  • Field experiences
  • Summary

Definition of Hi-Z faults

  • An energized primary conductor in contact with a quasi-insulating object, such as a tree, structure or earth…..thus a high impedance (or Hi-Z) fault
  • Not detected by conventional phase or ground overcurrent protection (fuses or overcurrent relays)
  • Little threat of damage to power system equipment, but potential safety and fire hazard
  • Hi-Z faults produce primary current levels of 0 to 100 Amps
  • Seldom documented on trouble reports

Causes of Hi-Z faults

  • Broken line on ground (Downed Conductor)
  • Broken pole allowing line to contact a ground or conducting surface
  • Broken pole or tree limb allowing primary to sag
  • Contact with tree limb or other objects (Intermittent Arcing)
  • Contaminated or failing equipment (insulators, transformers, conductors, etc.)

Importance of Hi-Z detection

Seasonal conditions impacts people & assets


Importance of Hi-Z detection

Extreme weather conditions impacts people & assets


Importance of Hi-Z detection

  • If not detected and isolated, live Downed Conductors can be fatal to public and line crewmen
  • Hi-Z faults often arc and can be a significant fire hazard
  • Detect failing insulation before complete device failure which can lead to power outages and loss of production
  • Inability to detect Hi-Z faults can cost utilities liabilitiesand customer service issues
  • Performance has been verified under normal conditions (noisy feeders, arc furnaces, arc welders, capacitor switching, line switching and load tap changing)

Hi-Z detection is about protecting people and assets


Misconceptions about Hi-Z faults

Misconception: Properly set, overcurrent protection will trip and clear all faults on distribution feeder

Reality:Hi-Z faults often draw less current (10 – 100 amps) than loads, making overcurrent protection unable to operate

Misconception: Sensitive ground protection typically used to detect low ground current, will clear Hi-Z faults

Reality:Unbalanced loads limit sensitivity of ground protection. Moreover, a down conductor can result in more balanced loads and reduced neutral current. This can easily mislead the low impedance protection element and no operation



Misconceptions about Hi-Z faults . . .

Misconception: Over time, fault current will increase and operate protection

Reality:In most cases, fault current decreases as conductor burns, moisture evaporates, sand fuses, etc. overcurrent protection seldom operates after first minute

Misconception: Faults always clear on my system

Reality:Engineering staffs believe Hi-Z fault rate is low, but line crews report many downed conductors are still hot when they arrive on scene

introduction to hi z







HiZ Fault Load Bolted Fault

Introduction to Hi-Z

Definition:A high-impedance (Hi-Z) fault is one that draws too little current to operate conventional overcurrent protection (fuses, relays, etc.).

Hi-Z Fault Current Levels

Conventional overcurrent protection is not able to detect Hi-Z

introduction to hi z15
Introduction to Hi-Z
  • Hi-Z arcing fault current is rich in harmonics and non-harmonics.
  • Hi-Z fault current is erratic but tends to decrease over time, often stopping completely after minutes.
  • Hi-Z faults persist seconds to minutes….. to days

Fault currents on various surfaces

Surface Fault current (A)Dry Asphalt 0Dry Sand 0Wet Asphalt 1Wet Sand 5Dry Sod 10Concrete (non-reinforced) 10Wet Sod 50Concrete (reinforced) 70

80-85% of all down conductors can be detected


Basics of Hi-Z Faults

  • Down Conductor occurs when live conductor breaks and falls on the ground
  • A break in the conductor usually leads to:
    • Drop in the load or
    • Momentary overcurrent due to falling conductor contacting a grounded object.
  • A Hi-Z fault often is accompanied by arcing at the point of the fault
  • Hi-Z fault without a broken conductor is termed as Intact Hi-Z condition and is caused by:
    • Failure of the conductor mounting system
    • Insulation failure
    • Inadvertent contact with external element (tree limb)

Characteristics of arcing faults

  • Little effect on voltage
  • Small fault current (10 – 100 Amps)
  • Current values will continue to fluctuate
  • Significant harmonic and non-harmonic current
  • No single parameter uniformly changing

Normal System Behavior

Hi-Z Fault Behavior


Research and development lead by Texas A&M University (TAMU)

  • Hundreds of staged fault tests since early 1990’s
    • At dedicated local facility
    • At multiple utilities across US
    • Important note: fault current was not artificially limited
  • Characterization of Hi-Z behavior
  • Multiple generations of prototypes
    • Staged faults assessed sensitivity to faults
    • Long-term installations assessed immunity to false trips
    • Long-term prototype installations established criteria for success and formed high-level system concepts


    • SOE
    • Oscillography
    • SCADA




Hi-Z & Arc detection

  • Apply on distribution breaker or recloser to detect Hi-Z faults
  • 4.16 to 34.5kV applications
  • Hi-Z detection will work on current alone - use relaying CTs
  • Voltage provides supplemental phase identification
  • Algorithm in service since 1992 on various hardware platforms

The following nine sophisticated algorithms are used:

  • Energy
  • Randomness
  • Expert Arc Detector
  • Load Event Detector
  • Load Analysis
  • Load Extraction
  • Arc Burst Pattern Analysis
  • Spectral Analysis
  • Arcing Suspected Identifier

Over 20 year matured algorithms

Hi-Z & Arc detection

  • Uses signature based expert pattern recognition system developed at Texas A&M University
  • Harmonic energy levels of currents in the arc is used for Hi-Z fault detection
  • The expert system techniques are designed to assure security and dependability

Hi-Z & Arc detection

Detection Parameters

  • Odd harmonics (3rd, 5th, …)
    • Largest increase
    • Smallest relative increase
  • Even harmonics (2nd, 4th, …)
    • Small ambient level
    • Affected by inrush
  • Non harmonics (1/2, 1-1/2, 2-1/2, …)
    • Small ambient level
  • Voltage
    • Enhance security
    • Phase Identification
  • Learns ambient harmonic level and adjusts frequently

Hi-Z & Arc detection

High Impedance Fault Detection Block Diagram


Implementation Strategy

Contrast in Detection Goals - Overcurrent vs. High Impedance

  • Sufficient current vs. low current
  • Equipment damage vs. safety/fire prevention

Electrical or Mechanical Detection Options

  • Electrical options applied one per feeder or recloser
  • Mechanical options applied in certain areas (schools and churches)
  • Mechanical options can detect sagging conductors

Implementation Strategy

Customer Service

  • A priority due to increasing competition
  • Hi-Z faults can cause service interruptions and deliver substandard power to users
  • Applying electrical Hi-Z detection, allows utilities to respond quicker to Hi-Z occurrences
  • Accurate, dependable and secure operation is very important
  • Inform customers of potential problems
  • Response procedures can be created
  • 95-98% complete fault detection possible(low impedance + high impedance)

Implementation Strategy

Feeder Selection

  • Unreasonable to apply Hi-Z detection on every feeder at once due to expense

Circuits with:

  • Past Hi-Z events
  • Population dense circuits
  • Fire prone areas
  • Older circuits with undersized conductors
  • 4 - 35kV circuits
  • Overhead construction

Arcing fault response strategy


Primary Response

Secondary Response

Arcing Suspected



Arcing Detected



Downed Conductor



  • No device can protect from initial contact
  • Disable reclosing after detection of Hi-Z

Hi-Z Based Feeder Sectionalizing

  • Coordination via settings & communication







  • Serves Washington DC and parts of Maryland
  • Covers an area of 640 sq. mi.
  • Provides power to over 2,000,000 customers
  • Distribution system of 620 13kV overhead feeders
  • Study based on 280 installed Hi-Z relays over a 2 year period
  • Hi-Z relays were set biased toward security
study methodology
Study Methodology
  • Candidate Faults for Study
  • Operator logs
    • Line Broken
    • Still Energized

-- OR --

  • Relay Hi-Z target

- Checked Weekly

Relays were set biased towards security


Study Results

  • Study based on 560 relay-years of operation
  • Several hundred broken wires recorded during study
  • 48 “Downed/Energized” faults recorded
  • From the remaining 48 “Downed/Energized” faults
    • 46 of the 48 relays indicated “Arcing” – 96%
    • 28 of the 48 relays reported “Downed Conductors” – 58%
    • 80% of the 28 “Downed Conductors” were tripped by Low Set Instantaneous & successfully reclosed

Only 2 false operations in 560 Relay-Years of Operation!


Other Installations

Experience to date of Hi-Z Algorithms

  • The ratio of “detected” downed conductors to the total population of downed conductors is 80-85%.
  • Investigation based on a periodic “arc detection” alarm lead to detect a motor failure at a customer site
  • Arcing due to loose transformer bushing was detected by Hi-Z algorithms
  • A house fire was successfully detected and feeder tripped and locked out
  • A downed conductor on an asphalt surface found paths through cracks in the asphalt, which lead to “down conductor” detection.
  • Intermittent arcing faults due to contact with tree limbs

Secure determination of Down Conductor & Arcing Conditions



  • HiZ faults happen . . . and result in:
    • Personnel hazard
    • Property damage
    • Poor customer service
  • Effective technology has been demonstrated to:
    • Reliably detect HiZ faults
    • Detect arcing conditions on system
    • Securely report Downed Conductors
    • Safely trip feeders with Downed Conductors