Arc Flash Hazard Can HRG Technology play a role in prevention

1
Arc Flash HazardCan HRG Technology play a role
in prevention
2
Overview

• Understand
• What is Arc Flash
• Bolted Fault and Arcing Fault
• Flash Hazard Analysis
• Ungrounded Systems
• Solidly Grounded Systems
• HRG Systems

3
Poll Question 1

• Have you considered using HRG technology as part
  of an arc flash reduction strategy?

4
What is Arc Flash?
NFPA 70E says an arc flash hazard is A dangerous
condition associated with the release of energy
caused by an electric arc
A hazard beyond shock and electrocution.
5
Two major electrical Faults Bolted and Arcing
Faults

• Bolted faults (low impedance and high current)
• Commonly caused by
• Improper connections after maintenance
• Installation errors
• Arcing faults (high impedance, lower current)
• Commonly caused by
• Careless cover or device removal
• Foreign object (tool) dropped into equipment
• Misalignment of moving contacts (parts failure)
• Dirt contamination or dielectric breakdown
• Entry of foreign body (rodent, snake, squirrel)

6
Bolted and Arcing Fault Characteristics

• Arcing fault incident energy produced is
• Greater at higher bolted fault current levels
• Reduced by dynamic impedance (air)
• And increased by the time duration of the arc
• The most controllable factor in reducing the
  incident energy is time
• Fuses or circuit breakers are the first line of
  defense in reducing arcing fault incident energy
• Calculating arc fault incident energy is a very
  complex engineering task

7
Electrical Arc Facts

• Arc is electric current passing through air
• Shock potential from contact with arc
• Temperature of arc plasma center is greater than
  5000F (some say much higher)
• Radiated heat burns
• Pressure wave generated from arc
• Impact to hearing, etc
• Gaseous copper is 44,000 times solid
• Molten metal expelled from equipment at high
  speed
• Arc fault results from something wrong or out of
  place

8
Some arc flash injury statistics
Five to ten arc flash explosions occur in
electrical equipment every day in the United
States, according to statistics compiled by
Cap-Schell, Inc., a Chicago-based research and
consulting firm that specializes in preventing
workplace injuries and deaths. Injuries from arc
flash events range from minor injuries to third
degree burns and potential death due to the
energy released. Other injuries include
blindness, hearing loss, nerve damage, and
cardiac arrest. The average cost of medical
treatment for survivors of arc flash incidents is
1,500,000
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Flash Hazard Analysis
Flash hazard analysis shall be done before a
person approaches any exposed electrical
conductor or circuit part that has not been
placed in an electrically safe work condition.
(NFPA 70E, Part II, 2-1.3.3)

• Desired output for each equipment
• Flash protection boundary distance
• Incident energy
• Hazard / risk category for PPE selection

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Limits of approach
Flash protection boundary An approach limit at a
distance from exposed live parts within which a
person could receive a second degree burn if an
electric arc flash were to occur. (NFPA 70E
proposed)
It is generally accepted that a second degree
burn results from exposure of incident energy of
1.2 cal/cm2
NFPA 70E, Fig. A-1-2.4
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Flash Protection Boundary
a distance from exposed live parts within which a
person could receive a second degree burn
D
Dfb
12
Incident Energy
The amount of energy impressed on a surface, a
certain distance from the source, generated
during an arc event. Incident energy is
measured in calories/cm2 or Joules/cm2.
(Ref NFPA 70E 2003 ROC, IEEE 1584)
13
Incident Energy
Incident energy is energy impressed on this
surface a working distance, D, from the arc
source.
D
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Analysis process steps

• 1 Collect system and installation data
• 2 Determine system modes of operation
• 3 Determine bolted fault current
• 4 Determine arc fault current
• 5 Find protective device characteristic and arc
  duration
• 6 Document system voltages and equipment class
• 7 Select working distances
• 8 Calculate incident energy
• 9 Calculate flash protection boundary

Source IEEE 1584
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Step 1,2System data and modes of operation

• Begin with single-line diagrams and people who
  know site
• Collect information needed for short circuit and
  coordination study
• Determine each operation mode and perform
  calculation for each (maximum fault may not be
  worst case)
• Modes
• Multiple feeders
• Multiple transformers with tie
• Generators in parallel or standby

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Step 3Determine bolted fault current

• Start with utility information
• Use information from Steps 1 2
• Calculate accurate 3-phase bolted fault
• High or low values may not give proper arc flash
  output
• Calculation spreadsheet available with IEEE 1584

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Step 4Calculate arc fault current
For V lt 1 kV, Log Ia K 0.662 Log Ibf
0.0966 V 0.000526 Gap 0.5588 (Log Ibf) V
-0.00304 (Log Ibf) Gap
For V gt 1 kV, Log Ia 0.00402 0.983 Log Ibf
Both cases, Ia 10(Log Ia)
480 V Examples 20 kA bolted gt 6 kA arc 50 kA
bolted gt 13 kA arc 80 kA bolted gt 20 kA arc
Calculate Normalized Incident Energy
Convert to Incident Energy
Gap is distance between electrodes. Typical
distances provided.
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Step 5Determine protective device duration
Use calculated arc current instead of bolted
fault current to determine duration. Notice
dramatic change below the instantaneous
level. Include delay plus device operating time
where relay is involved.
Time
Current
?
?
19
Determine PPE Hazard Risk Category
Category Cal/cm2 Clothing
0 (1.2) Untreated cotton 1 5 FR shirt FR
pants 2 8 Cotton underwear plus FR shirt FR
pants 3 25 Cotton underwear plus FR shirt FR
pants plus FR coverall 4 40
Cotton underwear plus FR shirt FR pants
plus double layer switching coat and
pants Source NFPA 70E, Table 3-3.9.3
From incident energy value Output category for
Personal Protective Equipment
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Example of PPE
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Analogy

• Automotive Industry
• Passive Safety Control
• Seat Belt
• Shoulder Harness
• Airbags

• Electrical Industry
• Passive Safety Control
• PPE
• Arc Resistant Gear

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Analogy

• Automotive Industry
• Active Safety Control
• Intelligent Speed Adaptation
• Center High Mounted Stop Lamps
• Rear End Collision Warning
• Variable Assist power steering
• Traction Control
• Anti-locking Braking System
• Emergency Brake Assist
• Corner Brake Control,
• Forward Collision Warning
• Lane Departure Warning
• Driver Training

• Electrical Industry
• Active Safety Control
• HRG?
• Training

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Analogy

• Automotive Industry
• Statistics
• 855,258 ACCIDENTS analyzed by light conditions
• In daylight, black cars were 12 more likely than
  white cars to involved in an accident. Followed
  by grey cars at 11,silver at 10
• At dusk, or Dawn, the risk factor for black cars
  jumped to 47 more likely than white cars to
  involved in an accident. Follwed by silver cars
  at 15.

• Electrical Industry
• Statistics
• 90 of all Faults start as ground faults.

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What are the main Hazards with Ungrounded /
Solidly Grounded?

• Ungrounded Method used to ground first power
  systems
• Very large transient over-voltage conditions may
  exist.
• Insulation not rated, therefore, hazard to
  personnel and equipment.
• Very difficult to locate ground fault.
• Good chance of second ground fault on a different
  phase due to prolonged ground fault.
• Solidly Grounded Replaced Ungrounded Systems
• Very high ground fault currents.
• Fault must be cleared, shutting down equipment.
• Generators may not be rated for ground fault .
• Tremendous amount of arc flash / blast energy.
• Equipment and people are not rated for energy.

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Do others agree?

• TO HRG OR NOT TO HRG?
• IEEE Std 242-2001 (Buff Book)
• Recommended Practice for Protection and
  Coordination of Industrial and Commercial Power
  Systems
• 8.2.5 If this ground fault is intermittent or
  allowed to continue, the system could be
  subjected to possible severe over- voltages to
  ground, which can be as high as six to eight
  times phase voltage. Such over-voltages can
  puncture insulation and result in additional
  ground faults. These over- voltages are caused
  by repetitive charging of the system
  capacitance or by resonance between the system
  capacitance and the inductance of equipment in
  the system.

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Do others agree?

• TO HRG OR NOT TO HRG?
• IEEE Std 141-1993 (Red Book)
• Recommended Practice for Electric Power
  Distribution for Industrial Plants
• 7.2.1 Accumulated operating experience
  indicates that, in general purpose industrial
  power distribution systems, the over- voltage
  incidents associated with ungrounded operation
  reduce the useful life of insulation so that
  electric current and machine failures occur
  more frequently than they do on grounded power
  systems.

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Do others agree?

• TO HRG OR NOT TO HRG?
• IEEE Std 242-2001 (Buff Book)
• Recommended Practice for Protection and
  Coordination of Industrial and Commercial Power
  Systems
• 8.2.5 Ungrounded low-voltage systems employ
  ground detectors to indicate a ground fault.
  These detectors show the existence of a ground
  on the system and identify the faulted phase,
  but do not locate the ground, which can be
  anywhere on the entire system.
• One disadvantage of the solidly grounded 480V
  system involves the high magnitude of
  destructive, arcing ground-fault currents that
  can occur. However, if these currents are
  promptly interrupted, the equipment damage is
  kept to acceptable levels.

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Do others agree?

• TO HRG OR NOT TO HRG?
• IEEE Std 141-1993 (Red Book)
• Recommended Practice for Electric Power
  Distribution for Industrial Plants
• 7.2.4 The solidly grounded system has the
  highest probability of escalating into a
  phase-to-phase or three-phase arcing fault,
  particularly for the 480V and 600V systems.
  The danger of sustained arcing for
  phase-to-ground fault probability is also high
  for the 480V and 600V systems, and low for the
  208V systems. For this reason ground fault
  protection shall be required for system over
  1000A. A safety hazard exists for solidly
  grounded systems from the severe flash, arc
  burning, and blast hazard from any
  phase-to-ground fault.

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What causes the Hazards inUngrounded Systems?

• System Capacitance
• Unable to discharge leading to transient
  over-voltages
• No direct return path for ground fault current
• Prolonged fault conditions due to inability to
  quickly locate fault
• NEC 250.21(B) Ground Detectors. Ungrounded
  alternating current systems as permitted in
  250.21(A)(1) through (A)(4) operating at not less
  than 120 volts and not exceeding 1000 volts shall
  have ground detectors installed on the system.

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Ungrounded Systems

• Ungrounded systems do not have an intentional
  connection from the source generator or
  transformer to ground
• Typically a three wire delta system
• Can be a four wire system where the source
  neutral is not connected to ground

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Ungrounded Systems

• Unintentionally grounded through system
  capacitance
• Such as cables, transformers, motors, surge
  suppressors, etc.

277V
Ground 0V
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Ground Faults

• Ground fault current distribution (minimal
  current)

480V
Ground AØ
33
Ground Faults

• Ground Fault voltage distribution (voltage rise)

34
Ground Faults

• Ground Fault current distribution (current rise)

35
What causes the Hazards in Solidly Grounded
Systems?

• Very low impedance in ground path
• High fault current
• High fault energy
• Ground Fault Coordination
• Long time delays on upstream devices
• High fault energy

36
Solidly Grounded Systems

• Grounded systems have an intentional connection
  from the source generator or transformer to
  ground
• Typically a four wire delta system
• Can be a three wire system where the source
  neutral is not connected to loads

37
Solidly Grounded Systems

• Intentionally grounded through ground wire

0O
277 O
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Bolted Ground Faults

• Ground fault current distribution on AF

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Arcing Ground Faults

• Ground fault current distribution on AF

40
Arcing Ground Faults

• Arcing ground fault Lower fault current, so
  OCPDs may not clear fault. Delay will cause
  severe equipment and personnel damage due to
  tremendous amount of energy released.

No transient over-voltages
High fault current
Arcing ground faults are approximately 38 bolted
faults.
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Locating Ground Faults

• Follow the Smoke!
• Direct return to source provides over-current
  conditions that allow for OCPD to operate, hence,
  clearing the fault.
• OK, IF the following condition is met (and you
  like repair work)
• Acceptable Damage
• People???
• Equipment???
• Costs???
• Who decides???
• Not OK, IF
• You do not want to accept damaging people
• You pay for equipment repairs

42
Coordination Problems

• Discussed Over-Voltage and Over-Current Hazards
  ...
• Now discuss time factor
• Energy is also a function of time
• E volts amps time
• Large radial systems have long time delays for
  coordination

43
Coordination Problems
IG Fault Current (A) Va 100V (typical) t
time (cycles)

• Typical Transformer
• 2500 kVA, 5 impedance
• Ground condition Ig23kA
• KWC 55,200
• Acceptable???

44
Coordination Problems
A) 100 Kilowatt Cycles Fault location
identifiable at close inspection - spit marks on
metal and some smoke marks. B) 2000 Kilowatt
Cycles Equipment can usually be restored by
painting smoke marks and repairing punctures in
insulation. C) 6000 Kilowatt Cycles Minimal
amount of damage, but fault more easily
located. D) 10,000 Kilowatt Cycles Fault
probably contained by the metal enclosure.
E) 20,000 Kilowatt Cycles Fault probably burns
through single thickness enclosure and spreads to
other sections. F) Over 20,000 Kilowatt
Cycles Considerable destruction.
45
Hazards w/ Ungrounded S-G
46
High Resistance Grounding

• How does HRG solve these hazards?
• Inserts a resistor between neutral and ground
• Eliminates 90 of Arc Flash / Blast Injuries

47
High Resistance Grounding

• What if no neutral exists (i.e. delta systems)?
• A grounding transformer is installed (either a
  zig-zag or a wye-delta) from all three phases to
  create an artificial neutral for grounding
  purposes only.

48
High Resistance Grounding

• Intentionally grounded through neutral resistor

277V
Vng0V
Ground 0V
49
High Resistance Grounding

• Compared to Ungrounded Systems (voltage rise)

480V
VngVan (277V)
Ground AØ
Additional return path, only difference between
Ungrounded and HRG!
50
High Resistance Grounding

• Voltages
• Normal Operation
• Vag 277V
• Vbg 277V
• Vcg 277V
• Vng 0V
• Fault conditions
• Vag 0V (Faulted phase is at ground potential)
• Vbg 480V
• Vcg 480V
• Vng 277V

51
High Resistance Grounding

• Importance of additional path versus Solidly
  Grounded

Resistor (HRG) in lieu of wire adds significant
amount of resistance to lower ground fault to a
predetermined value preventing destructive fault
currents and shut-down!
52
High Resistance Grounding

• Compared with Solidly Grounded (current rise)

55.4O 5.00A
277O 1.73A
5.83A
Ground AØ
Resistor in return path, only difference between
Solidly Grounded and HRG!
5.83A ? 3.00A 5.00A
53
High Resistance Grounding

• Currents
• Normal Operation
• Fault conditions

54
High Resistance Grounding

• Another advantage of return path ground fault
  location

Contactor shorts out part of the resistor
changing the resistance, hence, changing the
current. Ground fault current now is a pulse
signal that allows for detection!
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High Resistance Grounding
Meter reading will alternate from 5A to 10A every
2 seconds.

• Method to quickly locate ground faults.

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Ground Faults

• Damage to Power System Components
• Thermal Damage (Irms)2 t
• Mechanical Damage (Ip)2
• Comparison between S-G example and HRG

System Grounding HRG S-G
Ground Fault (A) 5 22,800
Damage to Equipment (1 sec) 1 per unit
(22,800 / 5)2 20.8x106 p.u.
Solidly-Grounded Systems have 20.8 million times
more damage than HRG!!!
57
Do others agree?

• TO HRG OR NOT TO HRG?
• IEEE Std 142-1991 (Green Book)Recommended
  Practice for Grounding of Industrial and
  Commercial Power Systems
• 1.4.2 Numerous advantages are attributed to
  grounded systems, including greater safety,
  freedom from excessive system over-voltages
  that can occur on ungrounded systems during
  arcing, resonant or near- resonant ground
  faults, and easier detection and location of
  ground faults when they do occur.
• 1.4.3 A system properly grounded by resistance
  is not subject to destructive transient
  over-voltages.

58
Do others agree?

• TO HRG OR NOT TO HRG?
• IEEE Std 142-1991 (Green Book)Recommended
  Practice for Grounding of Industrial and
  Commercial Power Systems
• 1.4.3 The reasons for limiting the current by
  resistance grounding may be one or more of
  the following.
• 1) To reduce burning and melting effects in
  faulted electric equipment, such as
  switchgear, transformers, cables, and rotating
  machines.
• 2) To reduce mechanical stresses in circuits
  and appartus carrying fault currents.
• 3) To reduce electric-shock hazards to
  personnel caused by stray ground-fault
  currents in the ground return path.

59
Do others agree?

• TO HRG OR NOT TO HRG?
• IEEE Std 142-1991 (Green Book)Recommended
  Practice for Grounding of Industrial and
  Commercial Power Systems
• 1.4.3 The reasons for limiting the current by
  resistance grounding may be one or more of
  the following.
• 4) To reduce the arc blast or flash hazard to
  personnel who may have accidentally caused or
  who happen to be in close proximity to the
  ground fault.
• 5) To reduce the momentary line-voltage dip
  occasioned by the clearing of a ground fault.
• 6) To secure control of transient
  over-voltages while at the same time avoiding
  the shutdown of a faulty circuit on the
  occurrence of the first ground fault (high
  resistance grounding).

60
Do others agree?

• TO HRG OR NOT TO HRG?
• IEEE Std 141-1993 (Red Book)Recommended Practice
  for Electric Power Distribution for Industrial
  Plants
• 7.2.2 There is no arc flash hazard, as there is
  with solidly grounded systems, since the fault
  current is limited to approximately 5A.
• Another benefit of high-resistance grounded
  systems is the limitation of ground fault
  current to prevent damage to equipment. High
  values of ground faults on solidly grounded
  systems can destroy the magnetic core of rotating
  machinery.

61
Do others agree?

• TO HRG OR NOT TO HRG?
• IEEE Std 242-2001 (Buff Book)Recommended
  Practice for Electric Power Distribution for
  Industrial Plants
• 8.2.5 Once the system is high-resistance
  grounded, over- voltages are reduced and
  modern, highly sensitive ground-fault
  protective equipment can identify the faulted
  feeder on the first fault and open one or both
  feeders on the second fault before arcing
  burndown does serious damage.

62
Design Considerations when applying HRG Systems

• HRG is the best Grounding Method available today
• First developed with resistor and pulsing
  contactor (Analog)
• Least Hazards of all grounding methods, but some
  still exist
• Elevated Voltages
• Trained Personnel
• Cables, TVSSs, VFDs Insulation
• Line-to-Neutral Loads
• Phase-to-ground-to-phase Faults
• Bypasses neutral grounding resistor
• Single-poling circuit breakers
• HRG Systems Resolve these Hazards

63
Design Considerations when applying HRG Systems

• NFPA 70 National Electrical Code (2005)
• 250.36/186 High-impedance grounded neutral
  systems in which a grounding impedance, usually
  a resistor, limits the ground-fault current to a
  low value shall be permitted for 3-phase ac
  systems of 480 volts to 1000 volts where all the
  following conditions are met
• 1) The conditions of maintenance and
  supervision ensure that only qualified persons
  service the installation.
• 2) Ground detectors are installed on the
  system.
• 3) Line-to-neutral loads are not served
• Continuity of power is required.(Removed 2008).

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Elevated Voltage Hazard

• Properly rated equipment prevents Hazards.

0V
480V
277V
480V
0V
Ground AØ
Maintenance must be aware of elevated voltages
and method to locate fault. IF NOT, DO NOT HAVE
TO MAINTAIN POWER. Allowed to trip (same as S-G)
but without the hazards.
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Elevated Voltage Hazard

• Properly rated equipment prevents Hazards.

0V
480V
277V
480V
0V
Ground AØ
Cables, TVSSs, VFDs, etc. and other equipment
must be rated for elevated voltages (Ungrounded
Systems).
66
Resolve Cable Insulation Issue

• 600V Cables
• Insulation thickness based on mechanical
  strength, not electrical
• Extra thickness exceeds 600V electrical rating
• Therefore, should be used on 600V systems (HRG)
• 1000V Cables
• Only CSA listed, not UL
• 5000V Cables
• Non-shielded Should be used on 2400V systems
  (HRG)
• Shielded Should be used on 4160V systems (HRG)
• 8000V Cables
• Non-shielded Should be used on 4160V systems
  (HRG)

67
Resolve NEC requirement
Add small 11 transformer and solidly ground
secondary for 1F loads (i.e. lighting).
68
Resolve NEC Requirement

• Advantages of 11 transformer
• Ability to retrofit HRG Systems
• Only 20 of facility / plant load is 1F
• No neutral required from main source and main
  switchgear (cost savings,)
• Significantly reduced risk of Arc Blast / Flash
  Hazard
• Only small portion of power system is solidly
  grounded
• Lighting Ballasts

69
Phase-to-Ground-to-Phase Fault

• Single-poling circuit breaker

2000A/3P/65kAIC
During phase-ground-phase fault, single-pole of
MCB has to clear the 480V fault at 65kA.
However, per UL 489, single-pole interrupting
rating is only at 20kAIC. HAZARDOUS?
70
Phase-to-Ground-to-Phase Fault

• For condition to occur, all of the following must
  be true
• 1) One fault must be on line side of MCB
• Very uncommon
• 2) Low impedance per ground fault
• Very uncommon
• Ground faults are usually arcing faults (high
  impedance faults per IEEE Std 241, 9.2.5)
• 3) Faults on different phases
• 4) No other over-current protective devices in
  fault path
• Very uncommon
• If so, they will open eliminating the
  single-pole interruption
• Although remote, HAZARD may still exists
• Should be considered during coordination study
• Detect ground faults per NEC 250-36

71
Resolving Hazard via New Technology

• First fault
• Sound Alarm
• Send signal
• Second fault
• Open feeder with lower priority
• Third fault
• Open feeder with lower priority

72
Poll Question 2

• Will you consider using HRG technology as part of
  an arc flash reduction strategy?

73
Additional Advancementsin HRG Systems

• Communications
• RS232 (Serial) / RS485 (Modbus, Profibus) /
  TCP/IP (Ethernet)
• Control and monitor relay remotely via existing
  SCADA system
• Data Logging Trending
• Most ground faults are intermittent, so when you
  go to locate via pulse, fault may have cleared
• Data log can link ground faults with equipment
  starting or running

74
Additional Advancementsin HRG Systems

• Filters Harmonics / Noise / RF
• Monitors fundamental voltage and current for
  ground faults
• Avoids nuisance tripping
• Monitors 3rd harmonic voltage and current
• High Harmonics may require de-rating resistor
• Low Harmonics may indicate ground fault near
  generator neutral
• Zone Selective Interlocking
• Allows coordination between interlocked
  protection relays on the same system
• Continuously measures system impedance
• Electrical systems are perpetual systems
• System capacitance may increase causing grounding
  resistor to be incorrectly sized
• Undesirable, higher fault current may flow
• Transient over-voltage may occur

75
Summary

• Hazards with Ungrounded Systems
• Severe transient over-voltages
• Cannot efficiently locate ground faults
• Hazards with Solidly-Grounded Systems
• Very high fault currents and time delays
• Causing severe arc blast / flash conditions
• Ground fault coordination problems

76
Summary

• High-Resistance Grounded Systems
• Best Grounding Method today
• Resolves Ungrounded hazards
• Resolves Solidly-Grounded hazards
• Technology continues to make HRG Systems safer
  than any other grounding method, but need help
• Continue to educate and train personnel (engr and
  maint.)
• NETA
• Update standards and guideline that hinder HRG
• NEC
• NFPA 70E and IEEE 1584