Download - Effective System Grounding
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Effective System Grounding
Presenter:
John DeDad
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Todays Agenda
Why the Concern over Ground Faults?
Electrical Grounding Options
Upgrading from Ungrounded to HighResistance Grounded
Application of Resistance Grounding
Advances in High Resistance Grounding
The Concerns with Solidly GroundedSystems
Controlling Time and Current to MinimizeHazard
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Why the Concern overGround Faults?
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0
5
10
15
20
25
30
35
40
0-99 100-199 200-299 300-399 400-499 500-599 1m - 1.9m over 2m
Industrial Commercial
Commercial locations such ashotels, universities and shoppingmalls = 72 incidentsManufacturing locations = 156
incidentsTotal occurrences = 228
Industrial Losses:
$ 120,000,000 total$ 769,230 average$ 120,000,000 totalCommercial losses:
$ 60,000,000 total$ 833.300 average
Total losses (industial andcommercial) $ 180,000,000
Why the Concern overGround Faults?
All figures correspond to losses associated withelectrical ground faults that occurredover a seven year period and were reported byone leading US based Insurance Company .
Losses related to ground faults:
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Business Interruption Costs Cost / Hour 12 Hours
Automotive $15,000 $ 180,000Food and Beverage $ 16,420 $ 197,040Plastic and Moulding $ 7,600 $ 91,200Machinery and Equipment $ 24,700 $ 296,400Metals / Mining $ 24,300 $ 291,600Ticket Reservations $ 72,000 $ 864,000
Property and Equipment Damage
Property Damage from $10,000 to $ 2,000,000
NFPA Equipment Average $ 46,720
Medical Costs and Miscellaneous
Employee Lost Time $ 85,200OSHA Fines $ 20,000 - $100,000
Range of Losses
Minor incident $ 50,000Major incident $ 3,000,000
Why the Concern overGround Faults?
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Least Effective Most Effective
Protection Prevention
Personal Protection
Wearing of PPE
Administration
Electrical Safety Training
Awareness
Hazard Category Labels
Engineering Controls
Arc Resistant Switchgear
Substitution
Reduction of Time or Fault
Current Available
Elimination
High Resistance Grounding
WebinarFocus
Why the Concern overGround Faults?
Risk Control of Ground Faults and Arc Faults
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Todays Agenda
Why the Concern over Ground Faults?
Electrical Grounding Options
Upgrading from Ungrounded to HighResistance Grounded
Application of Resistance Grounding
Advances in High Resistance Grounding
The Concerns with Solidly GroundedSystems
Controlling Time and Current to MinimizeHazard
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Ungrounded
Solidly Grounded
Resistance Grounded
Electrical Grounding Options
Industrial Power System Grounding Three Methods
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Popular for LV systems to 1950s
No intentional connection to
ground
Less than 2A ground fault current
=>no shutdown
Feeders must be de-energized to
locate ground fault Susceptible to voltage buildup 6-9
times above ground on
intermittent arcing faults
Electrical Grounding Options
Ungrounded Systems
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No ground fault:
A
B
C
IC0IC0 IC0
GND
277 V line-to-ground
277 V line-to-ground
277 V line-to-ground
Electrical Grounding Options
System Charging Current Definition
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Neutral rises above ground to rated L-N voltage (277V)
Unfaulted phases rise above ground to rated L-L voltage (480V)
N
60
B
C A
N
120
B
C A
G
No Ground Fault
Full Ground Fault on Phase B
VA-G= 277 V
VB-G= 277 V
VC-G= 277 V
VN-G= 0 V
VA-G= 480 V
VB-G
= 0 V
VC-G= 480 V
VN-G= 277 V
N
B
C A
Partial (50%) Ground Fault on Phase B
VA-G= 367 V
VB-G= 138 V
VC-G= 367 V
VN-G= 138 V
G
82
Electrical Grounding Options
Voltage Rise on Ground Fault
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With bolted ground fault:
Current of 3IC0 flows into fault from un-faulted phases
3IC0 defined as the System Charging Current
A
B
C
480 V line-to-line480 V line-to-ground
0 V line-to-ground
480 V line-to-ground
IF= 3IC0IC03 IC03
Electrical Grounding Options
System Charging Current Definition
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N
Vb
Vc Va
Vbc
Vca
Vab = VagVcb = Vcg
Ib = IaIc= 3Ico
Ia=3Ico60
30
30
Ic=3Ico
60
System Charging Current
Electrical Grounding Options
System Charging Current
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Current chopping from intermittent arcing ground fault can build up
voltage of system capacitance 6-9 times (2000V)
Ref. Donald Beeman, Industrial Power Systems Handbook,
McGraw-Hill, 1955, pp. 337-338 and 286-289.
A
B
C
IC0IC0 IC0
GND
Transient Voltage Escalation on Ungrounded Systems
Electrical Grounding Options
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Transient Voltage Escalation on Ungrounded Systems
Electrical Grounding Options
Photos courtesy of Job Garcia IEEE
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Least Effective Most Effective
Protection Prevention
Personal Protection
Wearing of PPE
Administration
Electrical Safety Training
Awareness
Hazard Category Labels
Engineering Controls
Arc Resistant Switchgear
Substitution
Reduction of Time or FaultCurrent Available
Elimination
High Resistance Grounding
Risk Control of Ground Faults and Arc Faults
Electrical Grounding Options
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System grounding
the connection of earth ground to theneutral points of current carryingconductors such as the neutral pointof a circuit, a transformer, rotatingmachinery, or a system, either solidlyor with a current limiting device.
Equipment grounding
the connection of earth ground to non
current carrying conductive materialssuch as conduit, cable trays, junctionboxes, enclosures and motor frames.
Grounding
What is System Grounding?
Electrical Grounding Options
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IEEE Std 142-1991 (Green Book)
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 asswitchgear, transformers, cables and rotating machines
2. to reduce mechanical stresses in circuits and apparatus carrying fault currents
3. to reduce electric-shock hazards to personnel caused by stray ground fault
currents in the ground return path
4. to reduce arc blast or flash hazard to personnel who may have accidentallycaused or who happen to be in close proximity to the fault current
5. to reduce the momentary line-voltage dip occasioned by the occurrence and
clearing of a ground fault
Why Consider Grounding your System?
Electrical Grounding Options
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Todays Agenda
Why the Concern over Ground Faults?
Electrical Grounding Options
Upgrading from Ungrounded to HighResistance Grounded
Application of Resistance Grounding
Advances in High Resistance Grounding
The Concerns with Solidly GroundedSystems
Controlling Time and Current to MinimizeHazard
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IEEE Std 242-2001 (Buff Book)
8.2.4. High-resistance grounding helps ensure a ground-fault of known magnitude, helpful for
relaying purposes. This makes it possible to identify the faulted feeder with sensitive ground-fault
relays.
IEEE Std 141-1993 (Red Book)
7.2.2. High-resistance grounding provides the same advantages as ungrounded systems yet
limits the steady state and severe transient over-voltages associated with ungrounded systems.
There is no arc flash hazard [for LV ground faults], as there is with a solidly grounded system,
since the fault current is limited to approximately 5A.
IEE Std 242-1986 Recommended Practice for the Protection and Coordination of
Industrial and Commercial Power Systems 7.2.5 Ungrounded systems offer no advantage over high-resistance grounded systems in terms
of continuity of service and have the disadvantages of transient overvoltages, locating the firstfault and burndowns from a second ground fault. For these reasons, they are being used lessfrequently today than high-resistance grounded systems
High Resistance Grounding IEEE Color Books
Upgrading from Ungrounded toHigh Resistance Grounded
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High Resistance Grounding
Upgrading from Ungrounded toHigh Resistance Grounded
Resistor limits the ground fault to 10 amps or less
Arc flash hazard on ground faults reduced
Faulted feeder remains in service Ground fault pulse locating provides valuable
troubleshooting tool
To prevent voltage escalation on the phase-to-ground
capacitance during intermittent arcing ground faults, theresistor current must exceed system charging current, IR 3IC0
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Upgrading from Ungrounded toHigh Resistance Grounded
Step 1 Requirements for Sizing the NeutralGrounding Resistor
Step 2 Determining the System ChargingCurrent
Step 3 Locate and Install the Neutral GroundingResistor
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What are the Requirements for Sizing the Resistor?
Upgrading from Ungrounded toHigh Resistance Grounded
The line-to-ground capacitance associated with system componentsdetermines the magnitude of zero-sequence charging current.
The resistor must be sized to ensure that the ground fault current
limit is greater than the system's total capacitance-to-groundcharging current. If not, then transient over-voltages can occur.
The charging current of a system can be calculated by summing thezero-sequence capacitance or determining capacitive reactance of
all the cable and equipment connected to the system.IEEE-32
The resistor must be built with a low coefficient of resistance/Temperature toensure that it carries the rated current during a ground fault condition
Clause 2.2
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Determining System Charging Current
Upgrading from Ungrounded toHigh Resistance Grounded
1. 1. Calculation See Application Guide High Res. Grounding
2. 2. Experience < 2 A 480 V and 600 V 2 7 A 2.4 kV and 4.16 kV < 20 A 13.8 kV
3. 3. Rule of Thumb (Conservative) 1 A / 2000 kVA 480 V and 600 V 1 A / 1500 kVA 2400 V 1 A / 1000 kVA 4160 V and higher
4. 4. Measurement
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Measuring the System Capacitive Charging Current.
Upgrading from Ungrounded toHigh Resistance Grounded
It is preferable to measure the magnitude of the charging current on existingpower systems for correct grounding equipment selection. The measured valuesmust be adjusted, to obtain the maximum current, if not all system componentswere in operation during the tests.
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Locating the Resistor
Upgrading from Ungrounded toHigh Resistance Grounded
Once we have determined the size requirement for the resistor the next step typicallywould be to connect the current limiting resistor into the system. On a wye-connectedsystem the neutral grounding resistor is connected between the wye-point of thetransformer and ground as shown below.
A
B
C
Neutral
Grounding
Resistor(NGR) NGRGNGR
CG
C
NGR
G
NGR
RIW
II
XR
I
ER
2
0
0
33
3
Where IG= Maximum
Ground Current (A)
Ohms
Ohms
Amperes
Watts
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Locating the Resistor
Upgrading from Ungrounded toHigh Resistance Grounded
On a delta-connected system, an artificial neutral is required since no star point existsthis can be achieved by use of a zig-zag transformer as shown
A
BC
N
ZIG-ZAG
Transformer
RNGR
NGRGNGR
CG
C
NGR
G
NGR
G
RIW
II
XR
I
ER
EIVA
2
0
0
3
3
3
VA
Ohms
Ohms
Amperes
Watts
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Todays AgendaWhy the Concern over Ground Faults?
Electrical Grounding Options
Upgrading from Ungrounded to HighResistance Grounded
Application of Resistance Grounding
Advances in High Resistance Grounding
The Concerns with Solidly GroundedSystems
Controlling Time and Current to MinimizeHazard
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Where service continuity is vital and where anorderly shutdown is essential
Where arc flash hazard reduction desired
Where Line to Neutral loads can be separated frombalanced phase to phase loads with isolationtransformers.
Where Maintenance personnel proactively locateand repair ground faults
High Resistance Grounding When to Choose
Application of ResistanceGrounding
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System charging current less than resistor current rating.
Rule of thumb charging current 0.5A/1000kVA Choose 5A continuous-duty resistor, tapped 2.5/5A or
5A/10A for ground fault pulse locating
Alarm pick-up level typically 50% resistor let-thru current
Optional individual motor feeder alarm relays set at 10%
pre-alarm setting (monitor winding insulation)
High Resistance Grounding - Low Voltage
Application of ResistanceGrounding
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High Resistance Grounding - Medium Voltage
Application of ResistanceGrounding
Typically in 5kV and below, but available for systems up to
13.8 kV.
To avoid ground fault escalation into a phase-to-phase fault,system charging current should be 5.5A
(J.R. Dunki-Jacobs)
5A-10A resistor typical (IR 3IC0)
Alarm pick-up level typically 50% resistor let-through current
Optional individual motor feeder alarm relays set at 10% pre-
alarm setting (monitor winding insulation)
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NEMA Std MG 1 Motors and Generators
Application of ResistanceGrounding
32.13
A synchronous generator shall be capable of withstanding, without damage, a 30-
second, three-phase short circuit at its terminals
32.34 The neutral of a generator should not be solidly grounded unless the generator has
been specifically designed for such operation. With the neutral solidly grounded, the
maximum line-to-ground fault current may be excessive, and in parallel systems
excessive circulating harmonic currents may be present in the neutrals.
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Generator Grounding IEEE Color Books
Application of ResistanceGrounding
IEEE Std 242-2001 (Buff Book)
Page 452: Solid grounding of a generator neutral is not recommended because this
practice can result in high mechanical stresses and excessive fault damage to the
machines.
IEEE Std 142-1991 (Green Book)
1.8.1.
Generators have low zero sequence impedance compared to transformers.
Thus a generator will have higher ground fault current than 3-phase fault current if
solidly grounded.
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Low Resistance Grounding of Parallel Generators
Application of ResistanceGrounding
GENERATORS 4160V
100A10 sec
40A
51G
40A1 Sec
G
52
52
51N
52
51N
51G
20A0.6 sec
10A0.25 sec
87G87GD
5A1 min
G
52
51G
87G87GD
5A1 min
G
52
51G
87G87GD
5A1 min
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High Resistance Grounding of Parallel Generators
Application of ResistanceGrounding
Must not solidly parallel the neutrals of generators otherwise excessive
triplen harmonic circulating currents
Use zig-zag grounding transformerGENERATORS 600V
600V
VOLTAGE
SENSING
MULTI-FEEDER
ALARM RELAY
15-20A, 3P
100 kAIC
5A
G G G G
To DCS
To DCS
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Todays Agenda
Why the Concern over Ground Faults?
Electrical Grounding Options
Upgrading from Ungrounded to HighResistance Grounded
Application of Resistance Grounding
Advances in High ResistanceGrounding
The Concerns with Solidly GroundedSystems
Controlling Time and Current to MinimizeHazard
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Safety Concerns with Traditional HRG Systems
Advances in High ResistanceGrounding
Unable to locate the ground fault in a timely manner resulting inexcessive damage
Arc Flash hazard when opening the main switchboard to trace thefault
Second ground fault resulting in destructive phase-to-phase faults
Loss of the Neutral Path resulting in loss of protection
Closing a main-tie onto a ground fault
Intermittent Faults
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MODBUS
TRIP TRIP
ZSCTZSCT
DSP HRG
. . . Several Feeders . . .
MotorMotor
Phase Indication
Advances in High ResistanceGrounding
Unable to locate the ground fault in a timely mannerresulting in excessive damage.
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MODBUS
TRIP TRIP
ZSCTZSCT
DSP HRG
. . . Several Feeders . . .
MotorMotor
Feeder Identification
Advances in High ResistanceGrounding
Unable to locate the ground fault in a timely mannerresulting in excessive damage.
Ad i Hi h R i t
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Options for Faulted Feeder:
1) Alarm Only (No Trip)
OR
2) Trip with Time Delay
MODBUS
TRIP TRIP
ZSCTZSCT
DSP HRG
. . . Several Feeders . . .
MotorMotor
Advances in High ResistanceGrounding
Second ground fault resulting in destructivephase-to-phase faults
Ad i Hi h R i t
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Advances in High ResistanceGrounding
Second ground fault resulting in destructivephase-to-phase faults
Photos courtesy of Schneider Electric Chile
Advances in High Resistance
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MODBUS
TRIP TRIP
ZSCTZSCT
DSP HRG
. . . Several Feeders . . .
MotorMotor
2nd Ground Fault:
Prioritize Feeders
Trips least important,
maintaining operation onmost important
Up to 50 Feeders
Advances in High ResistanceGrounding
Second ground fault resulting in destructive
phase-to-phase faults
Advances in High Resistance
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System Ground Monitor:
Continually monitors
circuit from Neutral to
Ground
Alarms if OPEN circuit
Alarms if SHORT circuit
MODBUS
TRIP TRIP
ZSCTZSCT
DSP HRG
. . . Several Feeders . . .
MotorMotor
Advances in High ResistanceGrounding
Loss of the Neutral Path resulting in loss of protection
Advances in High Resistance
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MODBUS
TRIP TRIP
ZSCTZSCT
DSP HRG
. . . Several Feeders . . .
MotorMotor
Remote Monitoring:
Tie into Internet
Monitor plant anywhere
in world
Notify maintenance or
local qualified electrical
contractor to locate
ground fault
Advances in High ResistanceGrounding
Intermittent Faults
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Todays AgendaWhy the Concern over Ground Faults?
Electrical Grounding Options
Upgrading from Ungrounded to HighResistance Grounded
Application of Resistance Grounding
Advances in High Resistance Grounding
The Concerns with Solidly GroundedSystems
Controlling Time and Current to MinimizeHazard
The Concerns with Solidly
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Low impedance bonding system
Bolted Fault --low impedanceat point of fault
277V
High fault current quickly trips breaker
Solidly Grounded Systems
The Concerns with SolidlyGrounded Systems
The Concerns with Solidly
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Popular for 3-wire LV systemssince 1950s
Solved overvoltage problem
System intentionally grounded(usually neutral)
Faults easy to locate
Permits line-to-neutral lightingloads
Solidly Grounded Systems
The Concerns with SolidlyGrounded Systems
The Concerns with Solidly
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Fault path has two parts:
1. Impedance of the fault, betweenthe live conductor and bonding
system (unpredictable)
2. Impedance of the bondingsystem (low)
BOLTED FAULTS (low impedance)quickly isolate faulted circuit
ARCING FAULTS (high impedance)do not quickly trip the breaker
Solidly Grounded Systems
The Concerns with SolidlyGrounded Systems
The Concerns with Solidly
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Potential for severedamage at point of faultdue to intense heat energyof the arc Arcing groundfaults are more common
than bolted faults
Solidly Grounded Systems
The Concerns with SolidlyGrounded Systems
Switchboard of a solidly grounded system at an
amusement park in Ontario Canada
The Concerns with Solidly
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Low level arcing groundfaults not detected byphase relays or fuses,until fault escalates
Sustained arcing faultscan release intense heat
and mechanical energy,causing severe damageand injury
Solidly Grounded Systems
The Concerns with SolidlyGrounded Systems
Switchboard of a solidly grounded system at an
amusement park in Ontario Canada
The Concerns with Solidly
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2000
10,000 KWC Acceptable
IG = Amperes
Va = 100V
t cycles
Arcing Fault Damage
The Concerns with SolidlyGrounded Systems
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.
The Concerns with Solidly
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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 andspreads to other sections.
F) Over 20,000 Kilowatt Cycles
Considerable destruction.
Arcing Fault Damage
The Concerns with SolidlyGrounded Systems
The Concerns with Solidly
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IEEE Std 242-2001 (Buff Book)
8.2.2. One disadvantage of the solidly
grounded system involves the high magnitude
of destructive, arcing ground-fault currents
that can occur.
IEEE Std 141-1993 (Red Book)
7.2.4. The solidly grounded system has the
high probability of escalating into a phase-to-
phase or three-phase arcing fault, particularlyfor the 480V and 600V systems. The danger
of sustained arcing for phase-to-ground
faultis also high for the 480V and 600V
systems, and low or near zero for the 208V
system.
IEEE Color BooksArcing Faults
The Concerns with SolidlyGrounded Systems
The Concerns with Solidly
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Least Effective Most Effective
Protection Prevention
Personal Protection
Wearing of PPE
Administration
Electrical Safety Training
Awareness
Hazard Category Labels
Engineering Controls
Arc Resistant Switchgear
Substitution
Reduction of Time or FaultCurrent Available
Elimination
High Resistance Grounding
Risk Control of Ground Faults and Arc Faults
The Concerns with SolidlyGrounded Systems
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Limit the fault current Limit the time
NGRs limit the fault magnitude.Ground fault relays trip breakersand limit how long a fault lasts
Mitigating Factors
Controlling Time and Current toMinimize Hazard
Controlling Time and Current to
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Low Resistance Grounding: 2.4kV 25kV
Low Resistance Grounding
Controlling Time and Current toMinimize Hazard
Controlling Time and Current to
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2.4kV to 25kV
When system charging current is high and requires high
current rated resistor not suitable for continuous operaion. Limits ground fault current to safe level
Faulted feeder trips in 0.25 1 sec
Resistor sized 50A to 200A
Provides sufficient ground fault current to selectively tripground fault relays
Arc flash hazard on ground faults reduced
Low Resistance Grounding on MV
Controlling Time and Current toMinimize Hazard
Controlling Time and Current to
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IEEE Std 142-1991 (Green Book)
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 apparatus carrying fault
currents
3. to reduce electric-shock hazards to personnel caused by stray groundfault currents in the ground return path
4. to reduce arc blast or flash hazard to personnel who may haveaccidentally caused or who happen to be in close proximity to the fault
current
5. to reduce the momentary line-voltage dip occasioned by the occurrence
and clearing of a ground fault
Low Resistance Grounding IEEE Color Books
Controlling Time and Current toMinimize Hazard
Controlling Time and C rrent to
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For ground fault coordination:
Trip settings range from 10% - 40% of resistor let-thru
current Time delay settings range from 0.2 1 sec with 0.3
sec coordination interval
Minimum trip setting (10%) must exceed the systemcharging current to avoid sympathetic tripping of un-faulted feeders
Low Resistance Grounding: Relay Trip Settings
Controlling Time and Current toMinimize Hazard
Controlling Time and Current to
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Designed and tested to IEEE Standard 32
CSA approved for Canadian applications
UL listed for US applications Installed in Canada to CEC rules 10-1100 thru 10-1108
Installed in the US to NEC articles 250.36, 250.186, and 450.5(B)
Resistors rated for :
Line-to-neutral voltage, let-thru current, allowable on time Grounding transformers rated for :
Line-to-line voltage, let-thru current, allowable on time
Codes and Standards for Neutral Grounding Devices
Controlling Time and Current toMinimize Hazard
Todays Agenda
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Today s Agenda
Why the Concern over Ground Faults?
Electrical Grounding Options
Upgrading from Ungrounded to HighResistance Grounded
Application of Resistance Grounding
Advances in High Resistance Grounding
The Concerns with Solidly GroundedSystems
Controlling Time and Current to MinimizeHazard
Controlling Time and Current to
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Least Effective Most Effective
Protection Prevention
Personal Protection
Wearing of PPE
Administration
Electrical Safety Training
Awareness
Hazard Category Labels
Engineering Controls
Arc Resistant Switchgear
Substitution
Reduction of Time or Fault
Current Available
Elimination
High Resistance Grounding
Risk Control of Ground Faults and Arc Faults
Controlling Time and Current toMinimize Hazard
The Concerns with Solidly
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Total Clearing Time is Critical
Reduce the Time, Reduce the Damage, Reduce the Incident Energy
-35 ms: no significant damage to persons or 1.27 Cal /cm2
Switchgear, which can often be returnedto use after checking the insulation resistances
- 100ms: small damage, requires cleaning and possibly 3.23 Cal/cm2
some minor repair likely
- 500ms: large damage both for persons and the 18.1 Cal/cm2
switchgear, which must be partly replaced.
The arc burning time is the sum of the time to detect the arc and the time to open the correct breaker.
*Based on 50kA maximum bolted fault current on a 480 volt solidly grounded system.
The Concerns with SolidlyGrounded Systems
Arc Detection and Mitigation
The Concerns with Solidly
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0 100 200 300 400 500 600 700
o
1
2
3
4
5
6
7
8
9
Current kA
An arc is developed within milli-
seconds and leads to the
discharge of enormous amounts
of destructive energy. The
energy in the arc is directly
proportional to the square of theshort-circuit current and the
time the arc takes to develop.
Reduce the Time,
Reduce the Damage,
Reduce the Incident Energy.
The Concerns with SolidlyGrounded Systems
Arc damage curve showing arc current versus arc time
The Concerns with Solidly
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Coordination for
ground faults
difficult unless
branch breakers
have ground fault
relays
NEUTRAL
PHASES
ZERO SEQUENCE CT
OVERLOAD GROUND FAULT
RELAY1200 A PICKUP
1 SECOND DELAY
FEEDER TRIPS ON
OVERLOAD
MAIN BREAKER TRIPS ON
GROUND FAULT!
175 A 400 A
The Concerns with SolidlyGrounded Systems
Coordination on Solidly Grounded Systems
Controlling Time and Current to
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System Charging Current 5 A
100 A10 sec
40 A
1 sec
40 A1 sec
20 A0.6 sec
10 A
0.25 sec
4.16 kV
51G
51N
52
52
52
51N
51N
80 A
1 sec
System Charging Current 10 A
200 A
10 sec
80 A
1 sec
40 A
0.6 sec
20 A
0.25 sec
13.8 kV
51G
51N
52
52
52
51N
51N
Examples of Coordinated Relay Settings
Controlling Time and Current toMinimize Hazard
Controlling Time and Current to
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ZSI offers an excellent solution to this problem. Itimproves arc flash safety upstream in the plantdistribution system without affecting servicecontinuity. ZSI is applied both to phase overcurrentdevices (on the short-timeprotection function), and to ground fault protectivedevices. It is available on electronic trip units and
relays of circuit breakers.
With ZSI, a breaker that senses a fault will trip withno intentional time delay unless it receives a restraintsignal from the breaker immediately downstream. Ifso restrained, the breaker will wait to time out beforetripping. The downstream breaker only sends a
restraint signal upstream if it also senses the fault,i.e. only for faults located downstream of bothbreakers.For the fault at point Y, the Sub-Feeder breaker willrestrain the Feeder breaker; and the Feeder breakerwill restrain the Main breaker. Hence the Main andFeeder will wait to time out. In the meantime, theSub-Feeder breaker will clear the fault.
Controlling Time and Current toMinimize Hazard
Zone Selective Interlocking (ZSI),
Controlling Time and Current to
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The final option for solidly groundedsystems is to employ arc detectiontechnology.
An arc is accompanied by radiation in theform of light, sound, and heat. Therefore,
the presence of an arc can be detectedbyanalyzing visible light, sound waves, andtemperature change.
To avoid erroneous trips, it is normal touse a short-circuit current detector along
with one of the aforementioned arcindicators.
The most common pairing inNorth America is current and light.
gMinimize Hazard
Arc Detection Technology
Controlling Time and Current to
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Arcing is accompanied by radiation in the form of light, sound, heatand electromagnetic waves as well as an associated pressurewave.
Controlling Time and Current toMinimize Hazard
Arc Detection and Mitigation
Controlling Time and Current to
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Two Direct Detection Methods
Pressure Arc Detector
Light Arc Detector
Detecting the pressure wavegenerated by the arc
Detection time 8ms
Detecting the arc flash throughoptical arc detection
Detection time 1ms
gMinimize Hazard
Arc Detection and Mitigation
Controlling Time and Current to
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gMinimize Hazard
Arc Detection and Mitigation Current and Light Schematic
Controlling Time and Current to
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Optical Sensor Schematic
Minimize Hazard
Controlling Time and Current to
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Ground Fault Protection, Zone Interlocking Protection (ZSIP) Remote Monitoring and Arc FlashMitigation all in one relay
Ground Fault, ZSIP and Arc Detection Example
Minimize Hazard
Controlling Time and Current to
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The combined use of high resistance grounding for protection from ground faults and its ability to prohibitthe escalation of the fault, the use ZSI to eliminate the delays associated with time and currentcoordination, and arc mitigation technology including pressure sensors and optical arc detection for phase-to-phase and three-phase arcing faults is an effective engineering approach to minimizing the impact ofground faults and the arc-flash hazard and to establish an effective and safe electrical grounding system.
80%
20%
Arc Flash Mitigation
Prevent - HRG Technology
Protect - ZSIP and OpticalDetection
gMinimize Hazard
Hazard Prevention and Protection
Controlling Time and Current to
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Arc Detection and MitigationProtection Type Clearance Time Incident Energy
MCGG Over-Current 3.1 seconds 37 Cal / cm2
MCGG Instantaneous 0.45 seconds 5.4 Cal / cm2
Pressure sensor 0.058 seconds 1.3 Cal / cm2
Optical Arc Detection 0.051 seconds 1.2 Cal / cm2
Assumes circuit breaker interrupting time of 0.05 seconds
gMinimize Hazard
Arc Detection and Mitigation
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Effective System Grounding
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What type ofgrounding system doyou employ?
Ungrounded
ResistanceGrounded
SolidlyGrounded
Upgrade to HRGwith first fault
time delay,second fault tripand feederidentification
Upgradeto relaywith ZSIPand ArcDetection
Add OpticalArcDetectionRelay
High Low
Add NGRMonitoring
Relay
HighResistanceGrounded
Eliminate Risk
Eliminate Risk
Substitute Risk
Eliminate Risk
HighResistanceGrounded
Effective System Grounding
Effective System Grounding
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