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© 2015 Eaton. All Rights Reserved..
1
Ground Fault Protection
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Ground Fault Protection
• All ground faults = short circuits
• But all short circuits are NOT = ground faults
• Bolted ground faults (cb or fuse)
• Low level ground faults = shock/burndown
• Need for additional protection
• Therefore ground fault protection
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Ground Fault Protection
• Ground Fault protection is essential for for
safety of personnel and equipment
• The degree of protection depends upon the
device selected.
• Ground Fault Protection Devices
• Ground Fault Circuit Interrupter (GFCI) – People
Protection
• Ground Fault Protection of Equipment (GFP) –
Equipment Protection
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GFCI Protection
• NEC Article 100 – GFCI Definition
• A device intended for the protection of personnel
that functions to de-energize a circuit or portion
thereof within an established period of time when a
current to ground exceeds the values established
for a Class A device.
• Note: Class A ground-fault circuit interrupters trip when the
current to ground has a value in the range of 4mA to 6mA.
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GFCI Protection
• Provides protection against electrical shock
and electrocution hazards.
• Incorporates a current sensing device that
monitors the current difference between the
ungrounded (hot) and neutral (grounded)
conductor.
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GFCI Protection
• GFCI Construction and Operation
• If the current difference between the hot and neutral
conductor exceeds 4-6 mA, the GFCI will trip.
ST
SS Circ Push-to-Test
20A GFCI
L
Sensor
N
120V
Load
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GFCI Protection
• Man completes circuit between the line conductor and
ground…current does not return through neutral back to
source…Sensor picks up leakage current and GFCI trips..
Man Saved.
ST
SS Circ Push-to-Test
20A GFCI
L
Sensor
N
R = 1000 OHMS
I = V/R
I = 120/1000 = 0.12 A
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GFCI Protection
• Man completes circuit between the line conductor and
neutral…current in equals current out…Sensor does not
pick up leakage current…GFCI does not trip….Man NOT
happy!!!
ST
SS Circ
20A GFCI
L
Sensor
N
R = 1000 OHMS
I = V/R = 120/1000 = 0.12 A
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GFCI Protection
• NEC 210.8(A) Dwelling Units
• Requires ground-fault circuit-interrupter
protection for personnel on all 125V,
single-phase, 15- and 20-ampere
receptacles installed in specified
locations
• Specified locations include bathrooms,
garages, outdoors, crawl spaces,
unfinished basements, kitchens, wet bar
sinks, boathouses, and laundry areas.
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20A CB
Bathroom
Receptacle
GFCI protection
must be provided
per NEC 210.8(A),
either by CB or
bathroom
receptacle
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GFP Protection
• NEC Article 100 – Ground-Fault Protection of
Equipment Definition
• A system intended to provide protection of
equipment from damaging line-to-ground fault
currents by operating to cause a disconnecting
means to open all ungrounded conductors of the
faulted circuit. This protection is provided at current
levels less than those required to protect
conductors from damage through the operation of a
supply circuit overcurrent device.
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GFP Protection – Why?
•
Rat causes
phase to ground
fault
•
I = V/R
277V
3,000A CB
A
I = 277/0.1
480V
C
I = 2,770A
•
B
480V
3000A CB will
not trip!
R = 0.1 OHMS
N
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Ground Fault Protection 230.95
• Requires ground-fault protection for each solidly
grounded wye service disconnect where:
• System voltage is more than 150V to ground, but not exceeding
600V phase to phase
• Rated 1000A or more
• Maximum ground-fault current setting – 1200A
• Maximum ground-fault time-delay setting – 1 second for
ground fault currents equal to or greater than 3,000A.
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Ground Fault Protection 230.95
• Ground Fault Protection is not required:
• For fire pumps (not permitted) or continuous process
industrials where a non-orderly shutdown would result in
additional hazards.
• 240V/120 single phase or 240V three phase systems.
• Services (or feeders) above 600V.
• Service with six disconnects or less where each disconnect is
less than 1000 amperes. A 4000 ampere service split into 5800 ampere switches would not require GFP.
• Delta, ungrounded, or resistance/impedance grounded
systems.
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Ground Fault Protection 215.10
• Ground Fault Protection is required on feeder
disconnect switches, in accordance with 230.95,
where:
• System voltage is more than 150V to ground, but not
exceeding 600V phase to phase
• Rated 1000A or more
• Exceptions:
• Fire pumps or continuous process industrials where a nonorderly shutdown would result in additional hazards.
• If ground-fault protection is provided on the supply side of
feeder.
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Ground Fault Protection 215.10
• Ground fault protection as required by 210.15
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Ground Fault Protection 215.10
• GFP not required on feeder equipment when it is
provided on the supply side of the feeder (except
as required for Health Care Facilities per Article
517)
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Ground Fault Protection 240.13
• Equipment ground fault protection, in accordance with
230.95, is required for each disconnect rated 1000A
or more, for 480/277V systems, that will serve as a
main disconnect for a separate building or structure.
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GFP Protection
• GFP Can
• Sense low magnitude ground faults
• Protect equipment against low magnitude ground fault
• GFP CANNOT
• Protect people or prevent ground faults and electric shock
• Protect against 3 phase, phase-phase or phase-neutral
faults
• Protect against high level ground faults
• Guarantee selective coordination – may adversely affect
selective coordination
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Ground Fault Protection
• Types of GFPs
• GFEP (Earth Leakage)
• Zero Sequence
• Residual (Neutral Ground Strap)
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Earth Leakage (GFEP)
• Earth Leakage (GFEP – Ground Fault Equipment
Protector) circuit breakers detect ground faults of
30 mA or more.
• Typically Rated 277V, 100A or less
ST
SS Circ Push-to-Test
20A GFEP
L
Sensor
N
277V
Load
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•
CT encloses
phase and
neutral
conductors
•
1 CT as shown
or 4 CTs (one
for each phase
and neutral)
•
CTs Sense
current
difference
•
Signals GF
Relay
•
ST Opens
Disconnect
•
Typically
available on CB
& Fused
Switches rated
400A or more.
Zero Sequence GFP
277V
ST
GF Relay
3000A BPS
A
480V
C
B
480V
N
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Zero Sequence GFP - Operation
277V
ST
480V
C
Current does
not return
through
neutral/CT
•
If current to
ground
exceeds
setting on GF
Relay, BPS
will shunt trip.
GF Relay
3000A BPS
A
•
B
480V
N
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Residual GFP
277V
ST
GF Relay
3000A BPS
A
480V
C
B
480V
N
Grounding
Strap
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Residual GFP - Operation
277V
ST
GF Relay
3000A S/F
A
480V
C
B
• Current does not
return through
neutral
• Residual CT
senses current
• If current through
grounding strap
exceeds setting
on GF Relay,
BPS will shunt
trip.
480V
N
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Calculating Ground Fault Energy
• kW-Cycles can be used to determine amount
of damage based upon ground fault energy.
• kW-Cycles = (I X E X t)/1000
• I = Ground fault current
• E = 100V of 277/480V system
• t = Arcing time in cycles
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GFP Protection: kW-Cycles
• Equipment Damage from ground-faults
can be measured by kW-Cycles
• 100 kW-Cycles – Spit marks on Equipment
• 2000 kW-Cycles – Limited damage.
Equipment can be put back in operation after
cleaning smoked and spotted areas and
repairing insulation if necessary
• 10,000 kW-Cycles – Serious damage.
Usually will not burn through. Equipment will
probably need parts replaced, such as bus
bars.
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GFP Protection: kW-Cycles
• 20,000 kW-Cycles – Serious damage.
Fault will probably burn through and
spread to other sections of
equipment. Equipment will need
extensive repairs and/or replacement.
• Over 20,000 kW-Cycles – Much
destruction of equipment…fire.
Equipment will need extensive repairs
and/or replacement.
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kW-Cycles vs. Fault Current
kW Cycles vs. Fault Current
100000
kW Cycles (at 100V)
• Protective
device
selected
can
determine
degree of
protection
as shown in
chart.
1600A
LVPCB 6 Cycle
Delay
1600A
BPS - 4
Cycle
10000
1600A
MCCB 3 Cycles
1000
1600A
Fuse
Curve
100
100
1000
10000
100000
Fault Current (Amperes)
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GFP Considerations – Current Limitation
This system offers:
1. Some degree of arcing and low magnitude
ground fault protection by the ground fault
relay operating the switch.
2. Current-limitation for high magnitude ground
faults and short-circuits by current-limiting
fuses, which provides component protection
for the switchgear.
This system offers:
1. Some degree of arcing and low magnitude
ground fault protection by the ground fault
relay operating the circuit breaker.
Note: This system does not provide currentlimitation for high magnitude ground faults
and short-circuits.
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GFP Considerations - Coordination
• A ground fault on a
feeder or branch
circuit can cause the
main device to open.
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GFP Considerations Coordination
• The system on the left
has a lack of selective
coordination – the shortcircuit on the branch
device causes the main
to open and results in a
black-out situation.
• The system on the right
has selective
coordination - only the
device nearest the short
circuit opens.
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GFP Relay & OCPD
Curves
• Effective time current
curve for line to ground
fault with 1600 ampere
fuse and ground fault
protection set at 1200
amperes (switch clearing
time of 4 cycles).
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GFP Relay & OCPD
Curves
• Effective time current
curve for line to ground
fault with 1600 ampere
circuit breaker and ground
fault sensor setting at 1200
amperes.
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GFP Considerations - Coordination
• Low Magnitude Ground
Fault
• Longer ground fault relay
delay permits larger feeder
fuse to coordinate with main
relay.
• High Magnitude Ground
Fault
• Fuses are selectively
coordinated.
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GFP Considerations - Coordination
• Low Magnitude Ground Fault
• A lack of coordination exists for
ground faults between 1200
amperes and 1800 amperes.
• High Magnitude Ground Fault
• Faults greater than 11,000A will
cause the 200A and 1200A
Power CB to open.
• Adding STD to the 1200 amp
Power CB would allow the two
CBs to selectively coordinate.
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Ground Fault Protection 517.17
• If the service or feeder is equipped with
ground fault protection in accordance with
230.95 or 215.10, the next level of feeders is
also required to have ground fault protection.
• Separation between the service and feeder
ground fault relays must achieve total selective
coordination.
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Ground Fault Protection 517.17
• Service Disconnect is
equipped with ground fault
protection per 230.95
• Feeders are equipped with
ground fault protection and
have manufacturer
recommended separation
between the two ground fault
curves.
• Complies with NEC 517.17
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Ground Fault Protection 517.17
• Main - 1200 A Fuse
• Feeder - 200 A Fuse
• GFP on Main & Feeder
• Meets 517-17
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© 2015 Eaton. All Rights Reserved..
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