wiring and grounding for safety
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Wiring, Grounding, and
Power DistributionEquipment
For safety
PCL
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Wiring & Grounding
Why is it important to be aware of building wiring,grounding, and power distribution equipment ?
Because:
Understanding the proper design and installation of buildingwiring, grounding, and power distribution equipment can
Assure a safe operating system
Help reduce power quality problems.
Understanding the most common wiring and groundingerrors can quickly help identify the source of power qualityproblems.
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The purposes of grounding are:
Source: National Geographic
1. Safety? reduce fire hazard
? prevent electric shock
? avoid equipment damage
2. Performance
? control stray currents
? create equal-potentialplane
?
reduce communicationnoise
3. Lightning and SurgeMitigation
?
control transients? remove static charge
? bond between all services
? equalize surge referencepoints
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NEC Code for Wiring and Grounding - Safety
The purpose of this Code isthe practical safeguarding ofpersons and property fromhazards arising from the useof electricity1 .....not
necessarily efficient,convenient, or adequate forgood service or futureexpansion of electricalservices.2
1. NEC 2002, 90.1(A), Pg
2. NEC 2002, 90.1(B)
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Power and Grounding Standards in IEEE Performance & Transient Protection
IEEE 141 - Industrial PowerDistribution (red)
IEEE 142 - Grounding (green)
IEEE 446 - Emergency &Standby Power (orange)
IEEE 1100 - Powering &Grounding Electronics(emerald)
IEEE 1159 - MonitoringPower Quality
IEEE 1250 - Service toCritical Loads
IEEE P1346 - SystemCompatibility in IndustrialEnvironment
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Grounding-Related Definitionsfrom National Electric Code
Ground:A conducting connectionintentional oraccidentalbetween an electrical circuit or equipment andthe earth, or some conducting body that serves in place ofthe earth.
Grounded conductor:A system or circuit (current carrying)
conductor that is intentionally grounded. The neutral wire.White Wire
Grounding conductor:A conductor used to connectequipment or the grounded circuit of a wiring system togrounding electrode(s). Not intended to conduct current.Green Wire
Premises wiring: Interior and exterior wiring, includingpower, lighting, . from service point of utility or aseparately derived power source.
(NFPA 70)
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480Y/277V 3, 4W
208Y/120V 3, 4WMain Service
Entrance Disconnect
N
G
A
C
B
GroundRod
Building Steel
GroundRing
ConcreteEncased Electrode
Underground
Water Pipe
Telephone
Cable TV
Solidly Grounded Wye System
Notes:
1. Effectively (Intentionally) Grounded System. Intentionally connected to earth through a ground
connection(s) of sufficiently low impedance and having sufficient current-c arrying capacity toprevent the buildup of voltages that may result in undue hazards to connected equipment or to
persons.
2. Grounded Conductor.A system or circuit conductor t hat is intentionally grounded.
3. Neutral.A grounded conductor intended to carry im balanced load current.
4. Bonded (Bonding). The permanent joining of metallic parts to form an electrically conductive path .
5. Grounding Electrode System (GES).A system c reated by intentionally bonding together G round
Rod(s), Underground Water Pipe, Concrete Encased Electrodes, Ground Ring and Building
Structural Steel. Provides a low impedance path which can carry lightning currents and system faultcurrents.
6. Grounding Electrode Conductor (GEC). Conductor used to connect the GES to the equipmentgrounding conductor. Keeps a;; equipment and utilities at a common ground potential .
7. Main Bonding Jumper. The connection between the grounded circuit conductor and the equipmentgrounding conductor at the service. Allows short circuit current to return to the source required for
protective device operation.
Grounded System1
Grounded Conductor2
(Neutral3)
Grounding Electrode System (GES)5
Main
Bonding
Jumper7
Grounded
Electrode
Conductor6
13,200-208Y/120VElectric Utility Transformer
13,200-480Y/277V
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Common Grounding Electrode ConductorFor Non-metallic Structure
NEC2002 HB,
Exhibit 250.15,Page189
Common GEC
sized based on
total secondary
cross-sectional
area and table
250.66
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PremisesGroundingTechniques(Service Entrance at 60 Hz)
250.58 Use one common grounding electrode for all enclosures and equipment within a building.
250.50 Bond all elements of grounding electrode system. water pipe, building steel, ground ring.
250.56 A grounding electrode of more than 25 ohms shall be augmented by one additional electrode.
Exhibit 250.29, NEC Handbook, Page 102
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Exhibit 250.30
Exhibit 250.31
Listed Bonding Devices
Exhibit 250.32 Bonding of ametal raceway that contains agrounding electrodeconductor to the conductor atboth ends, as required by
250.64(E).
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Steel Building Columns
? B vs. C: B = 80% C = 20%
? C vs. H: C = 90% H = 10%
? C vs. G: C = 95% G = 5%
100 FeetG
A
B
C
H
1 foot
KAUFMAN EXPERIMENT
Classic Kaufman Experiment in 1954 led to NEC 250.91, thou shall runthe equipment grounding conductor with or enclosing the circuitconductorsGround Fault Return Conductor
S
IAC
2.5-inch Steel Conduit
Ground Fault Return Comparisons
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Listed Bonding Devices
Exhibit 250.35 Grounding-and-bonding bushings used toconnect a copper bonding orgrounding wire to conduits.(Courtesy of Thomas & BettsCorp.)
Exhibit 250.36 A threadedgrounding bushing with setscrews used to ensure electricaland mechanical connection.
(Courtesy of Thomas & BettsCorp.)
Bonding Jumper Sizing
Service same as GEC
Branch Circuit same as EGC
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"NEC", Article 250.4(A)(5)
Basic Equipment Grounding
Effective grounding path. The path to ground from circuits,
equipment, and conductor enclosures shall:
a) Be permanent and continuous,
b) Have capacity to conduct safely any fault current likely to beimposed on it,
c) Have sufficiently low impedance to limit the voltage to ground
and to facilitate the operation of the circuit protective devices in
the circuit,
d) The earth shall not be used as the sole equipment ground
conductor.
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14
Types of Electric Service
UngroundedWye
480 V 3, 3W
Delta480 V 3, 3W
240 V 3, 3W
Grounded? Grounded Wye
480Y/277 V 3, 4W208Y/120 V 3, 4W
High-Impedance Grounded Wye480 V 3, 4W
Corner Grounded Delta480 V 3, 3W240V 3, 3W
Mid-Point Grounded Delta240 V 3, 3W &120/240 V 1,
3W
Single-Phase120/240 V 1, 3W
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480V 3, 3W
Main Service
Entrance Disconnect
N
G
A
C
B
GroundRod
BuildingSteel
Ground
Ring
Concrete
Encased Electrode
UndergroundWater Pipe
Telephone
Cable TV
13,200-208Y/120V
Electric Utility Transformer
GroundingImpedance
Impedance Grounded Wye System
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Corner Grounded Delta System
Corner
Ground
480V 3, 3W240 3, 3WMain Service
Entrance Disconnect
A
C
B
GroundRod
Building
Steel
GroundRing
ConcreteEncased Electrode
UndergroundWater Pipe
Telephone
Cable TV
13,200-240VElectric UtilityTransformer
G
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120/240 1, 3W
Main Service
Entrance Disconnect
L1
L2
GroundRod
Building
Steel
GroundRing
ConcreteEncased Electrode
UndergroundWater Pipe
Telephone
Cable TV
13,200-240V
Electric Utility
Transformer
Center-Tap
Ground
Grounded Split-Phase (Residential) System
G
N
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480V 3, 3W240 3, 3WMain Service
Entrance Disconnect
A
C
B
GroundRod
BuildingSteel
GroundRing
ConcreteEncased Electrode
UndergroundWater Pipe
Telephone
Cable TV
13,200-240VElectric UtilityTransformer
Ungrounded
G
Ungrounded Delta System
General Notes:
1. Ungrounded systems typically consist of a 3-phase 3-Wire 480 Voltor a 240 Volt Delta system.
2. An ungrounded system is a system which does not contain agrounded conductor (neutral) .
3. An ungrounded system is generally considered more reliable than agrounded system because short circuit faults involving only onephase and ground will not cause the overcurrent protection tooperate. The system will continue to operate with no problems andthe fault can be located at a convenient time.
4. The grounding electrode conductor provides a low impedance pathwhich can carry lighting currents into the earth and is connectedbetween the equipment grounding conductor and the groundingelectrode system (NEC 250.24).
5. The grounding electrode conductor keeps all equipment and utilitiesat a common ground potential by connecting the equipmentgrounding conductors and the service equipment enclosures to thegrounding electrodes in the earth {NEC 250.24, .28}.
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480V 3, 3W240 3, 3WMain Service
Entrance Disconnect
A
C
B
Ground
Rod BuildingSteel
GroundRing
ConcreteEncased Electrode
UndergroundWater Pipe
Telephone
Cable TV
13,200-240VElectric UtilityTransformer
GFDs
Notes:
1. The proper use of suitable ground fault detectors (GFDs) on ungroundedsystems can indicated when a single ground fault has occurred so it can becleared before a second ground fault occurs on another phase. Groundfaults on more than one phase can cause damage arcing overvoltages,equipment brownouts, and building fires [(NEC 250.21(3)(d)]
Ungrounded Delta System With GFDs
G
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480V 3, 3W
Main Service
Entrance Disconnect
G
A
C
B
Ground
Rod
BuildingSteel
Ground
Ring
ConcreteEncased Electrode
UndergroundWater Pipe
Telephone
Cable TV
13,200-208Y/120V
Electric Utility Transformer
Ungrounded Wye System
Ungrounded
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Ungrounded Wye System With GFDs
480V 3, 3W
Main Service
Entrance Disconnect
G
A
C
B
Ground
RodBuilding Steel
Ground
Ring
Concrete
Encased Electrode
UndergroundWater Pipe
Telephone
Cable TV
13,200-208Y/120V
Electric Utility Transformer
Ungrounded
GFDs
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Load Current
Equipment Grounding Conductor
~0 Volts
HOT
NEUTRAL
120 Vac
0 Volts 0 Volts
Imok
leakag
e
curren
tonly
? 250.80 All metallic enclosures (conduit,
raceway, panel boards) must bebonded.
Correctly Grounded EquipmentNormal Operation
THE GREEN WIRE SHOULD
NORMALLY SEE NO CURRENT
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Source
LoadGround Fault
Grounding Conductor
A low impedance path for the return offault current to trip the circuit breaker.
Bonded
Metallic enclosureBreaker trips in 2-3 cycles (lessthan .05 seconds) maybe
Phase to Ground Fault Path
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Incorrectly Grounded Equipment andGround Fault Path
Source Phase A
Phase B
Phase C
Grounded Circuit Conductor
LoadGround Fault
Code violation, no grounding conductor!
10 ohms
20 A
120 V
This breaker wont trip, prolonging the fault,because: I = E/R = 120/10 = 12 Amps
This enclosure isnot effectivelygrounded!
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Load Current
Equipment groundingconductor disconnected
violates NEC!
Ground Fault
HOT
NEUTRAL
120
Vac
0Volts
Volts?
Current Flow
x
~ 120V
Unknownearth resistance
Im not
ok!
Unsafe Practice of Isolating Equipment
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? Improper N-G bonds can cause unsafe situations aswell as performance problems, violates 250-24
TransformerGround
N
G
Hot
GroundedConductor(Neutral) Ground
Wire
Main Electrical
Panel
Distribution
Panel Receptacle LoadEquipment
ConduitGround
ConduitGround
EquipmentGround
40 A
? A
? A
? AServiceGround
Misapplied Neutral to Ground Bonds
N
G
N
G
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? Neutral to ground bonds are required at main service (NEC 250-24) and at
any separately derived power sources (NEC 250-30) and shall not be made
at any grounded circuit conductor (neutral) on the load side of the service
disconnect.
Proper Neutral to Ground Bond
TransformerGround
Hot
GroundedConductor
(Neutral)
Ground Wire
Main ElectricalPanel
DistributionPanel Receptacle
ComputerSystem
Separately
DerivedSystem
N-G bond per 250-30
N-G bond
per 250-24
NN
GG
N
G
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Neutral
Ground
3-Wire
Input
Ground
Transformer
Per 250.50 form a common premisesgrounding electrode system by connecting to
nearest effectively grounded:1. Building Steel2. Metal Water Pipe, or
3. Other Electrode
Ground
Neutral
Phase A
Phase BPhase C
Load
Separately Derived ac Systemper NEC 250.30
A neutral to groundbond between thesource and the firstdisconnect.
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Isolation TransformerNeutral to Ground Bond
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Neutral
Ground
3 Wire
Input
Ground
Transformer
Grounded
Phase A
Phase B
Phase C
Load
Fault
Common Building GroundingElectrode System
Separately Derived ac SystemInput-side Ground Fault
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Neutral
Ground
3-Wire
Input
Ground
Transformer
To Computer Room
Signal Reference Grid
Grounded
Phase A
Phase B
Phase C
Load
Fault
Common building grounding
electrode system
Separately Derived ac Systemwith External Ground Fault
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Grounding Application For SeparatelyDerived Systems
Exhibit 250.13, NEC Handbook, Page 187,
Neutral-ground bond established at transformer
Exhibit 250.14, NEC Handbook, Page 187,
Neutral-ground bond established at first disconnecting means
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N
G
ServiceEntrance
Grounding
Electrode
N
G
Local
Grounding
Electrode
G
Generator
3-PoleAutomatic
Transfer Switch (ATS)
3W+G Load
Grounding and Transfer withStandby Generator and 3-W Load
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N
G
ServiceEntrance
Grounding
Electrode
N
G
Local
Grounding
Electrode
G
Generator
3-PoleAutomatic
Transfer Switch
4W+G Load
N
Incorrect Grounding with Standby Gen, 4-Wire Loadand 3-Pole ATS
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N
G
ServiceEntrance
Grounding
Electrode
N
GG
Generator
4-PoleAutomatic
Transfer Switch
4W+G Load
Local
Grounding
Electrode
Correct Grounding with Standby Gen,4-Wire Load and 4-Pole ATS
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Wiring & Grounding Evaluation
? Check rms voltage levels
? Check for extra neutral-ground bonds
? Check for overloaded neutral conductors
? Check grounding electrode system
? Check isolated ground receptacle wiring
? Check overall circuit layouts
? Check for use of separately derived systems
?
Check for ground loops
? Apply protection to data lines and communications lines if ground loop
potential exists
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DETAILS, DETAILS
Neutral
Ground
3-Wire
Input
CASE #1
Where Is the Grounding Electrode Connection?
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GROUND, THERE IT IS !
HERES THE BLDG GRD?
Neutral
Ground
3-Wire
Input
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Problem: Erratic computer operation occurred in a large office buildingduring lightning storms.
Findings: Isolated ground conductors from all computers wereconnected to an isolated grounding electrode system. Voltages werebeing developed between the isolated computer ground and the buildingground.
Improper Isolated Ground
G
N
Service
Entrance
Grounding
Electrode
ITE
Equipment
IsolatedGround
ReceptacleDistribution
Panelboard
N
IsolatedGrounding
Electrode
Ground
Fault
Look for special quiet isolated ground!
IG
N
IG
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Problem: A new ten-story office building includeda 1/4x1 copper busbar installed from thebasement to the top floor. The busbar wasinsulated from the building and connected to a
ground rod driven through the basement floor. Allisolated ground receptacles throughout thebuilding were connected to this isolated groundbusbar.
Solution: The electrical inspector would not allowoccupancy of the building until the IG receptacleswere rewired.
Isolated Grounding System
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Problem: Four separate nearby buildings containedcontrol for chemical processing. There wereinterconnecting data cables between buildings. Each
of the four computer systems was separatelygrounded to its respective buildings groundingsystem. Operation of the computer systems waserratic.
Solution: The interconnecting data lines werereplaced with fiber-optic communication links inburied plastic conduits.
Multiple Interconnected Buildings
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Supplemental
Ground Rod
Properly InstalledMay Still Invite
Stray Ground
Currents
OPEN
CONCENTRIC
ONE OF
WORSTCONDITIONS
POSSIBLE
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CNCMachine
GroundingElectrodes
Building Premises
ServiceTransformer
Cable Tray (Raceway)
DedicatedGround Rod
ServiceEntranceSwitch
0A0B0C
Incorrect and Unsafe Installation of Isolated,Dedicated Ground Rod
Attempts
To IsolateFrame
Create anUnknownResistance
Path Backto Service
N
G
PVCIsolator
G
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SAFETYFIRST!
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?s
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