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Grounding & Bonding
September 18-20, 2007ERICO, Inc.
Frank Fitzgerald, PE
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“ERICO, Inc. has met the standards and requirements of the Registered Continuing Education Providers Program. Credit earnedon completion of this program will be reported to RCEPP. A certificate of completion will be issued to each participant. As such, it does not include content that may be deemed or construed to be an approval or endorsement by NCEES or RCEPP.”
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COPYRIGHT MATERIALS
This educational activity is protected by U.S. and International copyright laws. Reproduction,
distribution, display and use of the educational activity without written permission of the
presenter is prohibited.
© ERICO Inc., 2007
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Learning Objectives
At the end of this presentation you will be able to:
• Discuss the basics of electrical grounding and bonding.• Discuss electrical engineering principles and relevant specifications.• Discuss the basic design criteria for grounding systems.
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Outline
• Basic Electricity and Materials• Utility Grounding • C & I – Residential Grounding• Telecommunications Systems
Considerations
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Purpose of System Grounding
System Grounding: The intentional connection of a phase or neutral conductor to EARTH for the propose of controlling the voltage to EARTH or GROUND within predictable limits. It also provides for a flow of current that will allow detection of an unwanted connection between system conductors and ground which may instigate operation of automatic devices to remove the source of voltage. The control of voltage also allows reduction of shock hazard to person who might come in contact with live conductors.
IEEE 142-1991 (Green Book)
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Purpose of Grounding• Limit potential difference of neutral
for system stability• Allow for operation of relays and system
protections devices• Personnel safety
This is different from most grounding presentations; if you do the first two, you will get safety for free.
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Basic Electrical Theory Resistivity
Resistivity is the intrinsic property of a material to impede the flow of electrical current. It is the resistance factored by the length and cross sectional area. It is also referred to as volume resistivity.
lRA
=ρ
mmm
Ω=Ω
=2
ρ
A
l
R
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Volume Resistivity of Materials
10-8 10-2 104 1012102
All in mΩ
Metals Soils Wood Insulators
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SOIL RESISTIVITYLoamClaySand & GravelSurface LimestoneShaleSandstoneGranites, BasaltSlates
1 to 50 ohm-m20 to 100 ohm-m50 to 1000 ohm-m100 to 10,000 ohm-m5 to 100 ohm-m20 to 2000 ohm-m10,000 ohm-m10 to 100 ohm-m
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IMPEDENCE
Impedence : is the sum (and much more complex mathematical operations) of a circuit resistance elements and the REACTANCE which is caused by inductors and capacitive elements in the circuit. IT IS AFFECTED BY FREQUENCY f in Hz. Transients or faults in electrical systems look like very high frequency events, not 60 hz.
ωLX L =
ωCXC
1=
fπω 2=Inductive Reactance
Capacitive Reactance
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ELECTRICAL SYSTEMS - TRANSIENTS• ELECTRICAL SYSTEM
• Generators, Transformers, and Utilization devices• Use the earth as a REFERENCE for system stability• The EARTH is not a destination of currents• The System Neutral is the destination for currents• Earth is sometimes a conductor during transients
• NON SYSTEM TRANSIENTS• Lightning is a Non System Transient• The EARTH IS a destination for currents and they
are LARGE MAGNITUDE currents.
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UTILITY ELECTRICAL SYSTEM
• GENERATION– Transforms mechanical energy to electricity
P = 1,300MW @ 30 kV• TRANSMISSION
– P= V x I – Losses are proportional to I so V is increased, Iis
decreased for efficient transmission. V >130kV• DISTRIBUTION
– Requires lower voltage for practical handling of components. Substations transform voltage to 15-25kV and then to 480 or 240/120V
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1,000 MW 30 kVXFMR 500 kV
GENERATIONTRANSMISSION
DISTRIBUTION
500 kVXFMR 15-25 kV
SUBSTATION
15-25 kV
25 kV XFMR 480V 3Φ
25 kV XFMR 240/120V 1Φ
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Typical Utility System
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UTILITY GROUNDING SUMMARY
• ESTABLISH REFERENCE TO EARTH– Allows for Protection devices to operate– Keeps metallic and structures at the potential of the earth
• CREATES EQUIPOTENTIAL PLANE– Significantly reduces risk of step and touch
potential
• PROVIDES LOW IMPEDENCE PATH– Especially for SYSTEM and NON SYSTEM
transients
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AC Three Phase 3 PHASE VOLTAGE
-1.5
-1
-0.5
0
0.5
1
1.5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
Angle (/10)
Phase APhase BPhase C
Ground SystemReference
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Utility Substation Grounding
Substations have high fault currents and potential for voltages caused by that current. Fault Currents > 50kA
RIV ∗=
SAFTEY – At 50-60 Hz 60-100mA can cause ventricular fibrillation. Threshold voltage can be as low as 50 V
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TYPICAL SUBSTATION GROUDNING
• 4/0 Cu buried at 18”• 10-20’ grid, welded• Ground Rods every 20’
typical.• Extremely low R• IEEE 80
Typ
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• Provides a Molecular Bond Between Conductors– Current Carrying Capacity Equal to
that of the Conductor– Permanent - Cannot Loosen or
Corrode to Cause a High-Resistance Connection
– Requires No External Source of Power or Heat
– Can be Visually Checked for Quality
Exothermic Welding for Connections
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Exothermic Welding Process
Aluminum Reduces Copper Oxide, Forming Super Heated Copper and Aluminum Oxide (Slag)
3Cu2O +2Al --> 6Cu + Al2O3 + HEAT
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Mechanical / Compression Connections
• Joint Resistance (RJ )– Contact or Interface Resistance (Rc)
• Surface is Actually Consists of Many Points of Contact • Actual Area or Contact of Joint is Much Less than Surface
Area Brought Together• Dependent on Applied Pressure
– Spreading Resistance (Rs)• Diversion of Current Flow (Current Path is Not Uniform)
Rj = Rc+ Rs
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Comparison of Mechanical and Exothermic Connections
Molecular Bonds Guarantee Conductivity Across the Entire SectionContact and Spreading Resistance (Rs) Portion of Connection is Zero
Molecular Bond
MechanicalConnection
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C & I Grounding
• Generally C & I and Residential grounding methods establish a good earth reference
• Also grounding elements provide a path for fault currents
• All metal parts associated with the electrical system shall be connected to the ground reference for safety.
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NEC 250 Definition of Grounding
• Intentional permanent low impedancePath to carry fault current
• Capacity to carry ground fault current• Returns fault current to SOURCE,
NOT EARTH• Is connected to EARTH for system
REFERENCE• Used to allow operation of protection devices
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C & I and Residential System GroundingElements
• GROUNDED CONDUCTOR -NEUTRAL• GROUNDING ELECTRODE -SEVERAL• GROUNDING ELECTRODE CONDUCTOR• SERVICE BONDING JUMPER• EQUIPMENT GROUNDING CONDUCTOR
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Solidly Grounded 3 Phase Electrical System
Other GroundingElectrodes& Bonds
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NEW – MUST Be Bonded into GEC System
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Main Bonding Jumper
Neutral – GroundedConductor
Equipment Grounding Conductors
Typical Grounding Connections in small Main Service Panel
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Difference between Neutral (Grounded) and Grounding Conductor
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NEC 250 Resistance Requirement
• Section 250.56 requires < 25 Ω resistance for pipe, rod or plate electrodes OR
• Add another electrode > 6’ away from the first, WITHOUT FURTHER MEASUREMENT
⎟⎠⎞
⎜⎝⎛=
dl
lRO
4ln2πρ
Using this formula for resistance (from MIL Handbook 419)
rod of Resistance =ORdiameter Rod =d
length rod =lyResistivit Volume Soil =ρ
Source : http://www.mikeholt.com/newsletters.php?action=display&letterID=407
A ¾” by 10’ in 500 Ω M soil would have a resistance of over 160 Ω
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Practical Ground Resistance• Most rods have much less than 160 Ω• Typical rod resistances will be less than 25 Ω in most
soils because of the existence of proximate utility ground references.
Low ground resistance (< 10 Ω ) can be achieved with– Multiple Rods bonded together– Counterpoise system– Coupled rods– Conductive Enhancement Materials – Chemical Rods
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Beware of False Claims
UL 467 Bend Test Results Galvanize Rod
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Copper Electroplated Rod
UL 467 Bend Test Results - Cu Rod
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BONDING
• Interconnecting of all Ground Electrode Systems– Electrical Power Grounding System– Lightning Grounding System– Telecommunications Grounding System– Cable Grounding System
• Connect all conductive objects together both internal and external to the facility
• Provides near zero voltage difference during ground potential rise
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Other Grounding and Bonding Applications
• Signal Reference Grids• Grounding Bus Bars• Equipotential Mesh• Rebar Clamps• Swimming Pool Grounding• Wind Mill Grounding
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SRG for low impedance ground
• Stops noise from interfering with low voltage digital signals.
• Provides a lower impedance ground reference than signal conductors and shields.
• Minimizes voltage potential difference between interconnected equipment.
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SRG Impedance vs. Freq
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Signal Reference Grids Sensitive Electronics Grounding
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• Applications:– Reduce step & touch potential– Signal Reference Grid– Antenna ground screen– Electronic shielding
• Configuration:– Wire: #6 to #12 - 30 %CW, 40% CW, Cu– Mesh size: 2” x 2” up to 48” x 24”– 20’ maximum width - 500 lb maximum weight– Wire overhang for field splicing using PG style connection.
PREFABRICATED WIRE MESH
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EIA/TIA Ground Bars and Ground Plates
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TGB and TMGB Grounding Bus Bars
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Main Elements of Telecom Grounding and Bonding Equipment
• Bonding Conductor for Telecommunications• Telecom Main Grounding Busbar (TMGB)
-----------------------------------------------------• Telecomm Bonding Backbone (TBB)• Telecomm Grounding Busbar (TGB)• Telecomm Bonding Backbone
Interconnecting
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Bonding Conductor for Telecommunications
Bonding conductor for telecommunications
Equipment
TelecommunicationsEntrance Facility
Electrical Entrance Facility
TMGB
Grounding Electrode Conductor
Service Equipment
N
G
TBB
Outside the scope of this Standard
Within the scope of this Standard
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Equipment
TelecommunicationsEntrance Facility (TEF)
Telecommunications Main Grounding Busbar (TMGB)
Equipment
Telecommunications Room (TR)
Telecommunications Room (TR)
Telecommunications Bonding Backbone (TBB)
LEGEND
Cross connect
Grounding bar
Service equipment
Electrical Entrance Facility
Grounding Electrode System
Grounding Electrode Conductor Bonding
conductor for telecommunications
TGB
TGB
Metal Frame of Building
Equipment
Panelboard
Outside scope of this standard
Bonding conductor as labeled
Telecommunications Grounding Busbar (TGB)
Equipment
Equipment Room (ER)
Pathways
Equipment
Grounding Equalizer (GE)
EquipmentTGB
Telecommunications Room (TR)
TGB
Telecommunications Room (TR)
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Grounding and Bonding SUMMARY
• Discussed Basic Electricity• Understand Resistivity• Should know why Utilities Ground their
systems• Should know elements of commercial –
industrial- residential grounding• Telecom Bonding and Grounding