GROUNDING: PRACTICES AND ISSUES
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Transcript of GROUNDING: PRACTICES AND ISSUES
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BEYOND THE GREEN WIRE
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Is grounding science, art or black magic? Science – having the data and
knowledge to competently design a system
Art – always possible to manipulate a design for a more elegant solution based on factors such as budget or the environment
Black magic – the perception created with gaps in knowledge / understanding
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We must choose a perspective BYNUM’S 5 LAWS OF ENGINEERS An engineer is empirically oriented An engineer is data driven An engineer in one way or another sells
his/her time and expertise Liability concerns Due diligence issues
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Grounding, bonding, surge, lightning Cannot be treated discretely Each is an element of a coherent whole All may not be necessary but none can
be manipulated separately
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Why grounding / earthing? References a structure to the
environment – potentials rise and fall together
Set points for circuit breakers Fuse selection Defines and identifies ground reference
for ground fault and arc flash relays Coordination of the system
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Where to start?
Whether in design, forensics or rehab take a line from medical ethics
“First do no harm”
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Using the first two points of Bynum’s Law Engineers are compelled by training and
discipline to design TO something Anything else may be an educated
guess, informed guess or lucky guess… but still a guess
With grounding we design between what we want or need and what the environment allows
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Point #2 of Bynum’s Law
When the required data is not available the tendency becomes using a formerly successful “boilerplate” design or to “overdesign” effectively becoming a guess
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Standards
IEEE Std. 80 (1986) IEEE Std. 142-2007 (Green Book for
Commercial / Industrial Facilities) IEEE Std. 1100-2007 (Emerald Book for
Electronics)
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The perfect world
The engineer will have a certified “start point” in the form of results from a soil resistivity test
Fairly reliable predictor of grounding electrode performance
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Soil Resistivity Tests
4 Point Wenner Method Test developed in 1915 by Dr. Frank Wenner of NIST for U.S. Bureau of Land Management
Most accurate test Used by civil engineers to locate water
table but CE test protocol may be problematic for the electrical engineer
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Soil Resistivity Tests
Schlumberger Array developed by Conrad Schlumberger of France in the 2nd decade of the 20th century
These are typically “green field” tests
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Factors affecting resistivity Moisture Temperature (Sandy loam with a 15.2%
moisture content @ 32 deg. F [ice] has almost 3 times the resistivity of the same sample @ 32 deg. F [water])
Soil type – it has been reported the Metroplex has over 250 separately identifiable soil types
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Factors affecting resistivity Depth – varies with soil type, moisture
content and freeze line NOTE! There is a very real
correspondence between resistivity and corrosion issues
Electrolytes in the soil (active, pH issues)
Pollution issues
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Rules of thumb
Most useful when talking about thumbs Designs generally specify 18” deep and
18”-24” outside of slab (drip line) At 18” the conductor merely connects
the electrodes 30” and below (due to moisture content
and freeze line) the conductor begins to act as an electrode in and of itself
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Design Considerations
Types of Grounding Systems Facility Layout Conductor & Electrode Materials Special Considerations
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Types of Grounding Systems Solidly Grounded Low Resistance Grounding High Resistance Grounding Ungrounded
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System Layout
Grid Counterpoise Instrumentation – Dedicated System
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Conductor Materials
Copper or Tinned Copper Aluminum
Braided Concentric
Insulated Bare
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Electrode Materials
Galvanized Steel Copperclad Steel Zinc or Magnesium Solid Shapes Graphite Chemically Enhanced Ground Rod Ufer
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Special Considerations
Risk of Copper Theft Soil Reactivity Raw and Finished Products of Facility Depth of Conductors Cathodic Protection Systems Other Utilities
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Acceptance & Maintenance Grounding is not an “Install & Forget” Acceptance Testing & Documentation Scheduled Testing & Maintenance
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Why establishing a baseline is important.
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We have a problem.
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Summary
Grounding is a fundamental Infrastructure system
Yes, it can be expensive to get right. It is more expensive to correct problems
after construction. It is most expensive when a ground
system failure causes injury, equipment damage or loss of product.