Transcript here
BEYOND THE GREEN WIRE
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
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
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
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
Where to start?
Whether in design, forensics or rehab
take a line from medical ethics
“First do no harm”
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
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
Standards
IEEE Std. 80 (1986)
IEEE Std. 142-2007 (Green Book for
Commercial / Industrial Facilities)
IEEE Std. 1100-2007 (Emerald Book for
Electronics)
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
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
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
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
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
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
Design Considerations
Types of Grounding Systems
Facility Layout
Conductor & Electrode Materials
Special Considerations
Types of Grounding Systems
Solidly Grounded
Low Resistance Grounding
High Resistance Grounding
Ungrounded
System Layout
Grid
Counterpoise
Instrumentation – Dedicated System
Conductor Materials
Copper or Tinned Copper
Aluminum
Braided
Concentric
Insulated
Bare
Electrode Materials
Galvanized Steel
Copperclad Steel
Zinc or Magnesium Solid Shapes
Graphite
Chemically Enhanced Ground Rod
Ufer
Special Considerations
Risk of Copper Theft
Soil Reactivity
Raw and Finished Products of Facility
Depth of Conductors
Cathodic Protection Systems
Other Utilities
Acceptance & Maintenance
Grounding is not an “Install & Forget”
Acceptance Testing & Documentation
Scheduled Testing & Maintenance
Why establishing a baseline
is important.
We have a problem.
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.