NASA/TP–2014-217518

112 Pages·2015·2.69 MB·English

https://ntrs.nasa.gov/search.jsp?R=20150000774 2019-04-05T09:06:25+00:00Z
NASA/TP–2014-217518
Mobilization Protocols for Hybrid Sensors for Environmental AOP
Sampling (HySEAS) Observations
Stanford B. Hooker
National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Maryland 20771
April 2014
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NASA/TP–2014-217518
Mobilization Protocols for Hybrid Sensors for Environmental AOP
Sampling (HySEAS) Observations
Stanford B. Hooker
Goddard Space Flight Center, Greenbelt, Maryland
National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Maryland 20771
April 2014
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S.B. Hooker
Abstract
The protocols presented here enable the proper mobilization of the latest-generation instruments for measuring
the apparent optical properties (AOPs) of aquatic ecosystems. The protocols are designed for the Hybrid
SensorsforEnvironmentalAOPSampling(HySEAS)classofinstruments, butareapplicabletothecommunity
of practice for AOP measurements. The protocols are organized into eleven sections beyond an introductory
overview: a)cablesandconnectors,b)HySEASinstruments,c)platformpreparation,d)instrumentinstallation,
e) cable installation, f) test deployment, g) test recovery, h) maintenance, i) shipping, j) storage, and k) small-
boatoperations. Eachsectionconcentratesondocumentinghowtopreventthemostlikelyfaults, remedythem
should they occur, and accomplishing both with the proper application of a modest set of useful tools. Within
the twelve sections, there are Socratic exercises to stimulate thought, and the answers to these exercises appear
inAppendixA.Frequentlyaskedquestions(FAQs)aresummarizedinaseparatesectionaftertheanswerstothe
exercises in Appendix B. For practitioners unfamiliar with the nautical terms used throughout this document
plus others likely encountered at sea, an abbreviated dictionary of nautical terms appears in Appendix C. An
abbreviated dictionary of radiotelephone terms is presented in Appendix D. To ensure familiarity with many
of the tools that are presented, Appendix E provides a description of the tools alongside a thumbnail picture.
Abbreviated deployment checklists and cable diagrams are provided in Appendix F. The document concludes
with an acknowledgments section, a glossary of acronyms, a definition of symbols, and a list of references.
1. INTRODUCTION and then publishing, the SeaWiFS Ocean Optics Proto-
cols (Mueller and Austin 1992), hereafter referred to as
A number of international ocean color satellite sensors
the Protocols. The Protocols are a work in progress and
weredesignedandlaunchedinthelasttwodecadestosup-
were revised (Mueller and Austin 1995) and subsequently
port oceanographic studies and applications including the
expanded (Mueller 2000, 2002, and 2003) by having the
Ocean Color and Temperature Scanner (OCTS), the Sea-
scientific community decide the topics be updated. The
viewing Wide Field-of-view Sensor (SeaWiFS), two Mod-
Protocols set the requirements for all ground-truth—more
erate Resolution Imaging Spectroradiometer (MODIS) in-
properly sea-truth—observations, and although the Pro-
struments, the Medium Resolution Imaging Spectrometer
tocols were initially established for the SeaWiFS Project
(MERIS), and the recently launched Visible and Infrared
alone, follow-on ocean color missions advocated adherence
Imaging Radiometer Suite (VIIRS). All of these sensors
totheProtocolstominimizeuncertaintiesinsea-truthand
have contributed solutions to the general problem of in-
remote sensing data products.
verting optical measurements obtained principally in the
The central theme presented here is the incremental
visible(VIS)partofthespectraldomaintoderiveconcen-
pursuit of more accurate field observations to ensure ac-
tration estimates of biogeochemical parameters, and some
cess to state-of-the-art advances by making the hardware,
continue to provide coverage of the global biosphere.
software, and information solutions commercially or pub-
The worldwide deployment of commercial off-the-shelf
liclyavailable. Thecurrentchallengeinoceancolorremote
(COTS) radiometers has been the primary source of vali-
sensing is to extend the accomplishments achieved in the
dation data for ocean color remote sensing data products,
open ocean into much shallower coastal habitats (Hooker
because they are designed to sample the dynamic range of
etal.2007),e.g.,estuariesandrivers(Antoineetal.2013).
water types involved. The SeaWiFS Bio-optical Archive
andStorageSystem(SeaBASS)continuestoprovidelong- Thisrequirementisdrivenbythepresentfocusofsatellite
term access to these data for the global community, since observations, which is inexorably linked to launching new
itsinception(Hookeretal.1994). COTSinstrumentshave missions based on novel research topics to derive new sci-
also been used for vicarious calibration (Bailey et al. 2008 entificresultsfromanexpandedspectraldomain—i.e.,the
andAntoineetal.2008),whichisprimarilyanopen-ocean ultraviolet (UV) to the near-infrared (NIR).
problem because of the need for spatial and temporal ho- A principal objective of the procedures presented here
mogeneityduringsampling, atasimilarlevelofefficacyto is to prepare for the next-generation of ocean color satel-
custom hardware, e.g., the Marine Optical Buoy (MOBY) lites (NRC 2007 and NASA 2010)—which will emphasize
activity (Clark et al. 1997). expanded spectral (UV to NIR) and dynamic (open ocean
For the SeaWiFS Project, the first step in the pro- to rivers) ranges—with the most capable COTS instru-
cess of controlling uncertainties in calibration and valida- ments (hardware and software) in the shortest time possi-
tion datawasestablishing, through community consensus, ble. Thelatterisrequiredtoensurethatthescienceteams
1
Mobilization Protocols for HySEAS Observations
canstartcollectingthebaselineobservationsneededtobe-
gin formulating and testing the myriad details associated
with hypotheses, algorithms, and databases for the new,
spectrally more expansive, next-generation missions. At
thesametime,applicabilitytocurrent-generationmissions
mustbemaintainedwithsamplinginthelegacy(VIS)do-
main. Because of the emphasis on the near-shore environ-
ment, which is typified by shallow water depths and an
optically complex vertical structure, there is the added re-
quirementtodemonstratethatthenewcapabilitiescanbe
validated in waters with unprecedented multi-dimensional
heterogeneity (Doxaran 2012, Matsuoka et al. 2012, and
Fichot et al. 2013).
As part of an emphasis on innovation in global ob-
servations of the Earth system from space, NASA has a
present- and next-generation requirement to collect high-
quality in situ data for the vicarious calibration of ocean
color satellite sensors and to validate the algorithms that
use the remote sensing data. For ocean color, the physical
measurement is the spectral radiance emerging from the
ocean, the so-called water-leaving radiance, LW(λ), where Fig. 1. An example information flow wherein the
λ denotes wavelength (Hooker and Esaias 1993). “High Protocolsspecifyhowinsitu dataareacquiredwith
quality”referstomeasurementswithadocumenteduncer- the C-OPS field instruments and then processed
with PROSIT to create products to be stored with
tainty to within established metrics for producing climate
data records (CDRs). Derivations of LW(λ) are routinely the data in SeaBASS. PROSIT tools evaluate un-
certainties, which might lead to revised Protocols.
obtained by extrapolating near-surface in-water profiles of
the upwelled radiance Lu(z,λ) to null depth, z =0-.
In Fig. 1, reduced risk in one component is linked to
Contemporaneous satellite and in situ match-up data
subsequentcomponents,whichinturnbenefitfromthere-
(usuallycollectedtowithin60–180minintheopenocean)
ducedrisk. Intheiterativeflowoftheprocess,theevolving
are required for calibration and validation activities. The
cost of the system is reduced. Each turn through the cy-
applicable COTS field instruments and information sys-
tems infrastructure needed to derive and archive LW(λ) cle is leveraged within the community as a reduced risk
indecisionmaking, andexpressedintheapplicableNASA
data for next-generation baseline analyses include the fol-
programs as an overall cost benefit. The savings come
lowing:
from not having to acquire more data, because the qual-
• Thestate-of-the-artfieldinstrumentation,e.g.,anin-
ity of the data that were acquired were immediately suf-
waterCompact-OpticalProfilingSystem(C-OPS)or
ficient. In this regard, risk and uncertainty go hand in
above-water Biospherical Surface and Celestial Ac-
hand: datawithhighuncertaintiesareinherentlyriskyfor
quisition Network (BioSCAN) with applicable acces-
next-generation mission planning.
sories (http://biospherical.com/);
The purpose of the protocols presented here is to re-
• The Ocean Optics Protocols for acquiring the field
ducetheuncertaintiesinsea-truthobservationsneededfor
data and producing the agreed upon data products
calibrating and validating the next generation of NASA
(http://oceancolor.gsfc.nasa.gov/DOCS/);
oceancolorsatellitesbyimprovingthecommunityofprac-
• A data processing scheme for producing the data ticeandlinkingitdirectlytocommercial-off-the-shelfopti-
products, e.g., the Processing of Radiometric Ob- cal instrument systems and publicly available information
servations of Seawater using Information Technolo- systems. This will allow any scientist in the world to con-
gies(PROSIT)softwareprogram(HookerandBrown tribute to climate change research at the requisite quality
2014); and
level by simply following the procedures with sensors that
• An accessible repository for the original field data are available to all.
and subsequent processing results, e.g., SeaBASS at The majority of the procedures presented here are not
GSFC (http://seabass.gsfc.nasa.gov/). addressed in the Protocols, so the material is seen as an
A conceptual view of calibration and validation organizes important addition to developing the high-quality optical
the hardware, protocols, and processing as a triumvirate instruments plus the corresponding data acquisition, pro-
surrounding a central database (Fig. 1), so results or im- cessing, and analysis protocols needed for next-generation
provements obtained in one component can influence the ocean color calibration and validation. When combined
other applicable component(s) and lower the risk of being withthenewtechnologiesthathavebeendeveloped(Mor-
unable to properly maintain uncertainties. row et al. 2010a and Hooker et al. 2012), the procedures
2
S.B. Hooker
are expected to ensure the new technology is ready for and the answers to these exercises appear in Appendix A.
theglobalscientificcommunitytouseforlegacyandnext- FAQsfromstudentsandlessexperiencedpractitionersare
generationclimatechangeresearch. Theapproachadopted summarized in a separate section after the answers to the
here is to provide—by specific example—the implicit and exercisesinAppendixB.Forpractitionersunfamiliarwith
explicit tools, both in terms of hardware and procedures, the nautical terms used throughout this document plus
needed by any scientist to confirm the acquired field data others likely encountered at sea, an abbreviated dictio-
are at the requisite quality level. nary of nautical terms appears in Appendix C. An abbre-
The inspiration for the level of detail that is presented viated dictionary of radiotelephone terms is presented in
in each of the following sections was Mr. Horace Mann (4 Appendix D. To ensure familiarity with many of the tools
May 1796 to 2 August 1859), who was an American edu- that are presented, Appendix E provides a description of
cation reformer and abolitionist. Horace Mann is credited the tools alongside a thumbnail picture. Abbreviated de-
with saying (as adapted in 1880), ployment checklists and cable diagrams are provided in
Appendix F. The document concludes with an acknowl-
Habit is a cable; we weave a thread of it each day,
edgments section, a glossary of acronyms, a definition of
and at last we cannot break it.
symbols, and a list of references.
Thisperspectivewasadoptedbecauseitcreatesthenotion
The choices made to create the material presented are
thatsensibleprocedurespracticedtothepointofhabitin-
a combination of subjective and objective analyses, which
crementally reinforce the metrics for success. It also cre-
must necessarily change over time as the technology they
ates the imagery that the importance of a strong cable
are designed to protect change and as the tools change,
is discernible—as is its failure. Understanding these two
either in availability or in design. Consequently, the pro-
states is paramount to successful fieldwork. The level of
tocols are not viewed as static and this document is one
detail presented here is designed to expose all the proce-
version of many that will ultimately be produced to cor-
dural “threads,” so they can be woven together over time
rectly mobilize HySEAS instruments.
into strong protocols.
The procedures are applicable to both above- and in-
Protocols are not infallible; they are implemented to
water AOP sampling systems, but the emphasis is on the
make use of an instrument system, and system subcompo-
at-sea collection of in-water data, because it is the most
nents can fail of their own accord. Consequently, a com-
complicated and least forgiving—a ship can decide to not
plete protocol includes procedures for troubleshooting a
leave port as severe weather approaches, but once at sea,
faultandrestoringaninstrumenttonominalperformance.
the scientists and crews are obliged to withstand all the
The latter requires tools and, more importantly, tried and
naturalenvironmentprovides. Becausethismeansallper-
proven tools, so inventories of useful tools are also pre-
sonnelcanbeplacedincircumstancesassociatedwithhigh
sented. Given the sophisticated designs of modern instru-
risk to the safety of themselves and their equipment, the
ments, the total number of tools required to remedy all
material includes three levels of warning:
possible faults requires an impractical inventory for cost-
effective field campaigns, so only the most likely problems (cid:2) Cautionary explanations to avoid a needless degra-
are considered. Furthermore, the emphasis is on mobiliz- dationinperformanceappearseparatelywiththeso-
ing on a ship, and ships frequently have a large number of called dangerous bend graphic, shown to the left;
tools aboard.
(cid:2)(cid:2) If extra caution is warranted, because there is
a significant likelihood of compromising perfor-
1.1 Protocols Organization
mance, the text appears with double dangerous
The protocols presented here are organized into eleven bends; and
additional sections: a) cables and connectors, b) HySEAS (cid:2)(cid:2)(cid:2) If extreme caution is required, because there
instruments, c) platform preparation, d) instrument in-
is an inherent safety risk to personnel (and
stallation, e) cable installation, f) test deployment, g) test
secondly safety of equipment) that must be
recovery, h) maintenance, i) shipping, j) storage, and k)
respected, the text appears with triple dan-
small-boat operations. Each section concentrates on doc-
gerous bends.
umenting how to prevent the most likely faults, remedy
them should they occur, and accomplishing both with the Onmarineinstallations,safetyofequipmentmayalsocon-
proper application of a modest set of useful tools. The tribute to safety of personnel particularly if loss of vital
selection of what constitutes a “most likely” fault out of equipment places the ship in danger.
a large and evolving number worthy of inclusion is based The cautionary notes are provided to identify proce-
on experience. Similarly, the selection of “useful tools” is dures or aspects of the total environment—protocols, per-
also based on life experiences involving a large inventory sonnel, and hardware—that can lead to avoidable prob-
of contemplated, accepted, and rejected hardware. lems if they are misinterpreted or not adhered to. All
Withinthetwelvesections, thereareSocraticexercises cautions should be read carefully; adherence to the pro-
(i.e., questions posed to the reader) to stimulate thought, vided instructions will ensure the safety of personnel and
3
Mobilization Protocols for HySEAS Observations
Fig. 2. The two end members in platform sizes showing a) the ocean-class R/V Hakuho Maru (left), with
the height of a person standing on the bow setting the scale, and b) the small inflatable boat R/V Recon 18
(right). The former displaces almost 4,000tons with an overall length of 100m and two 1,085kW generators
developing 2,910hp for propulsion, whereas the latter is a FC 470 military 4.7m Zodiac fitted with a 30hp
Nissan outboard and custom-equipped for instrument deployments including 12VDC and 120VAC power.
equipment. In some cases, mitigation strategies for the range in possible deployment platforms. This approach
identified problem might be possible, but others are sim- produces five classifications for the platforms:
plyrestrictionsthatcannotbeeasilyovercomeandshould • Global- and ocean-class ships (55–90m or more),
not be circumvented.
• Regional- and local-class research vessels (20–54m),
Asanaidtothestudentandfirst-timepractitioner,So-
cratic learning exercises are provided throughout the en- • Coastal (small craft) research vessels (7–19m),
suing individual sections. The exercises are designed to • Small-boat operations (less than 7m), and
reinforce important aspects of the protocols by providing • Shoreline or shallow-water fixed structures, e.g., a
additional details in the answers (Appendix A) to logical
pier, dock, lighthouse, tower, etc.
questions. Theyarealsodesignedtoestablishaninquiring
Thesefiveplatformoptionsestablishthreegeneralcabling
perspective and encourage research, and are identified by
needs: long(55–90mships),medium(8–20mresearchves-
the following sequence:
selsandfixedstructures),andshort(small-boatandshore-
(cid:2)Exercise
lineoperations). Mobilizationprotocolsforthislargerange
which is followed by the section number associated with invesselsize(Fig.2),arevirtuallythesameasgivenbelow.
the material being considered.
Althoughabove-watersystemscanbedeployedonland (cid:2)Exercise 1.2 What types of in-water platforms are not
to make atmospheric or terrestrial observations, the per- mentionedinthefiveplatformclassificationsthatmight
spective adopted here is for oceanic data collection. This benefit from mobilization protocols?
does not result in any loss in applicability, because the
ship-based perspective shares common features with land 1.3 Deployment Options
andoffshorestructures,forexample,allrequirelocationor
global positioning system (GPS) information. At-sea de- There are four generalized deployment options for in-
ployments require additional considerations, however, be- water AOP sensors: a) free-fall profilers, b) winch and
cause of the harshness of the marine environment and the crane frames, c) buoys (moored or drifting), and d) au-
fact that a ship is in motion. Consequently, the unique tonomous vehicles (e.g., gliders). Of these, the emphasis
aspects of the oceanographic sampling problem can be ig- here is on free-fall systems, because next-generation sam-
nored if they are not relevant to an alternative structure pling requires shallow-water systems deployed from small
that is being used. vessels, whichismosteffectivelyaccomplishedwithafree-
fall profiler, e.g., a C-OPS instrument.
(cid:2)Exercise 1.1 What is likely the most significant ad-
vantage and disadvantage of deploying an autonomous (cid:2)Exercise 1.3 What are additional advantages of a free-
radiometricsystemonaterrestrialstructureasopposed fall profiler like C-OPS over the following deployment
to an offshore structure or research vessel (R/V)? alternatives: a) winch and crane frame, b) buoy, and c)
glider?
1.2 Platforms
The C-OPS instrumentation (Morrow et al. 2010b),
Five types of at-sea deployments are considered here whichisastate-of-the-artreplacementfortheSubmersible
basedonarbitrary,butsensiblyconstrained,sizesofawide Biospherical Optical Profiling System (SuBOPS) legacy
4
S.B. Hooker
instrument (Hooker et al. 2010a), illustrates a COTS in- (cid:2)(cid:2) It is not advisable to use a different sea cable than
water system that can be used across the full dynamic the one provided by the manufacturer, because
range of next-generation observational requirements, i.e., thebackplanefree-fallcharacteristicscanbenegativelyal-
from the open ocean to rivers and from the UV to NIR. tered, which can lead to degraded in-water data products.
The two types of deployment options for above-water
Thecablingthatisusedforallotherconnectionscanbe
instruments—whether on a structure, ship, or airborne
obtainedfromavarietyofsources(e.g.,directlyfromSub-
vehicle—are either manually or automatically pointed. A
Conn) as long as the wire gauge and connector specifica-
manual system is emphasized here, because above-water
tionsarecorrect. Thesubsequentsectionsdiscussthepro-
systems mounted on ships are usually manually pointed.
The BioSCAN instrument is the state-of-the-art re- tocols associated with the successful mobilization of each
placement of the Biospherical Surface Ocean Reflectance aspect of cables and connectors.
System(BioSORS)legacyinstrument(Hookeretal.2010b)
(cid:2)Exercise 1.4Whattypesofdeploymentmightlogically
and is selected for describing the mobilization procedures
require adifferent typeof cablefor thesurface reference
for an above-water system, because it is state of the art
and sufficiently similar to C-OPS in its basic components than the one provided?
that a unified approach to the protocols is facilitated.
Theprincipledeploymentoptiondiscussedbelowisthe 2. CABLES AND CONNECTORS
at-seamobilizationofaC-OPSin-waterinstrumentandan
For the discussion presented here, the principal pur-
above-water BioSCAN instrument. These two instrument
pose of an electrical cable and connector assembly is to
systemsarebothpartoftheBSIarchitectureforHySEAS
provide a safe and reliable connection for power and data
instrumentation(Sect.3). Althoughthisfocusesthemate-
telemetrybetweentwocomponents. Thechoiceofconnec-
rial presented here on two types of sampling systems, the
tortypeandcable,i.e.,thenumberofconductorsandtheir
majorityoftheprotocolspresentedareapplicabletoother
specificationplustheouterjacketingenclosingallwires, is
similarinstruments,deploymentoptions,orplatformsand,
an important aspect of maintaining the capabilities of a
therefore, easily adapted to other mobilization scenarios.
sampling system over the short and long term.
For cable assemblies that provide more functionality
1.4 Instrument Components
than power and telemetry alone, e.g., that are used to de-
In terms of the categories of components, the C-OPS ploy and recover an instrument like C-OPS, there are the
and BioSCAN instruments are purposely similar, and are followingadditionalrequirements: a)oneormorestrength
comprised of the following five subsystems: members to accommodate the anticipated tensile forces
1. A data acquisition computer; created when the instrument is pulled back to the surface
2. A deck box to provide power, control the instru- or deployment platform and recovered, and b) material
ments, receive records, transmit data, and receive specifications to set the overall buoyancy of the cable.
commands from the data acquisition computer;
(cid:2)Exercise 2.0 What is a principal concern for an elec-
3. A solar reference, with or without the Biospherical
trical cable that is designed to be used to deploy and
GPS (BioGPS) or Biospherical Shadowband Acces-
recover an instrument system?
sory for Diffuse Irradiance (BioSHADE);
4. Eitheranin-waterfree-fallingbackplanewithahand- Fieldworkisfrequentlystressful,andmostmistakesoc-
held sea cable or an above-water manually pointed cur when personnel are distracted (e.g., by an unantici-
frame, both with two optical sensors; and pated problem, sleep deprived from travel or work, lack of
5. The cabling that connects the first four components. familiarity with the workplace, etc.). A sampling system
thatisdesignedwithcomponentsandprocedurestofacili-
Consequently, the distinction between the two systems is
tatecorrectinstallationregardlessofthecircumstanceshas
with the fourth subsystem, and the two options both typ-
asignificantadvantageoveronethatcanbeeasilymiscon-
ically use two radiometers. In the case of the in-water
strued and assembled incorrectly—especially for the new
profiler, an irradiance and radiance sensor pair are used;
whereas for the above-water frame, two radiance radiome- practitioner.
ters are deployed. As noted earlier, a principle behind the protocols pre-
For the purposes of keeping track of the inventory of sented here is that safety, both for the personnel and the
parts involved, the sea cable for the C-OPS instrument equipment, is a primary concern. In some cases, there are
is considered part of the backplane subsystem, because it competing requirements for safety, and the adopted pro-
is specially made with a prescribed buoyancy and custom cedure might seem arbitrary, but that is not the case. In
wire-gauge specifications. It is coiled into a cable bucket fact,acomprehensivepointofviewhasbeenappliedtoall
to keep it organized and safe from ship operations. The oftheprotocolspresentedhereandtestedovermanyyears
bucket shields the cable from side impacts and can be of fieldwork spanning thousands of hours in the field and
moved to a sheltered location if necessary. thousands of successful data acquisition events.
5
Mobilization Protocols for HySEAS Observations
(cid:2)(cid:2)(cid:2) The perspective adopted here is that all cable 2.1 Electrical Grounding
connects and disconnects should be done with
In an electrical circuit, ground refers to the following:
thepoweroff,becausefieldwork—mostnotablyanoceano-
a) the reference point from which other voltages are mea-
graphicexpedition—involvesthehighprobabilityofwork-
sured;b)acommonreturnpathforelectriccurrent;orc)a
ing with wet components, as well as wet hands or feet,
direct physical connection to the Earth. For mobilization
which can result in unintended and potentially hazardous
applications, the latter two are the most applicable and
electrical pathways that might go unnoticed.
involveequipmentgrounding andearthgrounding. Equip-
ment grounding ensures the operating equipment within a
Itmightseempossiblethatthedirectcurrent(DC)lev-
structure has an unbroken connection to theEarth. Earth
elsassociatedwithmoderninstrumentconnectionscannot
grounding is the intentional and physical connection from
produce hazardous electric shocks to personnel and only
a circuit conductor, usually the neutral (or return), to an
alternating current (AC) sources are of concern. Person-
electrode placed in the earth or a ship’s hull. When this
nel might also believe the only concern for DC current is
connection is absent, the circuit has a floating ground.
if power is accidentally applied to the wrong pin or socket
Equipment and grounding systems must be kept sepa-
on an instrument—which could damage the instrumenta-
rate except for a physical connection between the two sys-
tion. If the integrity of the cables and connectors have
tems, usually at the point of power distribution. The pur-
been compromised or if their integrity is unknown, this is
pose of the ground connection is to provide a safe path for
ill advised and dangerous in all circumstances.
the dissipation of differences in unintended voltage poten-
(cid:2)(cid:2)(cid:2) If a cable or connector appears worn or phys- tials. Thevoltagepotentialscanarisefromthenormalop-
eration of equipment (e.g., electromagnetic and radio fre-
ically damaged—e.g., the outer insulation has
quencyinterference),faultconditions(e.g.,wiringmishaps
been cut or the body containing the pins or sockets has
thataccidentallycrosswires), ornaturalphenomena(e.g.,
been crushed—it should not be used and should be re-
static discharges and lightning strikes). The safe dissipa-
placed with a fully functional spare.
tion of unintentional voltage potential protects the people
Ifcabledamageistopicalandrestrictedinextenttothe and equipment in contact with the voltage potentials.
consequencesofnormaluse,e.g.,frayingoftheouterjacket For measurement purposes, the Earth serves (essen-
over a small area (Fig. 3), with no evidence that the in- tially) as a zero potential reference against which other
sulationintegrityhasbeencompromised,thefrayedjacket potentials can be measured. Consequently, an electrical
groundsystemshouldhaveanappropriatecurrent-carrying
should be wrapped with a brightly colored vinyl electrical
capability to serve as an adequate zero-voltage reference
tapetocallattentiontotheareabeingprotected. Atsome
level. In electronic circuit theory, a ground is usually ide-
point, normal use gives way to worn out and a cable must
alized as an infinite source or sink for charge, which can
be replaced. By inspecting and rectifying normal wear, a
absorb an unlimited amount of current without changing
cable lasts longer and it will be visually apparent (from
itspotential. Assuch,agroundisapathofleastresistance
the accumulation of brightly colored over wraps) when a
and ideally should have a resistance of 0Ω.
cable needs to be evaluated for replacement.
Because of variability in the material structure at the
point of contact with the Earth, the resistance to ground
Frayed
is usually not zero. A single standard resistance thresh-
Section
old for ground resistance that is recognized by all relevant
authorities does not exist. The National Fire Protection
Association (NFPA) provides some guidance on this issue
within the National Electrical Code (NEC) whereby if a
single rod, pipe, or plate electrode has a ground resistance
of 25Ω or less, a supplemental electrode shall not be re-
quired (NEC 2011).
(cid:2)(cid:2)(cid:2) A ground resistance of 25Ω or less ensures the
ground is significantly less than the resistance
Over Wrapped of the human body, which is on the order of 1–1,000kΩ.
Section
In facilities or applications involving sensitive equip-
Fig. 3. An example of a small cable section where
ment (e.g., the telecommunications industry), ground re-
the outer jacket is frayed and needs over wrapping
with brightly colored vinyl electrical tape as was sistance is typically set at 5Ω or less.
done below, but does not need to be removed from The goal in ground resistance is to achieve—and to
service and replaced, because the underlying insu- maintain over time—the lowest ground resistance value
lating jacket has not been compromised. possiblethatmakessenseeconomicallyandphysically. On
6