Danby DPR2260-2 User Manual

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112
Home Power #72 • August / September 1999
Code Corner
To Ground
or Not to
Ground:
That is
Not
the
Question (in the USA)
John Wiles
Sponsored by the Photovoltaic Systems Assistance Center,
Sandia National Laboratories
“Even most 12 volt PV systems shall be
grounded in some way,” sayeth ye
National Electrical Code (NEC).
This
Code Corner will begin with the code
requirements for a PV system from the
ground up.
Subsequent articles will discuss the code requirements for PV
hardware on the roof and for the components in between. The
terminology, the whys, and the hows of grounding PV systems
will be addressed. Proper grounding will enhance safety (both
user and equipment), improve performance, and may even
reduce costs. Article 250 (Grounding) was completely revised
in the 1999 edition of the
NEC
and references to Article 250
will refer to the new edition.
Why?
Research and experience have indicated that both grounded
and ungrounded electrical systems can be safe. Europeans
have a 100 year tradition of operating ungrounded electrical
power systems, and European codes reflect this. The
European Community also makes use of double-insulated
components and electronic ground-fault detectors in many
applications.
Reports in the U.S. have indicated that on ungrounded DC
electrical systems (non-PV) used by electrical utilities, when a
ground fault occurs that is undiscovered or unrepaired, a
second ground fault will frequently occur for the same reason
within two weeks. Double ground faults create problems
because overcurrent devices may not sense them and may
not offer protection. The utilities always rely on extensive
electronic ground-fault detectors in their ungrounded DC
electrical systems.
In the United States, the arguments for and against grounding
were carried out over several decades as the
NEC
was being
developed, just before the turn of the century. Unlike the
Europeans, the U.S. decided to require the use of grounded
systems. For those who want to achieve a good
understanding of the subject of grounding, the
NEC
Handbook
and the International Association of Electrical
Inspectors (IAEI)
Soares Book on Grounding
are
recommended (see Access). The IAEI book even gives some
of the history of grounding requirements in the U.S.
Electrical systems in the U.S. (including PV systems) are
generally solidly grounded to limit the voltage with reference
to ground during normal operation, and to prevent excessive
voltages due to surges from lightning or unintentional cross
connections with higher voltage lines.
In PV systems, the modules are usually mounted in high,
exposed locations where they are prone to picking up induced
surges from nearby lightning strikes. Utility-interactive
inverters are also subjected to surges on utility power lines. In
addition, systems using PV power to run computers with
hardwired modems are subject to surges from the telephone
line. Proper grounding effectively reduces these potential
problems and more.
Terminology
The term “grounded” indicates that one or more parts of the
electrical system are connected to the earth, which is
considered to have zero voltage or potential. The earth is
used as a reference because there is so much of it, and many
conductive surfaces are connected or in contact with it. Most
metallic objects such as metal building frames, as well as
other electrical/electronic systems (TV, telephones, etc.) are in
contact with or connected to earth. In some areas, the term
“earthing” is used instead of “grounding.”
Plumbing (including bathtubs and sinks) used to be solidly
grounded. But now it may be not connected to ground
because of the use of plastic pipes and drains. When faults
occur in electrical systems, those faults are frequently faults to
earth (ground faults). To better understand the grounding
requirements of the
NEC,
it is necessary to examine several
terms used in conjunction with grounding.
A
grounded conductor
is a conductor that normally carries
current and is connected to the earth. Examples are the
neutral conductor in AC wiring and the negative conductor in
many DC systems. Note that some DC systems such as
telephone systems connect the positive conductor to ground
rather than the negative conductor. A system is a “grounded
system” when one of the current-carrying conductors is
grounded.
An
equipment-grounding conductor
is a conductor that does
not normally carry current (except under fault conditions) and
is also connected to earth. It is used to connect the exposed
metal surfaces of electrical equipment together and then to
ground. Examples are the bare conductor in non-metallic
sheathed cable (Romex
®
) and the green, insulated conductor
in power cords for portable equipment. These equipment-
grounding conductors help to prevent electrical shocks and
allow overcurrent devices to operate properly when ground
faults occur.
A
grounding electrode conductor
is the conductor between a
common single grounding point in the system and the
grounding electrode. Splices are not normally made in this
conductor. The
common single grounding point
is a point
where the grounded conductor and the equipment-grounding
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Summary of Contents

Page 1 - (in the USA)

112Home Power #72 • August / September 1999Code CornerTo Groundor Not toGround:That is NottheQuestion (in the USA)John WilesSponsored by the Photovo

Page 2 - Code Corner

113Home Power #72 • August / September 1999Code Cornerconductors are connected to thegrounding electrode conductor.Agrounding electrodeis the metall

Page 3

114Home Power #72 • August / September 1999Code CornerPrior to the 1999 edition, the NEC did not have Table 690-7,and the voltage multiplication fac

Page 4

115Home Power #72 • August / September 1999Code CornerThe connection of the equipment-grounding conductor canrun from module frame to module frame a

Page 5

116Home Power #72 • August / September 1999Code Cornerprotection is required where this conductor may be subject tophysical abuse. However, if multi

Page 6 - Refrigerator

117Home Power #72 • August / September 1999Code Corneroperated by Sandia Corporation, a Lockheed MartinCompany, for the United States Department of

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