NEMA Standards Publication
Application Guide for Isolated Ground Wiring Devices
Published by:
National Electrical Manufacturers Association
1300 North 17th Street, Suite 1752
Rosslyn, Virginia 22209
www.nema.org
© Copyright 2007 by the National Electrical Manufacturers Association. All rights including translation into
other languages, reserved under the Universal Copyright Convention, the Berne Convention for the
Protection of Literary and Artistic Works, and the International and Pan American Copyright Conventions.
NOTICE AND DISCLAIMER
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development of this document.
The National Electrical Manufacturers Association (NEMA) standards and guideline publications, of which
the document contained herein is one, are developed through a voluntary consensus standards
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© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page i
CONTENTS
Section 1
1.1
1.2
1.3
1.4
Section 2
Foreword ....................................................................................................................................ii
APPLICATION CONSIDERATIONS ........................................................................................ 1
Why? ...................................................................................................................................... 1
Where? ...................................................................................................................................... 1
When? ...................................................................................................................................... 1
How? ...................................................................................................................................... 2
CHARACTERISTICS OF ISOLATED GROUND WIRING DEVICES ...................................... 3
Section 3
3.1
3.2
3.3
3.4
3.5
Section 4
4.1
4.2
4.3
Section 5
5.1
5.2
5.3
5.4
5.5
5.6
5.7
Section 6
6.1
6.2
6.3
MARKING AND IDENTIFICATION........................................................................................... 4
Ul Requirements........................................................................................................................ 4
CSA Requirements.................................................................................................................... 4
Federal Specification Requirements ......................................................................................... 5
Types ...................................................................................................................................... 5
Accessories ............................................................................................................................... 5
NATIONAL ELECTRICAL CODE REQUIREMENTS .............................................................. 7
General ...................................................................................................................................... 7
Basic Grounding and Bonding Requirements ........................................................................... 7
Isolated Ground Devices ........................................................................................................... 7
WIRING METHODS .................................................................................................................. 9
Metallic Raceway System.......................................................................................................... 9
Non-Metallic Raceway System................................................................................................ 10
Flexible Metallic Conduit, Armored Cable and Metal-clad Cable Systems............................. 11
Non-Metallic Cable Systems ................................................................................................... 11
Retrofit .................................................................................................................................... 12
Separately Derived Systems ................................................................................................... 12
Hospitals .................................................................................................................................. 12
MAINTENANCE, REPLACEMENT, SYSTEMS EXTENSIONS, TROUBLESHOOTING...... 13
Maintenance and Replacement............................................................................................... 13
System Extensions .................................................................................................................. 14
Troubleshooting....................................................................................................................... 14
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page ii
Foreword
This Application Guide for Isolated Ground Wiring Devices was developed by the Wiring Device Section
of the National Electrical Manufacturers Association.
In the preparation of this application guide, input of interested individuals and organizations has been
sought and evaluated. Inquiries, comments, and proposed changes are welcome and should be
submitted to:
Vice President, Engineering
National Electrical Manufacturers Association
1300 North 17th Street, Suite 1752
Rosslyn, Virginia 22209
The purpose of this application guide is to assist the user in proper installation and use of isolated ground
devices.
This standards publication was developed by the NEMA Wiring Device Section. Section approval of the
standard does not necessarily imply that all section members voted for its approval or participated in its
development. At the time it was approved, the group was composed of the following members:
BJB Electric L.P.—Ringgold, GA
Bryant Electric, Inc.—Milford, CT
BSafe Electrix, Inc.—Manhasset, NY
Cooper Wiring Devices—Long Island City, NY
Ericson Manufacturing Co.—Willoughby, OH
General Electric—Plainville, CT
Genlyte Thomas Group Llc—Garland, TX
Hubbell Canada L.P. —Pickering, ON
Hubbell Inc.—Orange, CT
Interpower Corporation—Oskaloosa, IA
Leviton Manufacturing Co., Inc.—Little Neck, NY
Lutron Electronics Company, Inc.—Coopersburg, PA
Pass & Seymour/Legrand—Syracuse, NY
Reliance Controls Corp.—Racine, WI
Schneider Electric/Square D Company—Lexington, KY
Technology Research Corp.—Clearwater, FL
The Watt Stopper, Inc.—Santa Clara, CA
The Wiremold Co.—West Hartford, CT
Thomas & Betts Corp.—Memphis, TN
Tyco Electronics/Amp—Harrisburg, PA
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page iii
Scope
This application guide covers wiring devices and accessories intended to help protect sensitive
equipment from malfunction due to noise on the equipment grounding path. Devices include receptacles,
single or duplex with straight blade or locking NEMA configurations, rated 15A to 60A and 125V to 600V.
Accessories include color-coded mating plugs and wall plates to identify isolated ground receptacles.
In addition to indicating the devices and accessories available for isolated ground installations, this guide
covers National Electrical Code® (NEC) requirements, applications for isolated ground circuits, and
trouble shooting and maintenance of such installations.
This application guide does not consider analysis of noise currents on the grounding system of power
supply circuits or their effects on sensitive electronic equipment.
NOTE—National Electrical Code and NEC are registered trademarks of the National Fire Protection Association.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page iv
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© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 1
Section 1
APPLICATION CONSIDERATIONS
1.1
WHY?
For an isolated ground system, an additional insulated/isolated ground conductor is required. When a
conventional receptacle is installed in a grounded metal box, the ground is common with the entire
building grounding system which may include the box, conduit, gas and water piping, ducting and
structural steel. It is grounded by using either a bonding jumper from the box to the green grounding
terminal or the self-grounding clip on the receptacle’s mounting strap. In a conventional receptacle, the
grounding contacts are connected to the mounting strap and the green grounding terminal. Thus, even
when a separate insulated equipment grounding wire is brought to the receptacle, it is still tied into the
entire grounding system. This occurs since the device strap is grounded to the outlet box.
The building grounding system can serve as a giant antenna and conductor of electrical noise. Noise, or
electromagnetic interference, is caused by numerous transient ground currents which produce random
electrical signals in the grounding system.
An isolated ground conductor is connected to the building grounding system at either 1) a sub-panel or 2)
the service panel or the separately derived source to keep the isolated ground receptacle relatively free of
electrical noise. The isolated ground network is intended to minimize sources of noise as it is connected
to the grounding system at a single point. Resonances and high impedance at noise frequencies can
occur in long runs of isolated ground wiring resulting in noise problems. Long runs can also result in an
impedance problem under fault conditions. Designers and installers should give careful consideration to
the point at which the isolated ground conductor is terminated to the building grounding system.
To achieve isolation of the receptacle ground from the normal grounding network, the grounding terminal
and contacts are isolated from the outlet box. The green grounding terminal is connected only to the
receptacle grounding contacts.
1.2
WHERE?
An isolated ground system is used where it is deemed necessary to minimize electrical interference on
the equipment grounding conductor. Applications are for power circuits supplying sensitive electronic
equipment such as point of sale terminals, accounting machines, word processors, electronic office
equipment, computers, and highly sensitive medical and communications equipment.
1.3
WHEN?
An isolated ground system should be used whenever the building grounding network is such that
transient currents can produce random electrical signals (voltages) on the grounding system of sensitive
electronic equipment. Such unwanted signals can be generated on the system by switching inductive
loads (magnetic coils, chokes, inductors) and by motor driven appliances and tools. Radio frequency
sources can also create unwanted signals that are picked up and carried by the grounding conductor.
Reduction of these signals can provide a corresponding reduction in the possibility of incorrect
commands or faulty readouts from the internal operation of the equipment. This can result in the benefits
of improved equipment performance, improved accuracy, and reduced downtime.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 2
1.4
HOW?
Design of isolated ground branch circuits, including the number of receptacles and the size of grounding
conductors, must comply with all applicable requirements of the NEC or local codes. In addition, the
designer should consider recommendations of the manufacturer of the utilization equipment, wiring
methods, the installation environment such as nearby or associated electrical equipment, and the lengths
of conductor runs.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 3
Section 2
CHARACTERISTICS OF ISOLATED GROUND WIRING DEVICES
Isolated ground devices help limit electrical noise on the equipment grounding circuit by providing two
grounding paths for the installation. The device grounding circuit terminal and grounding contacts are
isolated from the grounding path of the metal outlet box, device mounting means, and metal cover or wall
plate, thus establishing a separate, dedicated equipment grounding path for connecting noise-sensitive
equipment.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 4
Section 3
MARKING AND IDENTIFICATION
Proper identification of isolated ground receptacles is important to
the installer of the electrical system as well as the user of sensitive
electronic equipment. The installer must be able to properly identify
receptacles designed for use on isolated ground circuits and install
them correctly. The user needs to be able to easily identify
receptacles intended for use with sensitive electronic equipment.
Isolated ground receptacles are identified by an orange triangle on
the face as set forth by the NEC and nationally recognized
standards.
Although neither UL nor CSA requires the face of "Isolated Ground"
receptacles to be a specific color, most manufacturers conform with
the established industry standard orange face, orange face with an
orange triangle or a contrasting color face such as ivory, white,
brown, or gray with an orange triangle.
3.1
UL REQUIREMENTS
UL Standard ANSI/UL 498 requires that an isolated ground
receptacle be permanently marked with an orange-colored triangle
where visible after the receptacle and its cover plate are installed.
If the receptacle face is orange-colored, the triangle shall be
outlined in the contrasting color. The triangle has sides 5/32 inch (4
mm) or more in length.
A second UL requirement is that an isolated ground receptacle be
plainly marked on the device where visible during installation as
follows: "Isolated Ground" "CAUTION—Mounting means not
grounded. Grounding wire connection required," or an equally
definitive terminology following the word CAUTION. This
information is typically found on a label affixed to the back or side of
the isolated ground receptacle. It provides an important reference
to the installer that: 1) The device is intended for use on isolated
ground circuits and 2) The device cannot be self-grounded via a
grounded metal outlet box. FAILURE TO CONNECT THE
GROUNDING WIRE TO THE GREEN GROUNDING TERMINAL
WOULD RESULT IN AN UNGROUNDED RECEPTACLE.
3.2
CSA REQUIREMENTS
CSA Standard C22.2 No. 42 requires that an isolated ground
receptacle be marked with an orange-colored triangle on the face of
the receptacle. If the face of the receptacle is orange colored, the
triangle may be outlined in a contrasting color. The triangle has
sides 4mm or more in length, visible after the receptacle and cover
plate are installed. The wording "Isolated Ground" is marked in a
permanent manner on the device.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 5
3.3
FEDERAL SPECIFICATION
REQUIREMENTS
Federal Specification Sheet W-C-596/145,
recognizes a duplex hospital grade
receptacle with isolated ground. This device
is rated 15 Amp 125 Volts 50/60 Hertz, 2pole 3-wire. The face of the device is
required to be orange. The words "Isolated
Ground" may be shown in addition. Federal
specification isolated ground receptacles
must provide an isolated ground path when
used with metal covers or wall plates.
3.4
TYPES
In addition to basic type isolated ground
receptacles, hospital grade receptacles are
available with isolated ground. A green dot
on the face of a device indicates compliance
with hospital grade test requirements.
Transient voltage surge suppression
(TVSS) receptacles also are available with
isolated ground.
3.5
ACCESSORIES
A variety of accessories are available to aid
the identification of isolated ground outlets
and sensitive electronic equipment intended
to be plugged into those outlets. These
accessories are standard devices which
have been marked or color coded for
identification purposes to assist users. They
do not provide an isolated ground function.
Orange color-coded plugs installed on cordconnected sensitive equipment such as
computers, point of sale terminals,
electronic office equipment, and medical
and communications equipment remind the
user that the equipment should be plugged
into an isolated ground receptacle with an
orange face or orange triangle symbol.
Metal or plastic wall plates with the words
"Isolated Ground" on the face further
provide ease of identification of isolated
ground outlets.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 6
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 7
Section 4
NATIONAL ELECTRICAL CODE REQUIREMENTS
4.1
GENERAL
Two areas of the National Electrical Code are important to the understanding and application of isolated
ground devices. The first is the code's requirements for grounding and bonding. The other area is the
specific exceptions that permit isolated ground circuits and devices.
4.2
BASIC GROUNDING AND BONDING REQUIREMENTS
The basic code requirements for grounding and bonding are found in Article 250.
Specific grounding requirements for health care facilities are found in Article 517.
The underlying purposes of the code grounding requirements are:
•
•
•
To provide a low impedance path for fault current that will facilitate operation of the overcurrent
protective device;
To minimize voltage potential between grounded conductive surfaces to reduce shock hazard;
To provide a suitable path to earth for lightning.
These basic objectives are met through the specific application of a general philosophy requiring:
•
•
•
4.3
Connecting the grounded circuit conductor (neutral) and the equipment grounding conductor only at
the service entrance equipment;
Connecting the exposed metal parts of equipment to the equipment grounding system;
Bonding interior metal piping systems at the service entrance.
ISOLATED GROUND DEVICES
The code sections dealing with isolated ground devices are "exceptions" to the general rules concerning
the grounding system. They do not alter either the objectives or philosophy of the code. Rather, specific
provision is made for their installation consistent with the basic requirements.
The principle NEC provisions in Article 250 allows for isolated ground devices where such a device is
needed to reduce electrical noise. It must be run with the circuit conductors to an equipment grounding
conductor terminal at any point between the branch circuit source and the service or applicable
separately derived system.
Basic requirements for safety must still be maintained, including
• Grounding of exposed metal parts, boxes and wiring systems
• Metal face plates must be grounded.
Since the mounting system of isolated ground receptacles is isolated from the equipment ground wire,
Article 406 contains additional provisions regarding wall plates and non-metallic boxes.
• Isolated ground receptacles installed in non-metallic boxes shall be covered with a non-metallic face
plate.
• An exception is permitted to allow a metal face plate if the box contains a feature or accessory that
permits the effective grounding of the face plate.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 8
In metallic boxes the mounting system and wall plate are grounded through the normal means including
the use of self or auto-grounding clips.
Special precautions must be taken for application in patient care areas of health care facilities as
indicated in Article 517.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 9
Section 5
WIRING METHODS
5.1
METALLIC RACEWAY
SYSTEM
Since a metallic raceway system
serves as an equipment grounding
conductor, an additional grounding
conductor is normally optional. For
an isolated ground system, an
additional insulated/isolated ground
conductor is required.
In a metallic raceway system that
uses a supplemental equipment
grounding conductor and an isolated
ground conductor, the grounding
conductors should be readily
distinguishable from each other,
such as a green insulated conductor
for supplemental grounding and a
green with yellow striping insulated
conductor for the isolated ground.
Metal outlet boxes are used in
metallic conduit system branch
circuits. If a supplemental equipment
grounding conductor is used, it must
be bonded to each box.
The isolated ground conductor must
be connected to the grounding
terminal of the isolated ground
receptacle. This connection should
be made by means of an insulated
"pigtail" so that grounding continuity
is maintained when receptacles are
changed or removed, in accordance
with NEC Article 250.
The isolated ground conductor
should not be connected to the outlet
box since the isolation would be
defeated.
Care should be taken to ensure that
any uninsulated grounding conductor
in the box does not make contact
with the isolated ground terminal.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 10
5.2
NON-METALLIC
RACEWAY SYSTEM
Non-metallic systems with nonmetallic boxes by nature are not
grounded. Non-metallic systems are
required to have an equipment
grounding conductor to ground all
metallic parts of equipment supplied
by the circuit and grounding
terminals of receptacles on the
circuits. Metal wall plates should not
be used with isolated ground
receptacles mounted in non-metallic
boxes unless there is a feature or
accessory that effectively grounds
the face plate. If isolated ground
receptacles are the only receptacles
on the circuit, only an insulated/
isolated-ground conductor is
necessary. If other receptacles or
equipment are on the same circuit
with the isolated-ground receptacle,
then an additional equipment
grounding conductor is necessary.
The isolated equipment grounding
conductor is insulated to prevent
contact with grounded metal objects
other than the intended grounding
terminal. Care may be required to
maintain this insulation at a subpanel in accordance with NEC
Article 250.
If metal boxes, covers, wall plates,
or equipment are used in
conjunction with non-metallic
raceway, an equipment grounding
conductor is necessary to ground
metal boxes and all exposed metal
parts of the system, For isolatedground receptacles, a separate
insulated grounding conductor must
be run in the conduit system. The
grounding conductors must be
connected in the same way as
described in "Metallic Raceway
Systems" and, to preserve isolation
of the grounding circuit, must not be
interconnected.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 11
5.3
FLEXIBLE METALLIC
CONDUIT, ARMORED CABLE
AND METAL-CLAD CABLE
SYSTEMS
These wiring methods follow the
same rules as Section 5.1 Metallic
Raceway Systems.
The maximum permitted length of a
flexible metallic conduit without an
equipment grounding wire is six
feet. When flexible metallic conduit
is used in an isolated-ground
system, an additional insulated
grounding conductor must be
installed and identified as the
isolated-ground conductor.
Armored cable (AC) and metal-clad
cable (MC) have built-in equipment
grounding means which bond to the
outlet box via an appropriate box
connector. When these cables are
used as an isolated ground system,
an insulated conductor within the
cable must be used as the isolated
ground conductor.
Reidentification, when required, is
accomplished according to NEC
Article 250.
5.4
NON-METALLIC CABLE
SYSTEMS
Isolated ground can only be
obtained in systems using nonmetallic cable if the cable has an
insulated grounding conductor.
NOTE—Metal wall plates should not be
used with isolated ground receptacles
mounted in non-metallic boxes unless
there is a feature or accessory that
effectively grounds the face plate.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 12
5.5
RETROFIT
Metal outlet boxes must be grounded. A separate insulated isolated ground conductor must be installed
in a manner suitable for the wiring method used. NEVER use an isolated ground receptacle to replace a
conventional receptacle in a conventionally grounded (non-IG) system in an ungrounded system.
5.6
SEPARATELY DERIVED SYSTEMS
There are no requirements in NEC Article 250 in conflict with isolated ground principles. The isolated
ground conductor must terminate on the load side of the separately derived system rather than being
carried back to the service.
5.7
HOSPITALS
It should be noted that isolated ground circuits have no relationship to isolated power systems as
described in NEC Article 517.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 13
Section 6
MAINTENANCE, REPLACEMENT, SYSTEMS EXTENSIONS, TROUBLESHOOTING
6.1
MAINTENANCE AND REPLACEMENT
As with all electrical wiring devices, effective maintenance for isolated ground receptacles begins with
careful selection and proper initial installation. Guidance for choosing the most suitable unit for a specific
installation can usually be found in literature available from manufacturers of isolated ground devices. It is
vital for the safe performance of the device in its intended manner that it is installed completely according
to the instructions of the manufacturer, including circuit rating and polarity of the circuit conductors.
At any time when it is determined that an isolated
ground device is misapplied, improperly installed,
or damaged, it must be replaced or the improper
condition corrected. When not functioning
properly, an isolated ground receptacle may
present hazards to personnel or to the electronic
equipment it is thought to be protecting from
ground circuit noise. Periodically each isolated
ground receptacle should be checked electrically
for ground continuity and visually examined for
signs of damage. In addition, as for all
receptacles, thorough maintenance requires
ongoing verification that the device contacts firmly
grip an inserted plug; the installation remains
complete, with proper cover or plate, and securely
fastened to the outlet box; and that the device is
not warm to the touch due to loose connections or
excessive loading on the branch circuit. For
assurance that the entire isolated ground system
is safe and suitable for use, routine maintenance
procedures should be applied as well to the power
supply cords and extension cords that are
connected to isolated ground receptacles. Cords,
plugs, and connectors should be free of damage.
Strain relief clamps of plugs and connectors must
grip cord jackets tightly and across the full width
and depth of the clamp. Plugs and connectors
must be used only with devices of compatible
configuration. It is NEVER proper to modify a
connecting device to achieve unintended
connectability. Any sign of alteration of a blade or
a slot calls for immediate replacement.
© Copyright 2007 by the National Electrical Manufacturers Association.
Application Guide for Isolated Ground Wiring Devices 2007
Page 14
6.2
SYSTEM EXTENSIONS
Extensions of isolated ground circuits are accomplished in the same manner as for conventional circuits.
Downstream outlets may be hard-wired onto the system, either concealed or in surface raceways, so long
as the extension wiring conforms to isolated ground wiring principles. Portable cord extensions from an
isolated ground outlet should be made with care not to defeat the grounding isolation of the supply circuit.
For example, using an isolated ground outlet or extension to connect a water cooler or humidifier might
introduce noise onto the grounding wires of the entire circuit.
6.3
TROUBLESHOOTING
Troubleshooting an isolated ground circuit consists of reviewing recommended installation practices for
the wiring method used and verifying that they have been followed, plus carrying out a thorough
maintenance procedure to assure that all components of the grounding system are functional and safe.
It is beyond the scope of this guide to consider analysis of noise currents on the grounding system of
power supply circuits or effects of noise on sensitive electronic equipment.
§
© Copyright 2007 by the National Electrical Manufacturers Association.