NEMA Standards Publication Application Guide for Isolated Ground

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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 The information in this publication was considered technically sound by the consensus of persons engaged in the development and approval of the document at the time it was developed. Consensus does not necessarily mean that there is unanimous agreement among every person participating in the 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 development process. This process brings together volunteers and/or seeks out the views of persons who have an interest in the topic covered by this publication. While NEMA administers the process and establishes rules to promote fairness in the development of consensus, it does not write the document and it does not independently test, evaluate, or verify the accuracy or completeness of any information or the soundness of any judgments contained in its standards and guideline publications. NEMA disclaims liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on this document. NEMA disclaims and makes no guaranty or warranty, expressed or implied, as to the accuracy or completeness of any information published herein, and disclaims and makes no warranty that the information in this document will fulfill any of your particular purposes or needs. NEMA does not undertake to guarantee the performance of any individual manufacturer or seller’s products or services by virtue of this standard or guide. In publishing and making this document available, NEMA is not undertaking to render professional or other services for or on behalf of any person or entity, nor is NEMA undertaking to perform any duty owed by any person or entity to someone else. Anyone using this document should rely on his or her own independent judgment or, as appropriate, seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. Information and other standards on the topic covered by this publication may be available from other sources, which the user may wish to consult for additional views or information not covered by this publication. NEMA has no power, nor does it undertake to police or enforce compliance with the contents of this document. NEMA does not certify, test, or inspect products, designs, or installations for safety or health purposes. Any certification or other statement of compliance with any health or safety-related information in this document shall not be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement.

© 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



© 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.

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