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Information Technology Industry Council (ITI, formerly CBEMA)

Information Letter

GUIDELINES FOR GROUNDING INFORMATION TECHNOLOGY EQUIPMENT(ITE)

Ninety percent (90%) of the problems withITE installations are internal to the facility;only 10% are related to conditions on theutility electric service. Importantly, 75% ofthe problems arising within a facility arerelated to grounding making proper andadequate grounding the single-mostimportant factor in reliable ITE systemperformance.

Purpose: To establish guidelines forgrounding of power systems and equipmentfor ITE installations.

Scope: Computer room systems, distributedprocessing systems and stand alone systems.

Objective: To achieve a low impedance,common ground reference for allinterconnected ITE (InformationTechnology Equipment) to prevent orreduce the likelihood of:

1. electric shock hazard;

2. equipment damage fromtransient voltage events;

3. data contamination or upsetfrom noise voltage andcurrents in groundingconnections.

Items 1 and 2 are typically due to lowfrequency (<100kHz) events; item 3 istypically due to high frequency (>100kHz)events.

Safe grounding is a legal requirement underOSHA Subchapter S encompassing theNational Electrical Code (NEC) as well asstate and local code statutes.

Safety Code Requirements. Therequirements stated here are those of theU.S. 1996 National Electrical Code (NFPA70) most directly applicable to thegrounding of ITE. It is not an exhaustive list.Most jurisdictions adopt the National

Electrical code as a local requirement, butmay have additional requirements as well.The local electrical inspector has the powerto enforce electrical code requirementswithin his or her jurisdiction.

NEC 250-5 Alternating Current Systems tobe grounded.

(b) Ground AC premises wiring systems of50-1000 volts under any of thefollowing conditions:

(1) Where the maximum voltage toground on the ungroundedconductors does not exceed 150volts;

(2) Where neutral is used as a circuitconductor in a 3-phase, 4-wiresystem;

(3) Where the midpoint of one phase isused as a circuit conductor in a 3-phase, 4-wire delta connectedsystem;

(4) Where the grounded serviceconductor is uninsulated inaccordance with the Exceptions toSections 230-22, 230-30, and 230-41.

(d) Separately Derived Systems. Apremises wiring system whose power isderived from generator, transformer, orconverter windings and has no directelectrical connection, including asolidly connected grounded circuitconductor (neutral), to supplyconductors originating in anothersystem, if required to be grounded in (b)above, shall be grounded as specified inSection 250-26.

NEC 250-21 Objectionable Current overGrounding Conductors.

(b) If the use of multiple groundingconnections results in an objectionable

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flow of current, one or more of thefollowing alterations shall be made.

(1) Discontinue one or more, but notall such connections;

(2) Change the locations of thegrounding connections;

(3) Interrupt the continuity of theconductor or conductive pathinterconnecting the groundingconnections;

(4) Take other suitable remedial actionas permitted by the localjurisdiction.

(d) Limitations to Permissible Alterations.The provisions of this section shall notbe considered as permitting electronicequipment being operated on acsystems or branch circuits that are notgrounded as required by this Article.Currents that introduce noise or dataerrors in electronic equipment shall notbe considered as the objectionablecurrents addressed in this section.

NEC 250-23

(a) System Grounding Connections. ....Thegrounding electrode conductor shall beconnected to the grounded conductor(neutral) at any accessible point fromthe load end of the service drop...to andincluding the terminal or bus to whichthe grounded service conductor isconnected at the service disconnect.... Agrounding connection shall not be madeto any grounded circuit conductor(neutral) on the load side of the servicedisconnect.

In a separately derived system, (e.g.distribution transformer) a groundingelectrode conductor shall be connected tothe grounded conductor (neutral) inaccordance with NEC 250-26.

NEC 250-26 A separately derived systemrequired to be grounded by NEC 250-5 shallbe grounded as follows:

(a) A bonding jumper of appropriate sizeshall connect the equipment groundingconductor (green wire) of the derivedsystem to the derived system grounded

conductor (neutral). This connectionshall be made at any point on theseparately derived system from thesource to the first system disconnectionmeans or overcurrent device.

A grounding electrode conductor ofappropriate size shall be used to connectthe grounded conductor (neutral) to thegrounding electrode. ...This connectionshall be made at any point on theseparately derived system from thesource to the first system disconnectingmeans or overcurrent device....

(c) The grounding electrode shall be asnear as practicable to the system to begrounded and shall be:

(1) the nearest effectively groundedstructural metal member of thestructure;

(2) the nearest effectively groundedmetal water pipe;

(3) other electrodes specified in 250-81or 250-83 where (1) and (2) aboveare not available.

NEC 250-51 Effective Grounding Path. Thepath to ground from circuits, equipment andmetal enclosures shall (1) be permanent andcontinuous; (2) have capacity to conductsafely any fault current likely to be imposedon it; and (3) have sufficiently lowimpedance to limit the voltage to ground andto facilitate the operation of the circuitprotection devices.

The earth shall not be used as the soleequipment grounding conductor.

NEC 250-53 Grounding Path to GroundingElectrode at Services

(a) Grounding Electrode Conductor. Agrounding electrode conductor shall beused to connect the equipmentgrounding conductors, the serviceequipment conductors, and where thesystem is grounded, the groundedservice conductor to the groundingelectrode.

(b) Main Bonding Jumper. For a groundedsystem. an unspliced main bondingjumper shall be used to connect the

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equipment grounding conductor and theservice-disconnect enclosure to thegrounded conductor of the system ateach service disconnect.

NEC 250-54 Common GroundingElectrode. Where an ac system is connectedto a grounding electrode in or at abuilding,...the same electrode shall be usedto ground conductor enclosures andequipment in or at that building. Whereseparate services supply a building and arerequired to be connected to a groundingelectrode, the same grounding electrodeshall be used.

Two or more grounding electrodes that areeffectively bonded together shall beconsidered as a single grounding electrodesystem in this sense.

NEC 250-74 Connecting ReceptacleGrounding Terminal to Box.

Exception 4: Where required forthe reduction of electrical noise(electromagnetic interference) on thegrounding circuit, a receptacle in whichthe grounding terminal is purposelyinsulated from the receptacle groundingterminal shall be grounded by aninsulated equipment groundingconductor run with the circuitconductors. This grounding conductorshall be permitted to pass through oneor more panelboards without connectionto the panelboard grounding terminal aspermitted in Section 384-20 Exceptionso as to terminate directly at anequipment grounding conductorterminal of the applicable system orservice.

FPN: Use of an isolated equipmentgrounding conductor does not relievethe requirement for grounding theraceway system and outlet box.

NEC 250-75 Bonding Other Enclosures.

Exception: Where required for thereduction of electrical noise(electromagnetic interference) on thegrounding circuit, an equipmentenclosure supplied by a branch circuitshall be permitted to be isolated from a

raceway containing circuits supplyingonly that equipment by one or morelisted nonmetallic raceway fittingslocated at the point of attachment of theraceway to the equipment enclosure.The metal raceway shall comply withthe provisions of this article and shallbe supplemented by an internalinsulated equipment groundingconductor installed in accordance withSection 250-74, Exception 4 to groundthe equipment enclosure.

FPN: Use of an isolated equipmentgrounding conductor does not relievethe requirement for grounding theraceway system.

NEC 250-81 Grounding Electrode System.If available on the premises at each buildingserved, each of a-d below and any madeelectrodes in accordance with NEC 250-83must be bonded together to form agrounding electrode system...

(a) Metal underground water pipe locatedless than 5 ft. from its entrance to thebuilding (when augmented by at leastone of b-d below or made electrodes perNEC 250-83);

(b) Metal frame of the building (whereeffectively grounded);

(c) Concrete encased electrode (generallyformed by the rebar in the buildingfoundation);

(d) Ground ring.

NEC 250-83 Made and Other Electrodes.Where none of the above are available, oneor more of the electrodes in b-d below shallbe used.

(a) A metal underground gas piping systemshall not be used as a groundingelectrode;

(b) Other local underground systems andstructures;

(c) Rod or pipe electrodes;

(d) Plate electrodes.

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NEC 250-91

(b) The equipment grounding conductorrun with or enclosing the circuitconductors shall be one or more ... ofthe following: (1) copper or othercorrosion resistant conductor ...insulated, covered or bare; solid orstranded or busbar; (2) rigid metalconduit, (3) intermediate metal conduit,(4) electrical metallic tubing, (5)flexible metal conduit and fittingswhere listed for such service, (6) armorof Type AC cable, (7) copper sheath ofmineral insulated metal shielded cable,(8) metallic sheath and/or groundingconductors of Type MC cable, (9) cabletrays as permitted in Sections 318-3(c)and 318-7, (10) cablebus framework aspermitted in Section 365-2(a)....

(c) Supplementary grounding electrodesshall be permitted to augmentequipment grounding conductors(provided all of the NEC groundingconductors are installed per applicablesections of Article 250), but earth shallnot be used as the sole groundingconductor.

In other words, extra driven ground rod orrods may be used to augment the groundingsystem, but if used, must be connected usingan acceptable grounding electrode conductorto other grounding electrodes listed above.An isolated earth ground is prohibited by theNEC.

NEC 384-20 ....In panelboards, equipmentgrounding conductors shall not be connectedto a terminal bar provided for groundedconductors (neutrals) unless the bar isidentified for the purpose and is locatedwhere connection is made from thegrounded conductor to a groundingelectrode conductor as permitted by Article250.

NEC 645-15 Electronic computer/dataprocessing equipment (in an electroniccomputer/data processing room) shall begrounded in accordance with Article 250 ordouble insulated. Power systems derivedwithin listed electronic computer/dataprocessing equipment that supply electronic

computer/data processing systems throughreceptacles or cable assemblies supplied aspart of this equipment shall not beconsidered separately derived for thepurpose of applying Section 250-5(d). Allexposed, noncurrent carrying metal parts ofan electronic computer/data processingsystem shall be grounded.

OTHER POWER SYSTEMS

Power systems with the neutral conductorsolidly bonded to the grounding electrodeconductor at an on-premises point in thedistribution system, may not always be thecase particularly at locations outside NorthAmerica. Meeting the objective of this paperis still a requirement for acceptable systemperformance. Follow the recommendationsas closely as possible as permitted by localor national electrical codes.

Described below are five recognized powergrounding schemes. In internationalterminology, the Grounding Conductor (G)is termed the “Protective Earth Conductor(PE)”.

TN (Terra Neutral) Power Systems arethose systems having one point directlyconnected to Ground (Earth) by ProtectiveEarth Conductors. There are three types ofTN power systems:

TN-S (Terra Neutral-Separate) - SeparateNeutral and Protective Earth conductorsexist throughout the system.

TN-S Power System

L1

L2

L3

N

PE

Exposed Conductive Parts

FIGURE 1

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TN-C-S (Terra Neutral-Combined-Separate)– Neutral and Protective Earth functions arecombined in a single conductor in part of the

system.

L1

L2

L3

PEN

Exposed Conductive Parts

N

PE

TN-C-S Power System

FIGURE 2

TN-C (Terra Neutral-Combined) – Neutraland Protective Earth functions are combinedin a single conductor throughout the system.

TN-C Power System

L1

L2

L3

PEN

Exposed Conductive Parts

FIGURE 3

TT (Terra Terra) Power Systems are thosehaving one point directly connected toGround (Earth), but the exposed conductiveparts of the installation are connected togrounding electrodes independent of thegrounding electrodes of the power system.

TT Power System

L1

L2

L3

N

Exposed Conductive Parts

PE

FIGURE 4

IT (Impedance Terra) Power Systemshave no direct connection to Ground (Earth),the exposed conductive parts of theinstallation are directly connected toGround. Also called an impedance groundedsystem.

IT Power System

L1

L2

L3

N

Exposed Conductive Parts

PE

Impedance

FIGURE 5

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GROUNDING METHODS

General

1. In panelboards up to 100A servingIT equipment, an insulated groundingconductor the same size and run in the sameraceway as the feeder conductors should beused. In panelboards >100A, size theinsulated grounding conductor in accordancewith NEC Table 250-95, except thegrounding conductor should never besmaller than #4.

If the panelboard is not the point whereneutral and equipment ground areconnected, then separate neutral andequipment ground buses must be used.

2. Metal raceways (conduits, etc.)should not be used as the sole groundingconductor for ITE circuits. An insulatedequipment grounding conductor is alwaysrecommended for feeder circuits and branchcircuits serving IT equipment.

3. IT equipment should be poweredfrom dedicated branch circuits whereverpossible. A dedicated branch circuit not onlyhas its own breaker, but also its owngrounding conductor (ground) and groundedconductor (neutral) if used; neither of whichis to be shared with other circuits.

4. For cord connected equipment,dedicated branch circuits for ITE may beterminated in IG (isolated ground)receptacles. Doing so eliminates multipleground paths which may be a source ofnoise in the circuit. IG circuits are not apanacea for all branch circuit groundingconcerns. They are most effective whereserved from dedicated, separately derived,locally grounded sources. IG circuits willnot improve grounding conditions whenserved from sources which, due to improperwiring or faulty load equipment, alreadyhave currents flowing on groundingconductors.

5. In branch circuits serving ITE, agrounding conductor (green wire or greenwire with yellow stripe) the same size as thecircuit conductors shall be run in the samemetal raceway with the circuit conductors.

6. When IT equipment is served by aseparately derived system (such as atransformer), that system should begrounded with the neutral-ground bondmade as in Figure 6A. IT equipment usesthe equipment grounding conductor as alogic reference and that reference mustbe stable. For all applications, it ispreferred that distance "D" be as short aspossible.

FIGURE 6-A

N

G

RECOMMENDED

Conduit1st System Disconnect

PanelboardSeparately Derived SystemEnclosure

D

NEC 250-26(c)Grounding Electrode

NOT

FIGURE 6-B

N

G

RECOMMENDED

Conduit1st System Disconnect

PanelboardSeparately Derived SystemEnclosure

D

NEC 250-26(c)Grounding Electrode

It is not recommended to make the neutral-ground bond as in Figure 6B because currentflowing in the grounded conductor (neutral)between the panelboard and separatelyderived system (transformer) causesinstability (noise) in the ground reference.Furthermore, if the ground bus in thepanelboard is not insulated from thepanelboard, the panelboard, conduit andseparately derived equipment enclosureforma parallel neutral current return path.Parallel neutral current return paths also

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exist via equipment grounding conductors inbranch circuit-connected equipment.

7. Where IG equipment groundingconductors are used, they may be terminatedto a separate insulated grounding bus in thepanelboard. The insulated bus is thenconnected using an insulated equipmentgrounding conductor to the Xo groundingpoint of the transformer

8. Many UPS (uninterruptible powersupplies) with bypass circuits; MG (motorgenerators) with bypass circuits; standby oremergency engine generators; are solidlyinterconnected systems, not separatelyderived systems. The output groundedconductor (neutral) is solidly interconnectedto the service supplied (utility) systemgrounded conductor. (See Figure 7)

A BCNGTO CRITICAL LOADS

SOURCE

4 POLE TRANSFER SWITCH

SEPARATELY DERIVED SYSTEM

A B C N G

TO CRITICAL LOADS

SOURCE

SOLIDLY INTERCONNECTED SYSTEM3 POLE TRANSFER SWITCH

STD AC

STD AC

SOURCE

ALTERNATEAC

ALTERNATEAC

SOURCE

FIGURE 7

Computer Rooms (See Figure 8)

1. All panelboards, groundingconductor connections to informationtechnology equipment, grounded data cablespenetrating the room, transient suppressorplates, if used, and zero signal referencegrid, if used, should be bonded to a commonground reference.

2. A dedicated power source such as anisolation transformer or PDU (powerdistribution unit) should be used to power allequipment in a data processing room.Ground computer room power sources inaccordance with NEC 250-26 or 645-15.

3. Dedicated power sources andpanelboards serving computer rooms shouldbe located in, or as near as possible to, thecomputer room.

When multiple power sources (isolationtransformers, etc.) supply power to a dataprocessing room, bond together groundingelectrode conductors from each unless doingso results in objectionable current flow inthe grounding system.

4. Branch circuit grounding conductors,raceways, grounding electrode conductorsmay not have low enough impedance toeffectively ground high frequency signals. Ifsuch is the case, data contamination andmis-operation may result from noise voltagepropagating on grounding conductors.

If acceptable to the ITE manufacturer, aSRG (signal reference grid) may be used toprovide a nearly constant potential, lowimpedance, high frequency, signal referencegrounding system. An effective SRG may beformed using the 2 ft. x 2 ft. stringers of araised floor. The stringers must be boltedtogether and suitably plated to give lowresistance. A stringerless raised floor maynot be used for this purpose as the floorpanels do not form a solidly interconnectedgrid. As an alternative, a 2 ft. x 2 ft. SRGusing 2" strips of continuous sheet copper,aluminum, zinc plated steel, or any numberof pure and composite metals with goodsurface conductivity may be locally made orpurchased finished. The SRG is bonded tothe metal cabinets of all IT equipment usinga flat, braided copper strap as short aspossible, preferably 2 ft. or less in length,and for safety purposes, to the groundingconductor serving the dedicated ITEpanelboard(s). Such a grid will effectivelyground high frequency signals up to about20Mhz.

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FIGURE 8

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Distributed Processing Systems (SeeFigure 9)

Distributed processing systems and localarea networks (LAN's) are those withmultiple processing units separated andremote from each other. These individualprocessors may be powered from the sameor separate panelboards on the same electricservice, or separate services in separatestructures.

When processors or peripherals of adistributed processing system are located inseparate buildings powered from separateelectric services; if practicable, bondtogether the grounding electrode conductorsof each service using existing metallicstructures such as rigid metallic conduit,water pipes, etc. or pull a #4 or larger buriedcopper conductor so that earth is not the solegrounding connection between/amongservices. The metallic connection willattenuate steady state low frequencypotential differences between or amongservices. It will not adequately attenuatetransient impulse potential differences.Transient suppressors, optical isolators orfiber optic links are necessary to eliminateor reduce transient impulse activity totolerable levels.

When system components are located in thesame structure but powered from separatepanelboards and perhaps from distinctseparately derived systems, multiple groundpaths are present. These paths arise fromplanned connections (raceway and groundwire connections to the service entrance),incidental connections (raceway physicalconnections to structural steel, pipingsystems etc.) and data cable connectionsbetween/among IT equipment.

Dedicated circuits are stronglyrecommended for ITE circuits, and IGcircuits may be recommended to limitground paths and noise pickup in thosepaths. To further reduce ground noiseproblems, data cables can be electricallydecoupled using line drivers, multiplexors,modems, optical isolators, fiber optic links,etc. Any such practice must have theapproval of the ITE manufacturer to assure

it does not defeat EMI (electromagneticinterference) control mandated by the FCC.

Figure 9 Legend:

SE Service Entrance

XFMR Transformer

DP Distribution Panel

Solid Lines Power Wiring

Dashed Lines Data Cabling

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FIGURE 9

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DEFINITIONS

Ground: A conducting connection, whetherintentional or accidental, between anelectrical circuit or equipment and the earth,or to some conducting body that serves inplace of the earth.

Grounded conductor: A system or circuit(current carrying) conductor that isintentionally grounded. The neutral wire.(white, domestic; blue, international).

Grounding conductor: A conductor used toconnect equipment or the grounded circuitof a wiring system to groundingelectrode(s). This conductor is not intendedto be a current carrying conductor except infault conditions. The green wire (domestic),green with yellow stripe wire (international);or, the conduit or raceway are groundingconductors.

Grounding electrode conductor: Theconductor used to connect the groundingelectrode (NEC 250-81) to the equipmentgrounding conductor and/or to the groundedconductor of the circuit at the serviceequipment or at the source of a separatelyderived system.

Premises wiring: Interior and exteriorwiring, including power, lighting, control,and signal circuit wiring together with all itsassociated hardware, fittings, and wiringdevices, permanently or temporarilyinstalled, which extends from the servicepoint of utility conductors or source of aseparately derived system to the outlet(s).Such wiring does not include wiring internalto appliances, fixtures, motors, controllers,motor control centers, and similarequipment.

Raceway: An enclosed channel designedexpressly for holding wires, cables, orbusbars, with additional functions aspermitted by the NEC.

Separately derived system: See NEC 250-5(d) and NEC 250-26.

Isolation transformer: A transformerhaving electrical insulation and possiblyelectrostatic shielding between its windings

such that it can provide isolation betweenparts of the system in which it is used.

Power distribution unit (PDU): A device,usually isolation transformer based,equipped with input and output circuitbreakers, output receptacles or cablestogether with associated control and signalcircuits. Such devices are often safetyagency listed as electroniccomputer/information technology equipmentor part of such a system.

Terra: French term meaning earth orground. Used in international descriptions ofpower distribution systems.

References:

ANSI/IEEE Std 142-1991, "Green Book," IEEE Recommended Practice for Grounding of Industrial

and Commercial Power Systems .

ANSI/IEEE Std 446-1987, "Orange Book," IEEE Recommended Practice for Emergency and Standby Power for Industrial and Commercial Applications .

ANSI/NFPA 70 National Electrical Code, 1996 ,August 18, 1995, National Fire ProtectionAssociation, Batterymarch Park, Quincy, MA.

FIPS PUB 94, "Guideline on Electrical Power forADP Installations," September 21, 1983, U.S. Dept.of Commerce, N.I.S.T.

IEC 950, Safety and Energy Efficiency ofInformation Technology Equipment, Edition 2, 1995,International Electrotechnical Commission, GenevaSwitzerland.

IEEE Std. 1100-1992, "Emerald Book," IEEE Recommended Practice for Powering and Grounding of Sensitive Electronic Equipment .