cse-mag-200503

| PURE POWER // SPRING 200512

Grounding PointsSingle or Multi?Use of unconventional hybrid energy sources, along with the

increasing demand for uninterrupted power, is fueling unique

power quality issues

BY RAJAN BATTISH, P.E.Principal and VINCENT SATURNO, EIT, LEEP APRTKL Assocs., Baltimore

Perhaps no electrical engineering con-cept is as unclear, unexplained andmisunderstood as grounding. Many ofthese misunderstandings result frommethodologies and practices that havefloated around the building designindustry for years. In fact, some ofthese grounding approaches are evendirect opposites.

But one argument that consistentlyarises among engineers is the issueof single-point vs. multi-pointgroundingwhich is better? (For ageneral overview of grounding andwhy it is necessary, see GroundEssentials, p. 16).

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Single-point grounding means exactlywhat its name implies. Electrical,telecommunications and IT systemsare all grounded at a single point. Inmulti-point grounding, these systemsare grounded at multiple points.

SINGLE-POINT Figure 1 (p. 14) demonstrates a typicalsingle-point grounding system,complete with power andtelecommunications system grounds.The entire grounding network is takenback to building grounding at acommon point. The main electricalground bar (MEGB) is used as the hubof the grounding network to thebuilding. The MEGB is bonded to theneutral bus of the switchgear, which inturn is bonded to the ground bus, coldwater pipe, building steel, theswitchgear enclosure and ground rod.This achieves zero reference (seeGrounding Essentials) and puts theentire building grounding system atbuilding potential.

There are several advantages tousing this single-point design. Forexample, if a phase-to-ground faultoccurs at a piece of equipment alongthe electrical distribution system, a rel-atively controlled low-impedance pathis provided back to the source. Thefault current has limited routes back tothe source and does not have theopportunity to diverge to multiplepaths, creating parallel circuits. If mul-tiple paths were introduced, the faultcurrent would divide itself among thepaths based on the impedance of thecircuits.

Single-point grounding also limitsthe ground loops that occur whenmore than one conductive path existsbetween two points. If the electricalequipment is not only bonded toground bars, as in Figure 1, but to var-ious building steel columnsif avail-ableand if the grounding potential isdifferent in the building due to anelectrical storm, on a multi-pointgrounding system, noise could circu-late, cause error in equipment opera-

tion and become difficult to isolate.If power for IT equipment is served

from a separately derived source inde-pendent of the building ground sys-tem, noise might be generated on thesystem. This is a form of commonmode noise, in which the power-source ground is referenced to a differ-ent point from the equipment ground.These stray currentsin other words,noisecan find their way ontogrounded equipment, thus energizingthe equipment.

WHEN LIGHTNING STRIKESAnother advantage of a single-pointgrounding system is its effectiveness athandling ground potential rises, whichoccur when a lightning storm passesover the building and causes electricaldisruption. A lightning strike, or theelevation of a buildings potential dueto an electrical storm, can cause risesand falls of potential in the buildinggrounding system. When electricalcomponents are grounded at differentpoints, each point can have differentpotentials from other nearby points,thus establishing equipment at differ-ent potentials.

In this situation, a common groundreference is essential, and a single-point system provides a predictablegrounding method. The overall poten-tial will rise due to the lighting strike,but each component will have thesame potential because they are elec-trically tied to the building at the samepoint. The components potential willuniformly rise and fall. This goes along way in protecting electronicequipment from the effects of light-ning, while keeping within NEC guide-lines.

However, single-point grounding isnot without its drawbacks. Onedisadvantage is that it relies on acommon nodethe MEGBfor abuilding grounding system. In thesesystems, particular concern must begiven to properly made bondsbetween conductors and the bus, andcorrect installation of grounding

conductors to minimize highfrequency noise.

Another concern with single-pointgrounding is future testing andmaintenance of equipment. It isdifficult to isolate the MEGB fortesting or modifications withoutaffecting the equipment that uses theMEGB as a reference.

Finally, perhaps the most intriguingargument in the single- vs. multi-pointdebate concerns the ability to handlehigh frequencies10 MHz or more.Modern digital computer devices oftenproduce frequencies in the 100 MHz to300 MHz range. At these frequencies,the argument for single-pointgrounding tends to break down, dueto the length of the groundingconductors. When multiple pieces ofelectronic equipment are groupedtogether in one contiguous space, theyare effective sources of unwantedelectrical noise. The properties of thegrounding conductor are such that athigh frequencies, a conductor whoselength is 1/4 wavelength (or multiplesthereof) of an interfering frequencywill become an efficient antenna. Therule of thumb developed by EIA/TIAand BICSI is to specify conductorlengths at no more than 1/20wavelength of the highest frequencythreat. Single-point grounding willusually fall short in this regard.

MULTI-POINTUnlike single-point, the multi-pointsystem does not trace a singular pathback to building. Many existingbuildings use multi-point grounding bybonding the same pieces of electricalequipment to ground bars, buildingsteel, cold water pipes or otherelectrodes. One could say that multi-point grounding often uses a more themerrier approach.

Figure 2 (p. 14) represents a multi-point building grounding system inwhich ground bars in each electricaland telecom closet are bonded tobuilding steel and to the main groundbar. Both BICSI and EIA/TIA are



proponents of multi-point grounding. The EIA/TIA J-STD-607-A standard introduces the concept of groundingequalizers for telecommunications grounding. These aremeant to equalize potentials among ground systemcomponents. In the end, the goal of all multi-pointgrounding is the sameto provide multiple paths forground currents to flow and to equalize potentialsthroughout the building grounding system. And it can beargued that multi-point grounding achieves more effectivesafety than single-point grounding.

When it comes to implementation of groundingsystems, one way that multi-point grounding is appliedeffectively is the use of signal reference grids. Commonlyused in raised-floor applications in which many pieces ofelectronic and computer equipment are located in thesame room, an SRG is basically a network ofinterconnected ground conductors located beneath theraised floor. The equipment in the room is bonded to itvia conductive straps. In essence, the SRG acts as anequipotential plane to which the equipment is referenced.Charge can be easily dissipated to the grid from one ormore pieces of equipment, keeping the equipment at thesame potential.

The SRG can also be bonded to building steel or otherconductive paths within close proximity. Sensitive digitalequipment can be effectively bonded to the SRG atmultiple points, offering greater flexibility in equipmentlayouts because equipment can be grounded at anylocation in the room. This practice can minimizeequipment damage by limiting the potential differencesbetween pieces of equipment. But more importantly, itcan minimize touch potential.

Touch potential is the difference in voltage between anenergized piece of equipment and the feet of any personwho touches the equipment. A person who touches theequipment can receive a harmful or even fatal electricshock. When charge accumulates on the equipmentenclosuredue to static electricity, lightning storms orother reasonsit can be dispersed to the SRG, greatlyreducing the risk of touch potential.

Another major difference between the two types ofsystems is that where single-point grounding eliminatesground loops, multi-point systems may facilitate them. Ifthe building grounding system relies on multiplegrounding paths and numerous connections to buildingsteel, stray currents may be allowed to flow through thebuilding steel footings, then through the electrical system,and finally, back down to earth. Multiple connectionsprovide opportunities for stray currents to wreak havocon electrical, telecommunications and IT systems. If thepower and telecom grounding systems are intermingled,a fault or stray current on one system could locate a pathto the other and have adverse effects.

Figure 1

Figure 2



In the multi-point vs. single-pointdebate, both sides are able to offersubstantial evidence to support theircause. There are also detriments toeach design strategy.

In general, each accomplishesspecific goals while complying withNEC guidelines. However, the designercant always implementing just onestrategy while completing dismissing

the other. The design application andbuilding needs are factors instrumentalin selecting the best solution. Theanswer to the question, which isbetter, single or multipoint grounding?is not as clear-cut as the strategiesthemselves. More often than not,effective building grounding systemswill implement both strategies. Anelectrically complex building withintricate power and IT componentsshould rely on a hybrid system.

BEST OF TWO SYSTEMSSingle-point grounding should beutilized as the backbone of thebuilding grounding system. Provide amain ground bar to act a commondistribution point for ground risers andconnections. Tie the MEGB to theground bus of the main switchgear andthen go to the building from there.Separate ground bars for power andtelecommunications should be utilizedin each closet, while providing a singlepath back to the source (transformers).Tie telecommunication and ITgrounding systems to the powergrounding system and final connectionat the MEGB.

Multi-point grounding should beused almost as a grounding subsystemfor data centers and computer roomsfilled with high-frequency electronicequipment, where the benefits ofmulti-point grounding can beefficiently achieved with SRGs.

It is crucial, however, that this multi-point subsystem be tied to the single-point building grounding system. Itshould not be thought of as a separategrounding system. This type of hybridsystem will work in most applications.

This is the first of a two-part article.The second part, which will appear inthe Summer (June) issue, shows howthis hybrid design can create a reliableand effective system for building components. Exact design of an SRGwill be discussed.

Grounding Essentials

Electrical system grounding has three distinct purposes: to cause the operation ofovercurrent protection devices in the event of a fault condition; to provide zero ref-erence for the building electrical system; and to equalize potential differences in thesystem.

The National Electric Code (NFPA 70) stipulates that grounding must occur at thebuilding (premises wiring system) service entrance and at each separately derivedsourcein most cases, a transformer. At the service entrance, the ground and neu-tral are bonded together; then, the grounding conductor is taken from the neutralbus to ground rod(s), switchgear enclosure, building steel, an underground coldwater pipe or other available electrodes (NEC 250.30, 250.52).

Creating a neutral-to-ground bond at the service entrance creates a line-to-groundvoltage reference for the electrical system. This zero reference establishes a con-venient frame of reference for line-to-ground voltage measurements. The neutral-to-ground bond also creates an effective grounding system and minimizing the voltageto ground and can limit overvoltage stresses on conductors to electrical equipment.This allows for intended equipment performance by isolating potential fault.

Grounding at each separately derived system is also of benefit under a fault condi-tion, because electrons emanating from a sourcetransformer, generator or invert-erswill attempt to return to the source. Under a phase-to-ground fault condition,the current will travel back along the ground wire or ground pathsuch as conduitsand equipment enclosuresto the source. The source will provide current on thephase conductor(s) to meet the requirements of the short, thus causing the overcur-rent device to trip. The purpose of the ground wire in this case is to provide a lowimpedance path back to the source.

Note that the ground wire is not returning the current to ground. In this sense,ground wire is a bit of a misnomer. Many times, this is called an equipment orsafety ground, the latter being the most appropriate term, because it is meant toprovide personnel safety by isolating the fault in the system.

In many mission-critical applications, IT equipment operation is the main thrust ofdesign. But an often-overlooked aspect of data center reliability is the groundingsystem design and the need to provide an equipotential grounding system. If theelectrical and IT equipment are not properly grounded, there is a latent possibilityfor transients, EMI, RFI and static electricity affecting proper operation of the equip-ment. When noise is generated on the power source or in the enclosure housings,electronic equipment can have its data corrupted. Even though IT equipment mayappear to be functioning properly, there may be data errors or in extreme cases catastrophic failures.


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