SAFE GROUNDING CONTROLLED - Company Profile

1 Tech BriefPS-202207/05 Page 1 of 4 COPYRIGHT 1989 MILLER FREEMAN PUBLICATIONS, RIGHTS RESERVEDTHIS ARTICLE APPEARED IN THE ISSUE OFEOS/ESD TECHNOLOGY. IT HAS BEEN REPRINTED WITHPERMISSION OF MILLER FREEMAN PUBLICATIONS, 1050 COMMONWEALTH AVE., BOSTON, MA GROUNDINGOF STATIC- CONTROLLED WORKSTATIONSA lthough ESD-protective GROUNDING is often crucial, it is far from the onlyconsideration in workstation GROUNDING ; the avoidance of shock hazard topersonnel is even more Edward H. RussellMember, ESD Assn. GROUNDING Standards Subcommittee,and Quality Assurance Specialist, Santa Barbara, CAFigure 1. Earth GROUNDING in a typical 120-V, 60-Hz electrical systemconfiguration is used in electricalpower systems. The NationalElectrical Code recommends that theneutral circuit of all AC systems beterminated in some conducting bodyin the earth.

2 Here s why: such agrounding Maintains an arbitrary zero voltagereference for the AC system;2. Protects a building from lightning;3. Drains equipment s static 1 shows how earth GROUNDING isconfigured in a typical 120-V, 60-Hzelectrical system. This type ofsystem is the one most commonlyused at workstations in industrytoday and is, therefore, a primarysubject of this ESD Ground?One might think that electricalsystem grounds should not be usedfor ESD-protective GROUNDING , butthis is not necessarily the case. Afrequent alternative to using thesystem ground for ESD is a separateearth ground rod installed near theworkstation for ESD use, and this isindeed effective in draining awaystatic charges. However, such aground can be hazardous topersonnel working near electricalequipment; such a situation couldallow a potential difference betweenequipment and the separate addition to personnel hazards,an electrical overstress (EOS)hazard might exist between, say, asoldering iron (grounded through thepower system) and a worksurfaceusing the separate ESD grounddescribed above.

3 Fig. 2 illustratesthe potential , this is undesirableboth for people and parts. However,a properly grounded piece ofelectrical equipment placed on agrounded worksurface reduces thishazard. However, there is still thepossibility of ground current flowingthrough the worksurface itself, andstatic charges might not bleed offothers speak in terms of a commonpoint to which all electrical returnsare electronics, different groundsmay exist. Electromagnetic,electrostatic, signal, chassis andpower grounds may be addressedseparately in order to control noise,interference, and undesirable geographic areas whereelectrical storms are intense, specialrods are installed on buildings todivert lightning to earth throughheavy-gauge wire. A similarThe term GROUNDING is oftenmisunderstood; it s used freely andoften without the necessarymodifying specifics.

4 That sunfortunate because unlessgrounding is understood and itsprinciples properly applied, animproperly grounded workstation notonly can allow damage to sensitiveparts, but can present a shockhazard to humans as are various concepts ofgrounding. Some authorities relate itto a connection with the earth, while 2005 DESCO INDUSTRIES OwnedWe make our wriststraps with theindustrial environment in mind. Theyare designed and manufactured togive you years of service in thetough electronic we send you a sample?Quality Ground CordsPS-2022 Page 2 of 4An equipment ground conductoris often used in case a fault developsin the primary power-system 4 shows the relation betweenequipment ground (also referred toas safety ground) and earth methodology is designed tohandle situations in which 120-Vpower accidentally contacts theframe of electrical equipment.

5 So-called fault current will flow throughthe equipment ground, triggering thelocal circuit breaker. Installation ofGround Fault Circuit Interrupters, orGFCIs, increases safety due to related 3 shows how current flowthrough the worksurface problem can be solved bybonding the two earth groundstogether, eliminating the resistancebetween them. If electrical power isnearby or incorporated inworkstations, worksurface and wrist-strap ground circuits should beconnected only to the electrical -power-system ground to minimizepotential 4. The relationship between equipment (or safety) ground and 3. How current flow through a worksurface is 2. Separate ground systems present potential the highest quality ground cordsfor mats, work surfaces, andequipment call ESD Systems. Askfor a GradeWriststrapsFloor mat ground cordCommon point ground cordWrist strap coiled cord 2005 DESCO INDUSTRIES OwnedPS-2022 Page 3 of 4 Testing Is a MustElectrical systems should alwaysbe tested before depending on agrounding system.

6 Accurate,reasonably priced and easilyoperated systems analyzers areavailable. The analyzer used shouldboth detect flaws in the system andalso verify the wiring configurationper the National electrical aren t needed for thesetests unless discrepancies are maximize safety, the analyzershould be able to detect flaws suchas an equipment ground shorted toneutral in the AC outlet box. It shouldalso be capable of testingequipment-ground impedance byinjecting AC current into theequipment-ground conductor andmeasuring between it and the neutralconductor. ECOS Corp. (Oak Park,IL, Phone 208-383-2525, Fax 208-383-2137), a consultant on ACdistribution and GROUNDING ,recommends 1 Sbetween groundand It YourselfThough it should rarely benecessary, there are adequatemeans of measuring earth-groundresistance in AC systems.

7 Install a5/8-in. by 6-ft. copper rod in theearth near an electrical circuit. Usinga specialized ground-resistancetester, measure the earth sresistance between your auxiliaryearth ground and the AC system sground conductor. Ordinary metersare not recommended for thismeasurement due to the possibilityof invalid measurements; ground-resistance testers can guard againstearth currents (and consequentmeasurement inaccuracies); thetypical VOM resistance will varydepending on the nature andmoisture of the soil and the physicalimplementation of the systemground. For ESD purposes, themaximum resistance allowable forrapid static decay is about 1 MShowever, if the AC system groundmeets Code requirements, theresistance should measure less than100 5. GROUNDING strategy for workstations without AC 6. Block diagram of a ground-integrity GroundItem No.

8 13575 is designed for useas either a multiple GROUNDING blockor as a common point ground Mats,Grounds Wriststraps,and Tests at a TouchItem No. 41193 is a wriststrap testerthat also grounds your mat. Call us ifyou would like to see No. 13525 provides a secureground at each work station, andcomes complete with GROUNDING wireand CommonPoint Ground 2005 DESCO INDUSTRIES OwnedPS-2022 Page 4 of 4 Continuity of less than 1 Scan bemeasured with the wiring analyzer(s)previously described. If a workstationframe s resistance is too high, anapproach similar to that shown inFig. 5 should be used. In any event,for added safety, such metal framesshould always be bonded securely tothe equipment ensure the continuing safetyof people and ESD-sensitiveelectronics, all static-controlledworkstations should be periodicallytested for ground integrity.

9 However,even periodic testing can t detect animmediate break or degradation in agrounding circuit. When such abreak is eventually uncovered, it isoften too late for some parts, thusvarious constant monitoring systemsare available. Figure 6 shows ablock diagram of such a monitors sound an alarm if anopen circuit of ground-circuitdegradation occurs. Unfortunately,some of these monitors areundependable, so it s good policy toperiodically test not only groundintegrity but the associated monitorsas Kleronomos and , A Practical Approach toEstablish Effective GROUNDING forPersonnel Protection, IEEE IASA nnual Conference Record, Lee, electrical Safety inIndustrial Plants, IEEE Spectrum,June Daziel, electrical ShockHazard, IEEE Spectrum, National electrical Russell, Static-SafeWorksurfaces and Personnel Safety- Update, Evaluation Engineering,September Russell, Safely GroundingStatic-Control Worksurfaces, EOS/ESD Technology.

10 June Systems Makes GROUNDING of Work Surfaces and Floor MatsFast and without AC powercan be grounded to a common busrod mounted near the worksurfaces should beconnected to the ground bus withwire of 20-gauge minimum diameterand preferably larger. Heavystranded wire may be preferable to aless-durable solid-conductor. Theend of this GROUNDING bus should bebonded to a convenient AC-equipment-ground circuit. This typeis shown in Fig. conductors inworkstations wired with 120-VAC canbe used for GROUNDING , provided thecircuit has been tested and haspassed. In addition, the equipment-ground circuit should be bondedelectrically to a workstation s metalframe if that frame is to be used ascommon-ground point. Our ground cords are designed to hook directlyonto any nearby green wire utility for Tech Brief PS-2013 for details onproper GROUNDING and installation of ESDprotective surfaces or PS-2017 for propermaintenance and GROUNDING of wrist push andclinch snap for usewith multi layer or softconductive mats ItemNo.