understanding the differences between bonding, grounding, and earthing

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%he pro&le" e#perienced &y the plant in the e#a"ple is not unco""on. 7anu!acturers o! sensitive

"achines have discovered that isolated ground rods can decrease the nu"&er o! nuisance pro&le"s, such as

re&ooting, data errors, and inter"ittent shutdo'ns. %his decrease is due to the reduced a"ount o! voltage

transients or noise- on the ground rod, as co"pared to a co""on &uilding grounding syste". 8ecause o!

the reduction in data errors attri&uted to the ground rod, so"e "anu!acturers include isolated ground rods

in their installation instructions. So"e even i"ply the "achine 'arranty 'ill not &e honored i! the ground

rod is not installed.

During thunderstor"s or ground !aults, ho'ever, an isolated ground rod &eco"es a lia&ility, creating shock

ha$ard potential !or e"ployees and high potential rises on sensitive "achine co"ponents. Figure 11click

here to see Fig. 12illustrates the e#tre"ely large transient voltages that can develop &et'een driven ground

rods due to lightning currents and earth resistance. lthough ground !aults in the "achine itsel! "ay not

dra' enough current to trip overcurrent protective devices, they can create touch ha$ard potential !or

e"ployees.

rticle 9:;.:< o! the 9;;= E speci!ically prohi&its the use o! isolated ground rods, or earthing, as the sole

"eans o! equip"ent grounding, although so"e have used other sections o! the E to 5usti!y this practice.

%he E +and&ook- provides the !ollo'ing co""entary associated 'ith rt. 9:;.> 1?&5ectiona&le

urrents26

n increase in the use o! electronic controls and co"puter equip"ent, 'hich are sensitive to stray currents,

has caused installation designers to look !or 'ays to isolate electronic equip"ent !ro" the e!!ects o! such

stray circulating currents. irculating currents on equip"ent grounding conductors, "etal race'ays, and

&uilding steel develop potential di!!erences &et'een ground and the neutral o! electronic equip"ent.

solution o!ten reco""ended &y ine#perienced individuals is to isolate the electronic equip"ent !ro" all

other po'er equip"ent &y disconnecting it !ro" the po'er equip"ent ground. (n this corrective action, the

equip"ent grounding "eans is re"oved or non"etallic spacers are installed in the "etallic race'ay syste"

contrary to !unda"ental sa!ety grounding principles covered in the require"ents o! rt. 9:;. %he electronic

equip"ent is then grounded to an earth ground isolated !ro" the co""on po'er syste" ground. (solating

equip"ent in this "anner creates a potential di!!erence that is a shock ha$ard. %he error is co"pounded

&ecause such isolation does not esta&lish a lo')i"pedance ground)!ault return path to the po'er source,

'hich is necessary to actuate the overcurrent protection device.-

Bonding/grounding vs. earthing

(solated connections to earth are not required !or sensitive "achine operation. (ssues crop up 'hen

equip"ent &onding/grounding and earthing are con!used. (n the 0nited States, the ter" grounding- is

used to re!er to at least !ive or "ore grounding)related syste"s, including6

System type

%his re!ers to the "eans &y 'hich po'er source voltage relationships are esta&lished. 4o'er sources

!all into !our general categories6 %rans!or"ers, generators, electric utilities, and static po'er

converters. %hese syste"s "ay &e con!igured as 'ye or delta, and the "eans &y 'hich they are

inter!aced 'ith the grounding syste" deter"ines the syste" type. %he "ost co""on @)phase syste"

type is the solidly grounded 'ye, 'hich is esta&lished &y connecting a properly rated conductor 1also

kno'n as the "ain or syste" &onding 5u"per2 !ro" the A; ter"inal o! the source 1usually a

trans!or"er2 to the grounding syste".

Equipment grounding (bonding)

Resolving the issue

%he &est "eans o! equip"ent grounding is to route a grounding conductor, suita&ly si$ed, along the

erstanding the Differences Between Bonding, Grounding, and Earthing http://ecmweb.com/print/bonding-amp-grounding/understanding-diffe...

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sa"e route as the po'er and neutral conductors, !ro" source to "achine. %he E does allo' use o!

"etallic conduit and other su&stitutes, &ut so"e industry e#perts &elieve these syste"s are less

e!!ective and should &e avoided.

Grounding electrode (earthing)

%his ter" re!ers to the "ethod &y 'hich the !acility grounding syste" is connected and re!erenced to

earth. %he "ost co""on grounding electrode !or s"all !acilities is a "etallic ground rod, &ut earthing

syste"s !or larger &uildings can * and should * &e "ore ela&orate and include the "eans &y 'hich

to inspect and test these syste"s periodically. grounding electrode syste" that is &uried in earth or

encased in concrete and then !orgotten is o!ten the source o! increasing pro&le"s as the &uilding ages

and the grounding electrodes deteriorate.

Lightning abatement

So"e !acilities use air ter"inals 1also kno'n as lightning rods2 to direct lightning strikes a'ay !ro"

po'er equip"ent, &ut these devices are o!ten connected to the grounding syste" in such a 'ay that

they have the opposite e!!ect * unintentionally &ringing lightning energy into !acility structural steel,

lo')voltage trans!or"er 'indings, and, su&sequently, sensitive &uilding loads.

Signal-reference grounding

Sensitive electronic "achines rely on the grounding syste" !or re!erence o! lo')"agnitude signals.

%here!ore, it3s o!ten crucial to provide "ultiple grounding paths, rather than rely on a single

equip"ent grounding conductor &et'een the po'er source and the sensitive load. %his ensures that

spurious voltages on the grounding syste" are "aintained 'ell &elo' the level at 'hich they "ight &e

con!used 'ith sensitive "achine re!erence signals. %he &est guide !or signal)re!erence grounding is

(EEE Standard BB;;)9;;>, Reco""ended 4ractice !or 4o'ering and Crounding Electronic

Equip"ent.-

ote that earthing is not required !or sensitive "achine operation. 7odern aircra!t, !or e#a"ple, are packed

'ith sensitive co"puters and electronic devices, 'hich operate correctly 'ithout an attach"ent to earth.

%hey rely on a &onded "etallic syste" * the airplane !ra"e'ork, skin, structural supports, race'ays, and

grounding conductors * to serve as the ground re!erence. (! this &onded syste" rises in voltage 'ith respect

to earth, all "achines on&oard e#perience the increase together. %he net result is that the "achines see no

voltage potential di!!erences 'ith respect to each other. ?nce the airplane lands, any voltage potential

&et'een the plane and earth "ust &e discharged &y an electrode that &ypasses the ru&&er tires.

Resolving the issue

%he i""ediate solution to the e#a"ple plant3s illegal ground rod 1click here to see Fig. 2'as to re"ove theshock ha$ard. %his 'as done &y connecting a grounding conductor 1B/; copper2 !ro" the ground rod to the

nearest part o! the &uilding grounding syste" * in this case, the structural steel. %his connection eli"inated

the shock potential during stor"s &y reducing the resistance &et'een the ground rod and the &uilding

grounding syste".

%he ne#t step 'as to eli"inate the 'iring errors and install a ground 'ire !ro" the source to the

"achine 1click here to see Fig. !2. %he pri"ary reason that the isolated ground rod 'as e!!ective in

decreasing operating pro&le"s 'as the &uilding3s &onded syste" e#perienced voltage transients, i"posed

on it due to 'iring errors. ?ne co""on error is the i"proper connection o! neutral 'ires to ground &uses or

ground 'ires to neutral &uses. %his error allo's neutral currents to !lo' on the &onded syste", there&y

creating voltage transients. eutral 'ires are only allo'ed to &e connected to the &onded syste" at a service

entrance or at a step)do'n trans!or"er 1called a separately derived source &y the E2. otice in Fig. 9 that

the plant had installed &oth a voltage regulator and a noise suppression device ahead o! the "achine.

%hese devices are o!ten applied to solve the nuisance operating pro&le"s &rought on &y ground syste"

erstanding the Differences Between Bonding, Grounding, and Earthing http://ecmweb.com/print/bonding-amp-grounding/understanding-diffe...

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transients. Suppression devices are not a cure)all, ho'ever. (n !act, they3re so"eti"es unnecessary 'hen

'iring and grounding pro&le"s are corrected !irst.

?nce the spurious ground rod had &een connected to the rest o! the &onded syste", operating issues had to

&e addressed, 'hich involved correcting the 'iring errors identi!ied in the site survey. For the e#a"ple

!acility, these steps 'ere adequate. For other situations, you should re!er to the !ollo'ing checklist6

onnect the ground rod to the &onded syste" and install a grounding conductor !ro" the po'er

source to the sensitive load to eli"inate the sa!ety ha$ard and allo' an e!!ective ground)!ault return

path.

B.

orrect 'iring and grounding errors on the po'er syste" serving the sensitive "achine.9.

(nstall a step)do'n trans!or"er 1i.e., a separately derived source2 to serve only the process "achine.

Derive a ne' neutral to the ground &onding point at the load side o! the trans!or"er.

@.

ny re"aining operating pro&le"s are pro&a&ly caused &y co""unications ground loops. Cround

loops, 'hich are introduced &y co""unication 'iring &et'een sensitive "achines !ed !ro" di!!erent

po'er sources, "ay require "ore ela&orate correction sche"es, such as optical isolation.

understanding the differences between bonding, grounding, and earthing

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