5/26/2018 Understanding Standby Power System Grounding

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Understanding standby power system grounding

Knowing how to apply proper grounding and bonding to electrical systems and transfer

switches for common standby power system configurations can minimize power outages,

equipment damage, and injuries.

Consulting-Specifying nginner !CS"

David Chesley, PE, LEED AP, RCDD, Interface Engineering, Portland, Ore.12/1/2!11

Electrical syste" gro#nding is an often "is#nderstood area of electrical syste" design andconstr#ction that can ca#se havoc $hen "isa%%lied. &his %ro'le" is even "ore %rono#nced$ith stand'y %o$er syste"s. Circ#it 'rea(ers tri%, generators are dro%%ed offline, and cr#cialstand'y and life safety loads are lost 'eca#se of a hidden gro#nding %ro'le" "ade "anifestd#ring a %o$er o#tage, )#st $hen 'ac(#% %o$er is needed "ost.

Des%ite this harro$ing %ict#re, "ost gro#nding iss#es can 'e resolved $ith attentive design,and then chec(ed and avoided d#ring the installation and testing stages of constr#ction,

eli"inating "ost iss#es 'efore a facility is occ#%ied.

Know grounding and bonding terminology

*efore disc#ssing the challenges of gro#nding stand'y %o$er syste"s, (ey ter"s aree+%lainedas defined in the 2!11 -ational Electric Code -EC -0PA !to offer a 'etter#nderstanding of $hy $e gro#nd electrical syste"s. nless other$ise noted, the follo$ingdefinitions are cited fro" -EC, Article 1!!, 3Definitions.4

Ground:&he earth itself is ta(en as 3gro#nd4. *#ilding gro#nding electrode syste"s ares#n(en into the earth.

&he connecting of c#rrent5carrying e6#i%"ent to the earth is the definition of gro#nding. -ECArticle 27!.A2 states8

Grounding of Electrical Equipment. Normally, non-current-carrying conductive materialsenclosing electrical conductors or equipment, or forming part of such equipment, shall beconnected to earth so as to limit the voltage to ground on these materials.

&his is different fro" 'onding, $here ad)acent cond#ctive s#rfaces that do not nor"ally carryelectricity s#ch as chassis on a toaster, an +5ray, or a "illing "achine are connected toanother cond#ctive o')ect s#ch as the "etallic race$ay containing %o$er $iring, near'y

'#ilding steel, or an ad)acent "etallic $ater %i%e to create an electrically contin#o#s, lo$5i"%edance %ath. -EC Article 27!.A9 states8

Bonding of Electrical Equipment. Normally non-current carrying conductive materialsenclosing electrical conductors or equipment, or forming part of such equipment, shall beconnected together and to the electrical supply source in a manner that establishes aneffective ground fault-current path.

Grounding electrode:A cond#cting o')ect thro#gh $hich a direct connection to the earth isesta'lished.

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:ro#nding electrodes in %ractice incl#des gro#nd rods and cond#ctive "etallic $ater %i%ing,and concrete5encased '#ilding steel e.g., steel re'ar e"'edded in concrete fo#ndation indirect contact $ith the earth.

Grounding electrode conductor:A cond#ctor #sed to connect the syste"5gro#ndedcond#ctor or the e6#i%"ent to a gro#nding electrode or to a %oint on the gro#nding electrodesyste". &his cond#ctor connects the '#ilding gro#nding syste" to the earth 'y "eans of the

gro#nding electrode.

Grounded (grounding):Connected to gro#nd $itho#t inserting resistors or i"%edancedevices. :ro#nded e6#i%"ent or %anels can trace a contin#o#s cond#ctive %ath fro" theirchassis to an e6#i%"ent gro#nding cond#ctor, to the gro#nding electrode cond#ctor, to thegro#nding electrode to the earth gro#nd itself see 0ig#re 1.

Equipment grounding conductor (EGC):&he cond#ctive %aths installed to connectnor"ally non5c#rrent5carrying "etal %arts of e6#i%"ent together and to the syste" gro#ndingcond#ctor or to the gro#nding electrode cond#ctor, or 'oth.

&he E:C connects o')ects 'ac( to the gro#nding electrode syste" and to the earth.

Ground fault current path:0ro" -EC, Article 27!.2, an electrically cond#ctive %ath fro" the%oint of a gro#nd fa#lt on a $iring syste" thro#gh nor"ally non5c#rrent5carrying cond#ctors,e6#i%"ent, or the earth to the electrical s#%%ly so#rce.

Effective ground fault current path:0ro" -EC, Article 27!.2, an intentionally constr#cted,lo$5i"%edance electrically cond#ctive %ath designed and intended to carry c#rrent #ndergro#nd fa#lt conditions fro" the %oint of a gro#nd fa#lt on a $iring syste" to the electricals#%%ly so#rce and that facilitates the o%eration of the overc#rrent %rotective device or gro#ndfa#lt detectors on high i"%edance gro#nded syste"s.

All c#rrent tries to ret#rn 'ac( to its so#rce to co"%lete the circ#it. A live cond#ctor stri(ing an#ngro#nded cond#ctive "etal s#rface $ill go thro#gh $hatever, or $hoever, connects the"etal o')ect to earth, and 'ac( to the %o$er so#rce.

*eca#se having large a"o#nts of electrical c#rrent #nintentionally going thro#gh %eo%le isgenerally a 'ad thing, $e $ant the overc#rrent %rotection device to tri% as 6#ic(ly as %ossi'le,re"oving the fa#lt c#rrent 'efore da"age, in)#ry, or death can occ#r. &he 'est "eans ofdoing this for gro#nd fa#lt c#rrent is to %rovide a lo$ eno#gh i"%edance %ath s#ch thatgro#nd fa#lt c#rrent is high eno#gh in "agnit#de to 6#ic(ly tri% the overc#rrent %rotectivedevice.

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0ig#re 1 sho$s this in %ractice. A gro#nd fa#lt in an a%%liance, rather than ro#ting to the earththro#gh o#r #ns#s%ecting victi", ro#tes thro#gh the E:C 'ac( thro#gh the %anel'oard, and#lti"ately to the "ain service #tility transfor"er, co"%leting the circ#it.

;ore i"%ortantly, 'eca#se this is an effective gro#nd fa#lt c#rrent %ath, the fa#lt c#rrent ishigh eno#gh to allo$ the circ#it 'rea(er in the %anel'oard to o%en, c#tting the fa#lt c#rrent.

Separately derived system:A %re"ises $iring syste" $hose %o$er is derived fro" aso#rce of electrical energy or e6#i%"ent other than a service.

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&his ter", introd#ced in the 2!11 -EC, a%%lies to 'oth #tility services and alternate so#rcesof %o$er s#ch as a generator. &his cond#ctor is ho$ ret#rn gro#nd5fa#lt c#rrent goes fro" theE:C to the ne#tral and 'ac( to the %o$er so#rce, co"%leting the circ#it, and ho%ef#llytri%%ing the overc#rrent %rotection in "ini"al ti"e.

#rounding standby power systems

:ro#nding syste"s are created to allo$ overc#rrent devices to 6#ic(ly o%en $hen a line5to5gro#nd fa#lt occ#rs. 0or this reason, 'onding 'et$een the ne#tral and gro#nd '#s orchassis in the generator sho#ld not occ#r $hen a 95%ole transfer s$itch is #sed that directly'onds the "ain service %anel ne#tral to the ne#tral '#s of the generator.

0ig#re 2 sho$s the %ro'le" $ith this a%%roach. Ret#rn c#rrent fro" the electrical load travelsdo$n to the transfer s$itch and then s%lits $ith %art heading 'ac( to the #tility service and%art heading do$n to the generator. At the generator, this c#rrent then ret#rns thro#gh thene#tral5gro#nd 'ond 'ac( over the gro#nding syste" to the #tility service, leading to strayc#rrent %otentially traveling over "etallic race$ay, %i%ing, and even '#ilding str#ct#ral steel toret#rn to its so#rce.

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An added %ro'le" of connecting the ne#tral to gro#nd at 'oth the "ain service and thegenerator $itho#t transferring the ne#tral occ#rs $ith gro#nd fa#lt %rotection :0P, $hich isre6#ired 'y -EC Article 2!.19 on overc#rrent %rotection at >! ? and "ore than 1,!!! A.

:0P devices o%erate 'y "eas#ring the o#tgoing and inco"ing c#rrent and loo(ing atdifferences 'et$een the t$o, $ith the ass#"%tion that any c#rrent difference is stray ret#rnc#rrent that is traveling over the '#ilding gro#nding syste". If the "eas#red c#rrent difference

is high eno#gh, the :0P device is then set to tri% o%en, c#tting %o$er to do$nstrea" loadsand the gro#nd fa#lt ca#sing the stray ret#rn c#rrent.

@o$ever, if stray ret#rn c#rrent is ca#sed for other reasons, s#ch as a condition $here ret#rnne#tral c#rrent fro" the load can travel over the ne#tral5to5gro#nd 'ond at the generator, the:0P device can also tri%, 'eca#se the c#rrent difference can 'e the sa"e as in an act#algro#nd fa#lt, es%ecially if the generator is relatively close to the service.

If the :0P is then dialed to a higher c#rrent setting to "ini"ie interr#%tions, one is left $ithc#rrent reg#larly r#nning over the gro#nd syste" fro" stand'y loads, and has red#ced the%rotection fro" the :0P in the event of a real line5to5gro#nd fa#lt.

-ote that the c#rrent ro#ted fro" ne#tral to gro#nd at the generator can 'e si"ilar in"agnit#de to a line5to5gro#nd fa#lt at the stand'y loades%ecially if the i"%edance of the%ath fro" the load to the generator is relatively lo$. &h#s, setting the :0P %rotection to ahigher tri% setting can "ean that a real line5to5gro#nd fa#lt at the load "ay not ca#se the'rea(er to o%en, leading to a s#stained gro#nd fa#lt in the electrical syste".

&his leads #s to o#r first r#le of stand'y %o$er syste" gro#nding8

ule !o" #:0or a 95%hase, 5$ire syste", do not 'ond the ne#tral and gro#nd '#s togetherat the generator #nless the ne#tral is s$itched at the transfer s$itch together $ith the %hase

cond#ctors.

&ransferring the ne#tral can 'e acco"%lished 'y one of t$o "eans8

A 5%ole transfer s$itch that s$itches the load ne#tral 'et$een the #tility so#rce to the

stand'y so#rce A 95%ole transfer s$itch $ith overla%%ing contacts for the ne#tral that overla%s the

#tility and stand'y ne#trals very 'riefly at the ti"e of s$itching.

-ote that the 'onding of a generatorBs ne#tral and gro#nd '#s sho#ld not 'e conf#sed $iththe 6#estion of $hether a generator sho#ld have a gro#nding electrode syste". If the

generator is a se%arately derived syste", the -EC re6#ires a gro#nding electrode syste" atthe generator %er -EC Articles 27!.9! and 27!.72A.

If the syste" is not se%arately derived, the gro#nding electrode syste" "ay 'e installed as as#%%le"ental gro#nding electrode syste", %roviding it is also 'onded to the e6#i%"entgro#nd cond#ctor -EC Article 27!.7.

&his leads #s to the second r#le of stand'y %o$er syste" gro#nding8

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ule !o" $:A stand'y %o$er so#rce sho#ld have its o$n gro#nding electrode syste" tofacilitate gro#nd fa#lt c#rrent ret#rning to the generator if a line5to5gro#nd fa#lt occ#rs $hen agenerator is %o$ering load. @o$ever, this gro#nding electrode syste" "#st al$ays 'e'onded to the e6#i%"ent gro#nd cond#ctor that is also 'onded to gro#nding electrodecond#ctor at the #tility service disconnect.

&his raises the 6#estion of $hether there are ti"es 'y code or design $here the engineer

"#st design the generator as a se%arately derived syste" $ith a ne#tral5gro#nd 'ond at thegenerator.

One of the "ost co""on instances occ#rs $hen "ore than one level of :0P is #sed. &hisoften ha%%ens in health care facilities $here "ore than one transfer s$itch is #sed to isolatelife safety, critical, and e6#i%"ent 'ranches fro" one another, and a second level of :0P is#sed on the overc#rrent %rotection. Even $hen the generator does not have a ne#tral5gro#nd'ond and "#lti%le 95%ole transfer s$itches are #sed, ret#rn c#rrent fro" the life safety 'ranchret#rns on 'oth its o$n ne#tral and on the ne#tral #sed 'y the e6#i%"ent 'ranch see 0ig#re9.

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generator sho#ld 'e gro#nded as a se%arately derived syste", and transfer s$itches that cans$itch the ne#tral sho#ld 'e #sed for all transfer s$itches.

*eca#se it is "#ch less e+%ensive to s%ecify the generator $ith the %ro%er gro#nding andinstall the correct transfer s$itches initially than do so as a retrofit, consideration of thisa%%roach sho#ld 'e #sed if there is a %ossi'ility that a f#t#re #%grade co#ld create thiscondition that doesnBt e+ist initially in the '#ilding.

0or e+a"%le, if an o$ner advises the engineer on a %ro)ect $ith a s"all life safety generatorthat it "ay 'e re%laced in the f#t#re $ith a large generator ca%a'le of 'ac(ing #% "ost of the'#ilding, and that #%grade $o#ld re6#ire t$o levels of :0P for the non5life safety 'ranch, theengineer $ill $ant to s%ecify a 5%ole transfer s$itch for the life safety 'ranch to avoidneeding to re%lace that transfer s$itch d#ring the f#t#re #%grade.

*eca#se the %ro'le" of ne#tral c#rrent$hether #nder nor"al conditions or line5to5gro#ndfa#lt conditionse"erges $here "#lti%le transfer s$itches are #sed in one facility, $e cangeneralie R#le -o. 9 as follo$s8

Generali&ed ule !o" %:=hen #sing "ore than one transfer s$itch on a 95%hase, 5$iresyste" $here any one transfer s$itch "ay have t$o or "ore levels of :0P %rotection#%strea" of itself, all generators sho#ld 'e gro#nded as se%arately derived syste"s, andtransfer s$itches that can s$itch the ne#tral sho#ld 'e #sed for all transfer s$itches. In thedisc#ssion to this %oint, all "ention of :0P has 'een ass#"ed to 'e on the #tility side of thetransfer s$itches. =hile generators can 'e s%ecified $ith an o#t%#t circ#it 'rea(er $ith :0P,this is nor"ally avoided $ith good reason, 'eca#se a gro#nd fa#lt tri% $o#ld res#lt in the lossof the stand'y %o$er so#rce $hen it is critically needed.

Instead, $here a :0P $o#ld other$ise 'e re6#ired, generators sho#ld 'e s%ecified $ith agro#nd fa#lt ann#nciation feat#re on the o#t%#t circ#it 'rea(er, to notify via the generator

control %anel and ann#nciator $hen a gro#nd fa#lt condition has occ#rred $hile the facility isr#nning on 'ac(#% %o$er. &his allo$s the facility "anager to h#nt do$n the so#rce of thegro#nd fa#lt $itho#t c#tting %o$er to critical loads.

$he oddballs you must %now

&here are n#"ero#s cases $here a 95%hase, 95$ire syste" no ne#tral is #sed on thegenerator side.

Case !o" #'solidly grounded %-phase %-ire standby poer system: In this case, atransfor"er "ay 'e #sed on the load side of the 95%ole transfer s$itch to derive a ne#tral for

any line5to5gro#nd loads on the stand'y %o$er syste" see 0ig#re . @o$ever, the transfers$itch sees only %hase and gro#nd connections fro" the #tility, generator, and loadconnections to itself. *eca#se there is no $ay for line5to5gro#nd c#rrent to ret#rn on any %athe+ce%t the gro#nd %lane, $e do not see a s%litting of ret#rn gro#nd fa#lt c#rrent 'et$een ane#tral and gro#nd. &h#s, a :0P can "onitor )#st the %hase cond#ctors and tri% %ro%erly ifthe c#rrent does not ret#rn thro#gh the sa"e %hase cond#ctors.

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Case !o" $'solidly grounded %-phase %-ire standby poer system ith interloc*edcircuit brea*ers used for load transfer: *y definition, 'eca#se the circ#it 'rea(ers #nlessthey are 5%ole can only s$itch the %hase cond#ctors and not the ne#tral, $e $o#ld have thesa"e %ro'le" of ne#tral c#rrent fro" the stand'y load ret#rning on t$o %ath$ays. One $ayof avoiding this %ro'le" is to connect only 95%hase, 95$ire loads no ne#tral to the co""ondistri'#tion 'oard served fro" 'oth so#rces see 0ig#re 7.

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Case !o" %'high-resistance grounded %-phase %-ire standby poer system: In thiscase, a high5resistance so#rce 'et$een ne#tral and gro#nd at the generator is #sed to li"itany ret#rn gro#nd fa#lt c#rrent that "ay occ#r. &his is often #sed in ind#strial settings $herethe o$ner does not $ant to interr#%t %o$er in the event of the gro#nd fa#lt, '#t also $ants tored#ce the fa#lt c#rrent "agnit#de and th#s its inherent danger $hile tro#'leshooting thelocation and so#rce of the fa#lt. Conditions of -EC Article 27!.9 "#st also 'e "et.

;#ch of the control the '#ilding o$ner, engineer, and electrician have over resolvinggro#nding iss#es $ith the generator can 'e resolved $ith caref#l %lanning at the generatorand transfer s$itch. Caref#l foretho#ght and %lanning can do "#ch to "a+i"ie the gro#ndfa#lt %rotection of the %o$er syste", and "ini"ie %o$er o#tages d#e to stray c#rrents.

Chesley is an associate principal and senior electrical engineer at Interface Engineering. ehas been !ith the company since "##$. is engineering e%pertise includes buildingintegration, rene!able energy systems, telecommunications infrastructure, bac&up po!ersystems, energy metering, and building dashboards.

http://www.interfaceengineering.com/http://www.interfaceengineering.com/