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Overvoltages in the IT system
Posted JUL 26 2012 by EDV!D in E"E!#$ "D PO%E!& P!OTE'TIO"
(ith )
'O**E"T+
+ingle ins,lation -a,lt on a net(or. o/erated in IT earthed
system
Introd,tion to overvoltages
Eletrial net(or.s an be s,bet to overvoltages o- varying origins
+ome
overvoltages& s,h as di3erential mode ones 4bet(een live
ond,tors5 a3et all the
earthing systems
This setion is /arti,larly onerned (ith ommon mode
overvoltages& (hih
mainly a3et the IT system as the net(or. is then ,nearthed78
15 Overvoltages d,e to ins,lation -a,lts&25 Overvoltages d,e
to internal disr,/tive brea.do(n in the *V9LV trans-ormer&)5
Overvoltages d,e to lightning stri.ing the ,/stream LV
net(or.&:5 Overvoltages d,e to lightning stri.ing the b,ilding
in (hih the installation is
ho,sed
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These overvoltages are /arti,larly ta.en into ao,nt by standard
"; ' 1rst ins,lation -a,lt o,rs& the /hase=to=earth voltage o-
the so,nd
/hases is /ermanently bro,ght to the /hase=to=/hase voltage o-
the net(or.
LV e?,i/ment m,st th,s be designed to (ithstand a /hase=to=earth
voltage o- U0e
and not the /hase to ne,tral voltage U0 -or the time re?,ired to
tra. and eliminate
the -a,lt
This /arti,larly a//lies to8
$7 a/aitive >lters >tted on many eletroni devies@
installation PI* 4Permanent
ins,lation monitor5 (hen installed bet(een /hase and earth
bea,se the ne,tral is
not aessible
%hen hoosing a PI*& it is th,s im/ortant to veri-y the
voltage o- the net(or. to be
monitored delared by its man,-at,rer These reommendations are
s/ei>ed
in /arti,lar in standard IE' 60Arst -a,lt& a transient
overvoltage a//ears (ith a /ossible
/ea. o- 2B CU0 4U0 /hase to ne,tral voltage o- the LV net(or.5
On a 2)0 V9:00
V net(or.& this val,e is FF0 V& an overvoltage level
that is not dangero,s -or
e?,i/ment (ith an ins,lation o- 1&F00 V 4voltage onstraint
at /o(er -re?,eny on
the LV side as /er IE' 60)6:=:=::25
"ote that these overvoltages do not a,se /ermanent
short=ir,iting o- the s,rgelimiter
To/
Overvoltages d,e to intermittent ins,lation -a,lts
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;ig,re 1 = %hen the s,bstation -rames 4*V5 and the PE earth
onnetion 4LV5 are
onneted to the same earth onnetion& the LV load -rames are
bro,ght to the
/otential Ih*T !/
Internal disr,/tive brea.do(n bet(een the *V9LV (indings This
.ind o- overvoltageis at net(or. -re?,eny These overvoltages are
rare and their s,dden7 a//earane
means that the s,rge limiter& (hose ertain aring voltage is
set at least at 2
