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Earthing Systems
Design of Low Voltage Electrical Installation
Schneider Electric India Training Institude 2
Contents
General rulesDifferent types of Earthing systems
Schneider Electric India Training Institude 3
Compliance with natural standardsExample : in some countries in Europe TT is imposed for domestic useand schoolsIT is imposed in several areas.
there is no best system as every one has its advantages and weaknesses. They are all very good if you follow the rules. There is no miraculousearthing system
look carefully at : - the importance of continuity of service- the level of qualification of the maintenance team
take into account the features of some loads : motors are sensitive to high levels of current, some loads have a low insulation level
finally : think about the possibility of combining earthing systems. The use of a LV/LV transformer enables you to adapt the earthing system to fit the needs of the loads.
Earthing systems
General rules
Schneider Electric India Training Institude 4
Earthing systems
General rules according to IEC 364 § 312.2
system
situation of supply/earth :T = direct connection ofa point with the earthI = unearthed orimpedance-earthed
TN system
complementary letters
S = PE protection function separate from N or from the liveconductor (Ph) which is earthedC = protection function common to N (PEN)
TTI
TNT
1st letter 2nd letter
situation of installationframes/earth :T = frames directly earthedN = frames connected to the supplypoint which is earthed
Schneider Electric India Training Institude 5
Earthing systems
The different types
explanation of symbols according to IEC 617-11 (1983)
Neutral conductor (N)
Protective conductor (PE)
Combined protective andneutral conductor (PEN)
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the TT system has one point directly earthed, the exposed-conductive-parts of the installation being connected to earth electrodes electrically independentof the earth electrodes of the power system.
L1
L2L3N
PE
Earthingof system
Exposed-conductive-parts
L1
L2L3
PE
Earthingof system
Exposed-conductive-parts
Earthing systems
The different types
Schneider Electric India Training Institude 7
TT system
Situation on faultDangerous fault voltage (Ud)Mandatory disconnection on the 1st faultFault current to small to activate short-circuit protection in due timeUse of RCD (one per earth electrode)
RB RA
PEN
Id
RdUd
123
RB = RA = 10 ΩRd = 0 ΩId = Uo/(RB + RA + Rd)
= 230/20 = 11,5 AUd = RA x Id
= 11,5 x 10 = 115 V
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TT system
Analysis of a faultUtility is not responsible for fault protection which relies on private earth electrode and private PE conductorIn practice RA >> RB ⇒ fault voltage Ud ≈ ∆U in RA ⇒ Ud ≈ Uo(230V)If total equipotential bonding, touch voltage Uc ≈ 0 VAs Id is not high, PE cross sectional area may be reduced compared to Phase cross sectional area
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TT system
AdvantagesNo strong fault currentsNo additional requirements on cablesEasy extension
DrawbacksDisconnection on first faultUse of RCDNeeds 2 separate earth electrodes
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TN system
Situation on faultDangerous fault voltage (Ud)Mandatory disconnection on the 1st faultFault current may be high enough to activate short-circuit protection
Id=Ik1Ud=∆UPE
If SPE = Sph
Ud = 0,5Uph = 115 VIf SPE = 0,5 Sph
Ud = 0,66 Uph = 153 V
123PEN
Ud
RB
Id
Schneider Electric India Training Institude 11
TN system
Verification of the disconnection conditionsCircuit-breaker: Imag ≤ IdFuses: Ifu ≤ Id (Ifu = current on which fuse melts in due time)
Simplified algorithm
where
1
2
3
N
PE
IdRB
L
m)Lρ(1S0,8U
I ph0d +
=
PE
ph
SS
m =
mag
ph0max m)Iρ(1
S0,8UL
+=
Schneider Electric India Training Institude 12
TN system
Analysis of a faultUtility may be responsible for fault protection which relies on PE conductor (continuity, impedance)In case of feeder with great length, disconnection in due time may not be fulfilled
– to adjust magnetic setting at lower value– to increase cross-sectional area of cables– to use RCD (TN-S)
Schneider Electric India Training Institude 13
Earthing systems
The different types
L1
L2
L3
PEN
Earthingof system
Exposed-conductive-parts
TN-C system. Neutral and protective functions combined in a singleconductor throughout the system
Schneider Electric India Training Institude 14
Earthing systems
The different types
TN-S system : In which throughout the system, a separate protective conductor is used
L1
L2
L3
PE
Earthingof system
Exposed-conductive-parts
L1
L2
L3
PE
Earthingof system
Exposed-conductive-parts
Separate earthed phaseconductor and protectiveconductors throughoutthe system
Separate neutral and protectiveconductors throughout the system
N
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TN systemTN-C (Common) and TN-S (Separate)
Rules– TN-S has to be used if Sph < 10 mm² (Cu) or for flexible
cables– TN-C forbidden downstream TN-S– Multiple and regular earthing of exposed-conductive-parts
and PEN in TN-C– PE or PEN wire must run in vicinity of phase conductors
1
2
3
N
PE
TN-C TN-S
RB
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TN system
AdvantagesNo need of specific disconnecting devicesPE and N may be commonEasy location of faulty feeder
DrawbacksDisconnection on first fault Important fault currentVerification of installation after fault (ex: tightening torque)
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Earthing systems
The different types
L1L2L3
Earthingof system
Exposed-conductive-parts
PE
Impedance 1)
L1L2L3
Earthingof system
Exposed-conductive-parts
PE
Impedance 1)
312.2.3 IT systemthe IT system has all live parts isolated from earth or one point connected to earth
through an impedance, the exposed-conductive-parts of the electrical installationbeing earthed independently or collectively or to the earthing of the system. (SeeIEC 364-4-41, subclause 413.1.5)
1) the system may be isolated from earthThe neutral may or may not be distributed
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IT system
Insulation characteristicsFor 1 km of cable
– R1 = R2 = R3 = 10 MΩ– C1 = C2 = C3 = 0,3 µF
At 50 Hz: total for 1 km of cables321eq R
1R1
R1
R1
++=
MΩ 3,33Req =
3Cω1Xeq =
kΩ 3,54Xeq =
Ω 3540ωCR1
RZ222eq =
+=
123
equivalent to :
123
Zeq
C3 C2 C1
R3 R2 R1
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IT system
Situation of a single faultVery small fault currentNo hazardous fault voltageDisconnection not needed
RB=10WRdd=0WZeqeq=3540W(1 km of cable)
Idd=230/3540=65 mA
Udd=RAAxIddUdd=0,065x10=0,65 V
RB RA
Zct
Rd
Ud
123PE
CTDABd ZRRR
UI+++
≈
Schneider Electric India Training Institude 20
IT system
Analysis of the single faultIf used for public distribution, high chance to be permanently in situation of a first faultLimit of IT = single fault voltage exceeds 50 VAccording to interconnection of the exposed-conductive-parts, situation on double fault corresponds to TT or TNReal insulation of an LV electrical installation is always very poor !
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IT system
Insulation Monitoring DeviceTo signal the presence of a first insulation fault
123PE
IMD
RB RA
Z
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IT system
Insulation fault locationInjection of a specific zero sequence signalDetection of signal by “RCD type” device
123PE
Zero sequence supply
RB RA
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IT system
Double fault situationCase where all exposed-conductive-parts are interconnected (one single earth electrode)
– situation similar to TN system– disconnection by overcurrent protecting device– simplified algorithmIf no neutral distributed
If neutral distributed
with:
123PE
RB RA
IMDmag
NphOmax m)Iρ(1
)S or (S0,8U21L
+=
mag
phOmax m)Iρ(1
S0,8U23L
+=
PE
Ph
SSm =
Schneider Electric India Training Institude 24
IT system
Double fault situationCase where all exposed-conductive-parts are not interconnected (several earth electrodes)
– situation similar to TT system– disconnection by RCD– one RCD per separate earth electrode
RCD
123
IMD
RARB
B∆n R
50I ≤
Schneider Electric India Training Institude 25
IT system
AdvantagesContinuity of service (no disconnection on first fault)Optimum safety on single faultMay not require separate earth electrodes
DrawbacksSpecific devices needed (IMD, Voltage suppresser)Installation to be monitored by instructed personsNo distribution of neutral is recommended
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