Earthing Systems

1

Earthing Systems

Design of Low Voltage Electrical Installation

Schneider Electric India Training Institude 2

Contents

General rulesDifferent types of Earthing systems


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


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


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)


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


Earthing systems

The different types


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


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


TT system

AdvantagesNo strong fault currentsNo additional requirements on cablesEasy extension

DrawbacksDisconnection on first faultUse of RCDNeeds 2 separate earth electrodes


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


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

+=


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)


Earthing systems

The different types

L1

L2

L3

PEN

Earthingof system


TN-C system. Neutral and protective functions combined in a singleconductor throughout the system


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


L1

L2

L3

PE

Earthingof system


Separate earthed phaseconductor and protectiveconductors throughoutthe system

Separate neutral and protectiveconductors throughout the system

N


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


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)


Earthing systems

The different types

L1L2L3

Earthingof system


PE

Impedance 1)

L1L2L3

Earthingof system


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


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


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+++

≈


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 !


IT system

Insulation Monitoring DeviceTo signal the presence of a first insulation fault

123PE

IMD

RB RA

Z


IT system

Insulation fault locationInjection of a specific zero sequence signalDetection of signal by “RCD type” device

123PE

Zero sequence supply

RB RA


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 =


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 ≤


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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Earthing Systems