Earthing Systems

Welcome to the Schneider Electric Seminar on LV Power Concepts and Devices

Division - Name - Date - Language

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Agenda for the evening:Topic LV Earthing Systems LV Type Tested Panelboards Break for Salah LV Power Circuit Breakers LV Final Distribution Boards Wiring Devices Dinner Irfan Mufti Ibrahim Saleh

PresenterAsrar Mufti Shanker Shetty

Time6.15 to 6.50 pm 6.50 to 7.30 pm 7.30 to 7.50 pm 8.00 to 9.00 pm 9.00 to 9.30 pm

Mohammad Al-Amoudi 9.30 to 10.00 pm 10.00 pm


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LV Earthing Systems. as per IEC 60364 Installation Standard.

Fire protection

Risk analysis - the origins of fires in buildings

Studies carried out in Germany between 1980 and 1990Fire 37 % Lightning 1 % Explosion 1 %

Accidents 7 %

Cigarettes 6 %

Other 7 %

Electricity 41 %

41 % of fires are electrical in origin this risk is far from negligible it can be eliminated


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E92446

Fire protection

Main cause Ageing of the installation results in: less effective insulation the risk of very small leakage currents Presence of humidityLeakage currents

E92461

There is a real risk of fire starting at leakage currents of 300 mA


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E92462

Carbonisation of insulation (dust)

Small discharges

Low Voltage Earthing Systems

Basic principleT(ms) 10 000 5000 2000 1000 500 200 100 50 20 10 0,1 0,2 1 0,5 mA

Zone 1 : perception Zone 2 : unpleasant sensation Zone 3 : muscular contractions (reversible effects) Zone 4 : risk of ventricular

IEC 60479-1 Effect of current on the human body Time ms/current mA curve for AC current from 15 to 100 Hzb c1 c2 c3

fibrillation.(irreversible)

c1-c2 :prob. increases by 5% c2-c3 :prob. increases by 50% > c3 :prob. more than50 %

1

2

3

4

Body Impedance = 2000 Ohms.(Dry) = 1000 Ohms.(Wet) Max. Withstand Current = 25 mA

2

5 10

30 mA

100

500 2000 5000 (mA) 1000

UL (MAX. TOUCH VOLTAGE) = 2000x 0.025 = 50 V (Dry Conditions) = 1000x0.025 = 25 V (Wet Conditions)


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Protection of people

Standard IEC 60479-1Critical current thresholds

mA1A Cardiac arrest Irreversible cardiac fibrillation Breathing arrest

75 mA

30 mA

10mA

Muscular contraction


E92450

0.5 mA

Tingling

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Standard IEC 60479-1Effect of frequency

Current-sensitivity thresholds (mA) 500

100

30 DC 50 100 1000 (f)

E92451

The human body is most sensitive to frequencies in the 50 Hz/60 Hz range


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Basic principle

Maximum Touch Voltage Time (Protection of people according to IEC 364)Against Indirect Contact with Automatic Disconnection of Supply Maximum touch voltage time in UL = 50 V conditions prospective touch voltage (V) < 50 50 75 90 120 150 220 280 350 500 maximum protective device disconnection time(Seconds) AC current DC current 5 5 0.60 0.45 0.34 0.27 0.17 0.12 0.08 0.04 5 5 5 5 5 1 0.4 0.3 0.2 0.1 Maximum touch voltage time in UL = 25 V conditions (sockets/wet areas) prospective touch voltage (V) 25 50 75 90 110 150 230 280 maximum protective device disconnection time(Seconds) AC current DC current 5 0.48 0.30 0.25 0.18 0.12 0.05 0.02 5 5 2 0.80 0.50 0.25 0.06 0.02


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Basic principleProtection of people, direct contactDefinition Contact of persons or livestock with live parts which may result in electric shock


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Basic principleProtection of people, direct contactE36914

Types of protection

Insulation Distance

IP2X or IPXXB

TBT < 25 V

30 mA


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Standard IEC 60364Indirect contact Contact of persons or livestock with exposed conductive parts in case of a fault


E92454

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Basic principleProtection of people according to IEC 364

Protection against indirect contact with automatic disconnection of supply Earthing of all the exposed conductive parts of electrical equipment and all accessible conductive parts 2 simultaneously accessible exposed conductive parts must be connected to the same earth electrode Automatic disconnection by a protective device of the circuit in which a dangerous insulation fault occurs The protective device must operate within a time that is compatible with "Maximum Touch Voltage & Time-Safety requirements"


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Earthing Systems - General rules according to IEC 60364 312.2s

The Three Earthing Systems1. 2. 3.

T T I

T N T

1st letter Situation of supply T = Direct connection of Transformer Neutral with the earth I = Neutral unearthed or Impedance-earthed

2nd letter Situation of installation frames T = Exposed frames directly earthed N = Frames connected to the supply point which is earthed, either by a separate Protective Earth conductor (S). Or combined with the Neutral (C)


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Earthing systems

The different typess explanation of symbols according to IEC 617-11 (1983) Neutral conductor (N) Protective conductor (PE) Combined protective and neutral conductor (PEN)


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Earthing system techniqueTT systemL1 L2 L3 NE36886

Definition The neutral point of the LV transformer is directly connected to an earth electrode

Rn


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Earthing system techniqueTT systemL1 L2 L3 NE36886

Definition The neutral point of the LV transformer is directly connected to an earth electrode The exposed conductive parts of the installation are connected to an electrically separate earth electrode

PE Rn Ru


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System earthing arrangements

Earth electrode Equivalent electrical circuit "Deep" earth is equipotential in nature whatever the distance!11 1000 km

"Deep" earth the earth does not act as an insulator

"Deep" earthE92452

1

15

10

10

5


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E92453

"Deep" earth


Earth electrodeWell designed network At AA

L1 L2 L3 N PE

I1 + I2 + I3 + IN = IPE Well designed network

IPE = 0

IPE = 0

I1 + I2 + I3 + IN = 0

Current in the neutral does not depend on current IPE equal to unbalanced load currents and/or 3rd order harmonics (3 k)E92457

IN = Iunbalance

+ I3k


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Earth electrodeFaulty distribution systemL1 L2 L3 N PE

Faulty distribution system IPE 0

A

I1 + I2 + I3 + IN 0

IPE 0

Current in the neutral does not depend on current IPE equal to unbalanced load

currents and/or 3rd order harmonics (3 k)

E94409

IN = Iunbalance

+ I3k

Measurement of current IPE can be used for protection of persons (values depend on the earthing arrangement) protection against fire hazards However, it is necessary to detect the "true" IPE


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TT systemEarth-fault studyValue of fault current:Uo = 230 V L1 L2 L3 N

Id = Uo / (Rn + Ru) = 230 / (10 + 10) = 11.5 A Ud = Ru x If = 10 x 11.5 = 115 V > UL = 50 V The fault current generates a dangerous touch voltage The SCPD is usually not suitable for eliminating this type of fault

400 V/230 V

Id = 11,5 A

Exposed conductive part

Ud = 115 V Load Rn 10 E95420

Ru 10


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TT systemEarth-fault studyUo = 230 V

SolutionL1 L2 L3 N

The SCPD is usually not suitable for this type of fault (ST setting at 25 A) A residual current device specially designed for the protection of persons Tripping conditions: Max touch voltage < Safety curve Ru x I n < UL (I n is the setting of RCD) I n = UL / Ru = 50 /10 =5A

400/230 V

SCPD 25 A

I

n = 5A

Exposed conductive part

Load Rn 10 E95421

Ru 10


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TT systemMaximum disconnecting timesStandard IEC 60364 converts the exposure-time/current curves of standard IEC 60479-1 into tables presenting the disconnecting-time versus the nominal AC-voltage (Uo)50 V < Uo 120 V 120 V < Uo 230 V 230 V < Uo 400 V Uo > AC 0.2 DC 0.4 AC 0.07 DC 0.2 AC 0.04 400 V DC 0.1

Disconnecting time (s) AC TT system 0.3

DC 5

From table 41 A of standard IEC 60364


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SEAs and devices

Earth-leakage protection RCD technologies electromechanical no auxiliary power required electronic integrated in SCPD (no auxiliary power required) separate from the SCPD auxiliary supply required RCDs are immune to nuisance tripping to DC currents (class A as defined by IEC 755)


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Associated switchgear in TTEarth leakage protectionE37522

RCD electromechanical own current


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RCD electromechanical own current electronic integrated in the voltageoperated short-circuit protection device


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RCD technologies electromechanical own current electronic integrated in the voltageoperated short-circuit protection device separate from the short-circuit protection device auxiliary supply


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SEAs and devices

Devices for the TT systemOperating principleOperating principle of residual current devices requiring no auxiliary supply (electronic) Detection MeasurementNo aux. power required

Tripping

Tripping

Detectio n

Earth-leakage relay

E37508

M

Measurement


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SEAs and devices

Devices for the TT systemSelection of solutions Electromechanical technology for final distribution Application: protection of life and property in all sectors (industrial, commercial and residential) Main characteristics: continuity of service and safe if neutral conductor is cut

E37540Division - Name - Date - Language

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SEAs and devices

Devices for the TT systemSelection of solutions Electronic technology for power distribution Application: general protection from the main low voltage switchboard to the secondary switchboard in industrial and large commercial buildings Main characteristics: high-performance solutions wide range of settings (discrimination) miniaturisation solutions for complex installations qualified personnel (lead-sealable relays)


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SEAs and devices

Devices for the TT systemEarth-leakage protection DiscriminationRCD1

vertical discrimination

setting I n1 > 2 I n2RCD2E95454

time-delay settings RCD1 > RCD2 Caution. For an RCD not integrated in the SCPD, RCD2 disconnecting time = tripping time + time delay horizontal discrimination


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SEAs and devices

ApplicationsCoordination of RCDs

Discrimination rulesCB1

Two conditions:RCD1

1 2

I n (RCD1) > 2 I n (RCD2) t (RCD1) > t (RCD2) + t (CB2) (including the disconnecting time)

CB2

E94442

RCD2

2 To implement condition , it is necessary to know the total breaking time guaranteed for the CB2 + RCD2 combination or to run tests on the combination


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SEAs and devices


Discrimination rulesCB1 RCD1

Two conditions:

1 2

I n (RCD1) > 2 I n (RCD2) setting (RCD1) setting (RCD2) +1

CB2 RCD2

E94442

Condition 2 is automatically obtained for (RCD2) +1 if RCD2 is combined with a circuit breaker/switch disconnector from the Multi 9 or Compact ranges


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RCDs


Discrimination rules with Vigirex upstreamCB1 Vigi RCD1

Two conditions: 1 I n (RCD1) > 1.5 I n (RCD2) 2 setting (RCD1) setting (RCD2) +1

CB2 RCD2

E94442

Condition 2 is automatically obtained for (RCD2) +1 if RCD2 is combined with a circuit breaker/switch disconnector from the Multi 9 or Compact ranges


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Main features of the TT system Protection of persons: fault current is dangerous fault current is too weak to trigger the short-circuit protection devices protection must be practically instantaneous It is provided by a specially designed RCD device Fire protection: fault current is limited "naturally" managed by RCDs for the protection of persons Continuity of service: ensured by discrimination between the RCDs


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TT Earthing SystemConclusion Fault current limited Dangerous touch voltage First fault tripping Human Protection ensured. No Risk of Fire. Continuity of Service simple design use of RCDs system easily extensible.


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TN systemDefinition The neutral point of the LV transformer is directly connected to an earth electrode The exposed conductive parts of the installation are connected by the PE to the same earth electrode

L1 L2 L3 N PE


E95416

Rn

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TN-S systemDefinition (cont.) The PE and neutral conductor are separate

L1 L2 L3 N PE


E95417

Rn

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TN-C systemDefinition (cont.) A common conductor is used for both the PE and the neutral conductors (PEN)

L1 L2 L3 PEN


E95423

Rn

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TN-C-S systemDefinition (cont.) In this TN sub-system:L1 L2 L3 PEN L1 L2 L3 N PE

the upstream part is TN-C

(with PEN) the downstream part is TN-S

(with PE and N) Note. A TN-S system may not be used upstream of a TN-C system

E95424

Rn


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TN systemEarth-fault studyUo = 230 V L1 L2 L3 N PE

Consider the PH & PE Conductor are Copper, 50 m Long with a X-section of 35 mm2. The Fault Current Id =U0/(RPE +RPH) RPE= RPH= . L/S =0.025 -mm2/m for Cu. RPE= RPH=0.025 x 50/35 = 32.14 m Id = 230/(2 x 0.3214) = 3578 A.

400 V/230 V

Id

The fault current is equal to a Ph/N shortcircuit

Exposed conductive part Fault Rn

Uc

E95425

This Fault Current will generate a Touch Voltage Uc = RPE x Id = 3578 x 0.03214 = 115 V. Since the fault current depends on the Length of the Lines, it is necessary to check that the Fault Current is more than the Protection Operating Threshold of the CB i.e Id > Ia


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TN systemEarth-fault study (cont.)Uo = 230 V L1 L2 L3 N PE

The Value of the fault current is: Id = 0.8.Uo. SPH .(1 + m).L where m=Sph/Spe L=Length of the Cond. Lmax = 0.8Uo. SPH .(1 + m).Ia If the length of the conductor is greater than Lmax., it is necessary to; Reduce Ia.

400 V/230 V

Id

Exposed conductive part Fault Rn

Uc

Increase Spe. Install an RCD.

E95425


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TN systemImplementationUo = 230 V

TN-SL1 L2 L3 N PE

PE separate from the neutralProtection SN

of the neutral

400 V/230 V

= SPH < SPH

disconnected, not protectedSNExposed conductive part Load

disconnected, protected


E95426

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TN systemImplementation (cont.)TN-CUo = 230 V L1 L2 L3 PE N

PEN = protective conductor and neutral conductor Protection of the PEN SPEN = SPH the PEN must not be disconnected The exposed conductive parts of the substation, the LV neutral and the exposed conductive parts of the loads are connected to the same earth electrode

400 V/230 V

Exposed conductive part Load


E95427

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TN systemMaximum disconnecting times The disconnecting time depends on the distribution-system voltage Uo

50 V < Uo 120 V Disconnecting time (s) AC TN system 0.8 DC 5

120 V < Uo 230 V 230 V < Uo 400 V Uo > AC 0.4 DC 5 AC 0.2 DC 0.4 AC 0.1

400 V DC 0.1

Drawn from table 41 A of standard IEC 60364


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SEAs and devices

Devices for the TN-S systemProtection by short-circuit protection devicest

t < 0.4 s

Protection: for a given cross-section and material (e.g. copper or aluminium), the fault current Id depends on the length of the conductorsId I

E95442

circuit breaker protection:t

setting of magnetic relay / ST

< 30 msE95449

Im

Id

I


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Earthing system technique

TN system- Checking of the tripping conditions: The max. Length of any circuit of a TN-earthed installation is

0.8.Uo.Sph .(1+m).Ia L = Length of the Conductor. Sph= Cross-Sectional area of Ph. Cond. = resistivity in Ohm-mmsq/metre (22.5 mohm for Cu) m = Ratio between Sph and SPE Ia = Trip Current setting for Inst. Operation of CB. If the condition is not met reduce the magnetic setting install an RCD - LS (up to 250A) Increase Cross-Sectional area of the Cond. L max =

see pg. G20 for Tables.


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SEAs and devices

Devices for the TN-S systemProtection by short-circuit protection devices If the conditions for correct protection are not met

Circuit breaker low setting of magnetic relay/ST or installation of a standard RCD or increase the conductor cross-section


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SEAs and devices

ApplicationsProtection of property Protection of motors (TN-S system)

R

MERLIN GERIN

a low insulation fault can cause a short-circuit an RCD with a current setting between 3 and 30 A avoids this risk


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E94458

M

SEAs and devices

Devices for the TN-S systemSummaryEarth-fault studyUo = 230 V L1 L2 L3 N PE

400 V/230 V

SCPD tripping at 160 A RCD

The fault current is equal to a phase/neutral short-circuit The fault current generates a dangerous touch voltage The circuit breaker trips Check the loop impedance

Exposed conductive part LoadE95428

Earth-leakage protection set to 300 mA is recommended if there is a risk of fire

Rn


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SEAs and devices

Devices for the TN-S systemProtection by short-circuit protection devices (SCPD)t D1 D2 1 I

Discrimination by circuit breakers current LT and ST settingsIm2

E95450

Im1 t D1 D2

time t

intentional delay of the LT and ST upstream protectionI

E95451

t D1 D2 I2 tE95452

energy

comparison of energies (ST)

I


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SEAs and devices

Devices for the TN-S systemProtection by short-circuit protection devices (SCPD)TN-C / 3P 3D TN-S / 4P 3D, 4P 4D

Merlin Gerin type ranges circuit breakers

Masterpact

Compact

Multi 9 Circuit breakers also provide overload protection for all low-voltage system earthing arrangements


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SEAs and devices

Main featuresTN-S system Protection of persons: fault current is dangerous fault current is usually high enough to trip the SCPDs tripping must be practically instantaneous It is ensured by the magnetic settings on the SCPDs if the fault current is not high enough, RCDs may be used to ensure protection Fire protection: fault current is high it must be managed by additional RCDs Continuity of service: ensured by discrimination between the short-circuit protection devices and among RCDs


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SEAs and devices

Main featuresTN-C Protection of persons: fault current is dangerous fault current is usually high enough to be tripped by the SCPDs tripping must be practically instantaneous = same as TN-S It is ensured by the magnetic settings on the SCPDs if the fault current is not high enough, the installation must be resized

E37544

Fire protection: cannot be provided (TN-C not allowed where there is a risk of fire) Continuity of service: ensured by discrimination between the SCPDs = same as TN-S


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TN systemConclusion High fault currents Dangerous touch voltage

Tripping after first fault cost savings check on tripping conditions calculations required for extensions


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IT systemDefinition The Neutral point of the LV transformer is Isolated, not connected to an earth electrode The exposed conductive parts of the loads are connected by the PE conductor to a common earth electrode

L1 L2 L3 N PE


E95429

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IT systemDefinition (cont.) The Neutral point of the LV transformer is Isolated and not connected to an earth electrode The exposed conductive parts of the loads are connected by the PE conductor to a common earth electrode or to separate earth electrodes

L1 L2 L3 N PE

PE

PE


E95430

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IT systemEarth-fault study Under Normal operation, the System is earthed by its System Leakage Impedance.

L1 L2 L3

PE


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IT systemEarth-fault study (cont.) System leakage impedanceL1 L2 L3 PE

If=U/Zt =230/3500 =0.065 AUc=10 x0.065= 0.6V Uc< UL(50V) The touch voltage is not

dangerous There is no risk of fire The fault does not cause tripping but it must be indicated


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IT systemSignalling the first fault Detection principle:L1 L2 L3 PE

emission of a specific zero

sequence signal


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IT systemSignalling the first fault (cont.)L1 L2 L3 PE

Detection principle: emission of a specific zero sequence signal Fault-clearance principle: detection by toroid and indication of the faulty outgoer


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SEAs and devices

Devices for the IT systemIMD 1st faultI inj L1 L2 L3 N PE

Principle injection of current tracking generator measurement of IR IMD:(Insulation Monitoring Device) DC current: direct measurement of IR AC current: calculation of IR

I inj I inj I inj

RI

eI inj

(IR) Insulation ResistanceE95435Division - Name - Date - Language

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SEAs and devices

Devices for the IT systemIMD 1st fault (cont.)L1 L2 L3 N

Principle of the FTD (*) detection of fault current Type of FTD (*): portable fixed

PE

E95436

(*) Fault Tracking Device


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SEAs and devices

Devices for the IT systemIMD & FTD Merlin Gerin range IMD (*)

FTD (**)

(*) Insulation Monitoring Device (**) Fault Tracking Device


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SEAs and devices

Devices for first faultRCDs Typical leakage currents following a first faultSystem leakage capacitance (F) First-fault current 1 5 30 70 mA 360 mA 2.17 A I n setting 300 mA 1A 5A

Standardised rule IEC 60364-5-53: The RCD current settings must be greater than twice the first-fault current


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SEAs and devices

Protection plan for second faultIT system with interconnected exposed conductive parts (ECP)Study of the 2nd earth faultL1 L2 L3 N PE Id1 Id2

The SCPD protection trips protection is ensured by the same circuit breaker as for TN-S, mais 4P 4t is compulsory Check the loop impedance Merlin Gerin circuit breakers are appropriate for protection in IT systems


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SEAs and devices

Devices for second faultIT system with ECPs not interconnectedStudy of 2nd earth faultL1 L2 L3 N

Same principle as TT system (length of conductors) Protection provided by RCDs (same switchgear as TT)

CPI


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SEAs and devices

Protection plan for second faultIT systemMaximum disconnecting times for IT systems for the 2nd fault50 V < Uo 120 V 120 V < Uo 230 V 230 V < Uo 400 V Uo > AC 0.4 DC 5 AC 0.2 DC 0.4 AC 0.1 400 V DC 0.1

Disconnecting time (s) AC IT system 0.8

DC 5

Drawn from table 41 A of standard IEC 60364


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SEAs and devices

Main characteristics of the IT system Protection of persons: the insulation fault is not dangerous

Protection is ensured by the IT system itself, however a maintenance strategy is required A second fault is dangerous and protection must be ensured by the magnetic setting of the SCPD s or the RCDs Fire protection: the fault current is close to zero Continuity of service is total


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IT systemConclusion First-fault current is very weak First-fault touch voltage is very weak Dangerous touch voltage in the event of a double fault Tripping after the second fault Optimal safety when first fault occurs Continuity of service when first fault occurs Use of IMD for fault tracking Check on tripping conditions Calculations necessary for extensions


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Earthing system techniqueIT systemConclusion First fault current very weak First fault touch voltage very weak Dangerous touch voltage if there is a double fault Second fault tripping optimal safety when first fault occurs continuity of service when first fault occurs use of PIM for fault tracking checking of tripping conditions calculations necessary for extensions


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Fire protection

Standard IEC 60364

IE C 603 64E94550

Standard IEC 60364, section 3-32, defines premises presenting a risk of fire (BE2) or explosion (BE3) Standard IEC 60364, section 4-48, deals with premises where there is a risk of fire imposes use of a 500 mA RCD device recommends use of a TT or IT system for the electrical installation on such premises prohibits use of a TN-C system

In TT, IT and TN-S systems, a 300 mA RCD eliminates the risk of fire


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Selection of a system earthing arrangement6 selection criteria Protection of persons Protection of equipment Continuity of the power supply Effects of disturbances Easy implementation Economic analysis


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Selection of a system earthing arrangementConclusion Facility managers need a dependable electrical distribution IEC 364 offers solutions which: optimally protect persons (systems, RCDs, neutral switching, etc.)

E37521

minimise the risk of fire (TT, IT systems, RCDs) protect property by limiting leakage/fault currents (IT, TT, TN-S with

RCDs) Mixing of system earthing arrangements is the means to provide optimum solutions to the needs of operators

IEC 364 means a dependable, high-performance installation


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Low Voltage Earthling Systems

Earthing Systems Comparison.E00000

Criterion Protection of people Protection against Fire Ease of Implementation Continuity of service Upgradable installation Cost Saving

TT XXXX XXXX XXX XX XXXX XXXXXX=Excellent

TN-S XXX XXX X XX XX XXXXXX=Good

TN-C XX X X XX XX XXXXXX=Average

IT XXXX XX X XXXX XX XX=Caution


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Earthing Systems

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