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Rabu, 11 April 2012

Implementation of the TN system


Preliminary conditions

At the design stage, the maximum permitted lengths of cable downstream of a protective circuit-breaker (or set of fuses) must be calculated, while during the installation work certain rules must be fully respected.
Certain conditions must be observed, as listed below and illustrated in Figure F38.
  1. PE conductor must be regularly connected to earth as much as possible.
  2. The PE conductor must not pass through ferro-magnetic conduit, ducts, etc. or be mounted on steel work, since inductive and/or proximity effects can increase the effective impedance of the conductor.
  3. In the case of a PEN conductor (a neutral conductor which is also used as a protective conductor), connection must be made directly to the earth terminal of an appliance (see 3 in Figure F38) before being looped to the neutral terminal of the same appliance.
  4. Where the conductor ≤ 6 mm2 for copper or 10 mm2 for aluminium, or where a cable is movable, the neutral and protective conductors should be separated (i.e. a TN-S system should be adopted within the installation).
  5. Earth faults may be cleared by overcurrent-protection devices, i.e. by fuses and circuit-breakers.
The foregoing list indicates the conditions to be respected in the implementation of a TN scheme for the protection against indirect contacts.


FigF38.jpg













Notes:
  • The TN scheme requires that the LV neutral of the MV/LV transformer, the exposed conductive parts of the substation and of the installation, and the extraneous conductive parts in the substation and installation, all be earthed to a common earthing system.
  • For a substation in which the metering is at low-voltage, a means of isolation is required at the origin of the LV installation, and the isolation must be clearly visible.
  • A PEN conductor must never be interrupted under any circumstances. Control and protective switchgear for the several TN arrangements will be:
  - 3-pole when the circuit includes a PEN conductor,
  - Preferably 4-pole (3 phases + neutral) when the circuit includes a neutral with a separate PE conductor.

Fig. F38: Implementation of the TN system of earthing


Protection against indirect contact

Three methods of calculation are commonly used:
  • The method of impedances, based on the trigonometric addition of the system resistances and inductive reactances
  • The method of composition
  • The conventional method, based on an assumed voltage drop and the use of prepared tables
Methods of determining levels of short-circuit current
In TN-earthed systems, a short-circuit to earth will, in principle, always provide sufficient current to operate an overcurrent device.
The source and supply mains impedances are much lower than those of the installation circuits, so that any restriction in the magnitude of earth-fault currents will be mainly caused by the installation conductors (long flexible leads to appliances greatly increase the “fault-loop” impedance, with a corresponding reduction of short-circuit current).
The most recent IEC recommendations for indirect-contact protection on TN earthing systems only relates maximum allowable tripping times to the nominal system voltage.(see Figure F12
The reasoning behind these recommendations is that, for TN systems, the current which must flow in order to raise the potential of an exposed conductive part to 50 V or more is so high that one of two possibilities will occur:
  • Either the fault path will blow itself clear, practically instantaneously, or
  • The conductor will weld itself into a solid fault and provide adequate current to operate overcurrent devices
To ensure correct operation of overcurrent devices in the latter case, a reasonably accurate assessment of short-circuit earth-fault current levels must be determined at the design stage of a project.
A rigorous analysis requires the use of phase-sequence-component techniques applied to every circuit in turn. The principle is straightforward, but the amount of computation is not considered justifiable, especially since the zero-phase-sequence impedances are extremely difficult to determine with any reasonable degree of accuracy in an average LV installation.
Other simpler methods of adequate accuracy are preferred. Three practical methods are:
  • The “method of impedances”, based on the summation of all the impedances (positive-phase-sequence only) around the fault loop, for each circuit
  • The “method of composition”, which is an estimation of short-circuit current at the remote end of a loop, when the short-circuit current level at the near end of the loop is known
  • The “conventional method” of calculating the minimum levels of earth-fault currents, together with the use of tables of values for obtaining rapid results
These methods are only reliable for the case in which the cables that make up the earth-fault-current loop are in close proximity (to each other) and not separated by ferro-magnetic materials.
Method of impedances
For calculations, modern practice is to use software agreed by National Authorities, and based on the method of impedances, such as Ecodial 3. National Authorities generally also publish Guides, which include typical values, conductor lengths, etc.
This method summates the positive-sequence impedances of each item (cable, PE conductor, transformer, etc.) included in the earth-fault loop circuit from which the short-circuit earth-fault current is calculated, using the formula:
I=\frac{Uo}{\sqrt{\left ( \sum R \right )^2 + \left ( \sum X \right )^2 }} 
where
(ΣR) 2 = (the sum of all resistances in the loop)2 at the design stage of a project.
and (ΣX) 2 = (the sum of all inductive reactances in the loop)2and Uo = nominal system phase-to-neutral voltage.
The application of the method is not always easy, because it supposes a knowledge of all parameter values and characteristics of the elements in the loop. In many cases, a national guide can supply typical values for estimation purposes.
Method of composition
This method permits the determination of the short-circuit current at the end of a loop from the known value of short-circuit at the sending end, by means of the approximate formula:
Isc=I\frac{Uo}{U+Zs\ Isc}
where
Isc = upstream short-circuit current
I = end-of-loop short-circuit current
Uo = nominal system phase voltage
Zs = impedance of loop
Note: In this method the individual impedances are added arithmetically(1) as opposed to the previous “method of impedances” procedure.
(1) This results in a calculated current value which is less than that it would actually flow. If the overcurrent settings are based on this calculated value, then operation of the relay, or fuse, is assured.
Conventional method
The maximum length of any circuit of a TN-earthed installation is: \frac{0.8\ Uo\ Sph}{\rho \left ( 1+m \right )Ia}
This method is generally considered to be sufficiently accurate to fix the upper limit of cable lengths.
Principle The principle bases the short-circuit current calculation on the assumption that the voltage at the origin of the circuit concerned (i.e. at the point at which the circuit protective device is located) remains at 80% or more of the nominal phase to neutral voltage. The 80% value is used, together with the circuit loop impedance, to compute the short-circuit current.
This coefficient takes account of all voltage drops upstream of the point considered. In LV cables, when all conductors of a 3-phase 4-wire circuit are in close proximity (which is the normal case), the inductive reactance internal to and between conductors is negligibly small compared to the cable resistance. This approximation is considered to be valid for cable sizes up to 120 mm2.
Above that size, the resistance value R is increased as follows: 
Core size (mm2)Value of resistance
S = 150 mm2R+15%
S = 185 mm2R+20%
S = 240 mm2R+25%
The maximum length of a circuit in a TN-earthed installation is given by the formula:
Lmax=\frac{0.8\ Uo\ Sph}{\rho \left ( 1+m \right )Ia}
where:
Lmax = maximum length in metres
Uo = phase volts = 230 V for a 230/400 V system
ρ = resistivity at normal working temperature in ohm-mm2/metre
(= 22.5 10-3 for copper; = 36 10-3 for aluminium)
Ia = trip current setting for the instantaneous operation of a circuit-breaker, or
Ia = the current which assures operation of the protective fuse concerned, in the specified time.
m=\frac{Sph}{SPE}
Sph = cross-sectional area of the phase conductors of the circuit concerned in mm2SPE = cross-sectional area of the protective conductor concerned in mm2.
(see Fig. F39)


FigF39.jpg
 



















Fig. F39: Calculation of L max. for a TN-earthed system, using the conventional method
Tables
The following tables give the length of circuit which must not be exceeded, in order that persons be protected against indirect contact hazards by protective devices
The following tables, applicable to TN systems, have been established according to the “conventional method” described above.
The tables give maximum circuit lengths, beyond which the ohmic resistance of the conductors will limit the magnitude of the short-circuit current to a level below that required to trip the circuit-breaker (or to blow the fuse) protecting the circuit, with sufficient rapidity to ensure safety against indirect contact.
Correction factor m
Figure F40 indicates the correction factor to apply to the values given in Figures F41 to F44, according to the ratio Sph/SPE, the type of circuit, and the conductor materials.
The tables take into account:
  • The type of protection: circuit-breakers or fuses
  • Operating-current settings
  • Cross-sectional area of phase conductors and protective conductors
  • Type of system earthing (see Fig.F45 )
  • Type of circuit-breaker (i.e. B, C or D)(1)
The tables may be used for 230/400 V systems.
Equivalent tables for protection by Compact and Multi 9 circuit-breakers (Merlin Gerin) are included in the relevant catalogues.


CircuitConductor materialm = Sph/SPE (or PEN)
m = 1m = 2m = 3m = 4
3P + N or P + NCopper10.670.500.40
Aluminium0.620.420.310.25
Fig. F40: Correction factor to apply to the lengths given in tables F41 to F44 for TN systems

Circuits protected by general purpose circuit-breakers (Fig. F41)


Nominal cross-
sectional
area
of
conductors
Instantaneous or short-time-delayed tripping current Im (amperes)
mm2506380100125160200250320400500560630700800875100011201250160020002500320040005000630080001000012500
1.51007963504031252016131098766544          
2.51671331048367524233262117151312101087754        
426721216713310783675342332724211917151312118754      
640031725020016012510080635040363229252320181613108654    
10  41733326720816713310483676053484238333027211713108754  
16    42733326721316713310795857667615348433327211713118754
25      41733326020816714913211910495837467524233262117131087
35       467365292233208185167146133117104937358473629231915129
50        49539631728325122619818115814112799796349403225201613
70           4173703332922672332081871461179373584737292319
95             45239636231728326319815812799796350403225
120               4574003573202502001601251008063504032
150                4353883482722171741361098769544335
185                 45941132125720616112810382645141
240                   400320256200160128102806451
Fig. F41:  Maximum circuit lengths (in metres) for different sizes of copper conductor and instantaneous-tripping-current settings for general-purpose circuit-breakers in 230/400 V TN system with m = 1

Circuits protected by Compact or Multi 9 circuit-breakers for industrial or domestic use (Fig. F42 to Fig. F44)


SphRated current (A)
mm212 34 61016 20253240 506380100125
1.51200600400 300 20012075604837302419151210
2.5 1000666500 333200125100 8062504032 252016
4   1066 800533320200160128100806451 4032 26
6   1200800 480300240192150120967660 4838 
10        800500400 320250200160127100 80 64
16           800640512400 320 256 203160128102
25               800625500400317250200160
35                   875700 560444 350280224 
50                     760 603475380304
Fig. F42:  Maximum circuit lengths (in meters) for different sizes of copper conductor and rated currents for type B (1) circuit-breakers in a 230/400 V single-phase or three-phase TN system with m = 1


SphRated current (A)
mm212 34 61016 20253240 506380100125
1.5600300200150100603730241815129765
2.5 5003332501671006250 4031 252016 1210
4   533400 2671601008064 504032 25 2016 13 
6   600400 24015012096756048 3830 24 19 
10      677400250200 1601251008063 50 40 32 
16           640400320256200 160 1281018064 51 
25             625500400312250200159 12510080 
35               875 700560437350 280222 175140112 
50                 760594475380 301 237190152 
Fig. F43:  Maximum circuit lengths (in metres) for different sizes of copper conductor and rated currents for type C (1) circuit-breakers in a 230/400 V single-phase or three-phase TN system with m = 1
(1) For the definition of type B and C circuit-breakers refer to chapter H


SphRated current (A)
mm212 34 61016 20253240 506380100125
1.54292141431077143 2721171311953
2.5714357238179 11971 45 36 2922 18 1411 76
4 571381 286 190114 71 804636 29 23 18 14119
6 857571429286171 107 120 69 54 43 34 27 21 1714
10    952714476284 179 200 114 89 71 57 45 36 2923
16           762457 286 320183143 114 91 73 57 4637
25          714446500286 223 179 143 113 89 7157
35            625700400313 250 200 159 125 80100
50              848543424339271 215 170 136109
Fig. F44: Maximum circuit lengths (in metres) for different sizes of copper conductor and rated currents for type D (1) circuit-breakers in a 230/400 V single-phase or three-phase TN system with m = 1
(1) For the definition of type D circuit-breaker refer to chapter H

Example
A 3-phase 4-wire (230/400 V) installation is TN-C earthed. A circuit is protected by a type B circuit-breaker rated at 63 A, and consists of an aluminium cored cable with 50 mm2 phase conductors and a neutral conductor (PEN) of 25 mm2.
What is the maximum length of circuit, below which protection of persons against indirect-contact hazards is assured by the instantaneous magnetic tripping relay of the circuit-breaker?
Figure F42 gives, for 50 mm2 and a 63 A type B circuit-breaker, 603 metres, to which must be applied a factor of 0.42 (Figure F40 for m=\frac{Sph}{SPE}=2).
The maximum length of circuit is therefore:
603 x 0.42 = 253 metres.
Particular case where one or more exposed conductive part(s) is (are) earthed to a separate earth electrode
Protection must be provided against indirect contact by a RCD at the origin of any circuit supplying an appliance or group of appliances, the exposed conductive parts of which are connected to an independent earth electrode.
The sensitivity of the RCD must be adapted to the earth electrode resistance (RA2 in Figure F45). See specifications applicable to TT system.


FigF45.jpg
 













Fig. F45: Separate earth electrode


High-sensitivity RCDs

(see Fig. F46)
According to IEC 60364-4-41, high sensitivity RCDs (≤ 30 mA) must be used for protection of socket outlets with rated current ≤ 20 A in all locations. The use of such RCDs is also recommended in the following cases:
  • Socket-outlet circuits in wet locations at all current ratings
  • Socket-outlet circuits in temporary installations
  • Circuits supplying laundry rooms and swimming pools
  • Supply circuits to work-sites, caravans, pleasure boats, and travelling fairs 


FigF46.jpg
 











Fig. F46: Circuit supplying socket-outlets


Protection in high fire-risk location

According to IEC 60364-422-3.10, circuits in high fire-risk locations must be protected by RCDs of sensitivity ≤ 500 mA. This excludes the TN-C arrangement and TN-S must be adopted.
A preferred sensitivity of 300 mA is mandatory in some countries (see Fig. F47).


FigF47.jpg
 










Fig. F47: Fire-risk location


When the fault current-loop impedance is particularly high

When the earth-fault current is limited due to an inevitably high fault-loop impedance, so that the overcurrent protection cannot be relied upon to trip the circuit within the prescribed time, the following possibilities should be considered:
Suggestion 1 (see Fig. F48)


FigF48.jpg
 










Fig. F48: Circuit-breaker with low-set instantaneous magnetic tripping

  • Install a circuit-breaker which has a lower instantaneous magnetic tripping level, for example: 2In ≤ Irm ≤ 4In
This affords protection for persons on circuits which are abnormally long. It must be checked, however, that high transient currents such as the starting currents of motors will not cause nuisance trip-outs.
  • Schneider Electric solutions
  -  Type G Compact (2Im ≤ Irm ≤ 4Im)
  -  Type B Multi 9 circuit-breaker
Suggestion 2 (see Fig. F49)


FigF49.jpg












Fig. F49: RCD protection on TN systems with high earth-fault-loop impedance

  • Install a RCD on the circuit. The device does not need to be highly-sensitive (HS) (several amps to a few tens of amps). Where socket-outlets are involved, the particular circuits must, in any case, be protected by HS (≤ 30 mA) RCDs; generally one RCD for a number of socket outlets on a common circuit.
  • Schneider Electric solutions
  -  RCD Multi 9 NG125: IΔn = 1 or 3 A
  -  Vigicompact REH or REM: IΔn = 3 to 30 A
  -  Type B Multi 9 circuit-breaker
Suggestion 3
Increase the size of the PE or PEN conductors and/or the phase conductors, to reduce the loop impedance.
Suggestion 4
Add supplementary equipotential conductors. This will have a similar effect to that of suggestion 3, i.e. a reduction in the earth-fault-loop resistance, while at the same time improving the existing touch-voltage protection measures. The effectiveness of this improvement may be checked by a resistance test between each exposed conductive part and the local main protective conductor.
For TN-C installations, bonding as shown in Figure F50 is not allowed, and suggestion 3 should be adopted.


FigF50.jpg
 










Fig. F50: Improved equipotential bonding

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