## Power

Tujuan pembuatan blog "Gogeneration" ini adalah sebagai sarana untuk berbagi ilmu pengetahuan dan mencerdaskan anak bangsa, dengan mengumpulkan tutorial dan artikel yang terserak di dunia maya maupun di literature-literature yang ada. Semoga dengan hadirnya blog "Gogeneration" ini dapat membawa manfaat bagi kita semua. dan saya ingin sharing tentang power plant dan substation khususnya di electrical, mechanical , automation, scada. walaupun sudah lebih dari sepuluh tahun menggeluti dunia itu tapi masih banyak hal yang harus dipelajari. dengan blog ini saya berharap bisa saling sharing, Blog ini didedikasikan kepada siapa pun yang mencintai ilmu pengetahuan

## 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.

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 mm2 R+15% S = 185 mm2 R+20% S = 240 mm2 R+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)

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.

 Circuit Conductor material m = Sph/SPE (or PEN) m = 1 m = 2 m = 3 m = 4 3P + N or P + N Copper 1 0.67 0.50 0.40 Aluminium 0.62 0.42 0.31 0.25
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) mm2 50 63 80 100 125 160 200 250 320 400 500 560 630 700 800 875 1000 1120 1250 1600 2000 2500 3200 4000 5000 6300 8000 10000 12500 1.5 100 79 63 50 40 31 25 20 16 13 10 9 8 7 6 6 5 4 4 2.5 167 133 104 83 67 52 42 33 26 21 17 15 13 12 10 10 8 7 7 5 4 4 267 212 167 133 107 83 67 53 42 33 27 24 21 19 17 15 13 12 11 8 7 5 4 6 400 317 250 200 160 125 100 80 63 50 40 36 32 29 25 23 20 18 16 13 10 8 6 5 4 10 417 333 267 208 167 133 104 83 67 60 53 48 42 38 33 30 27 21 17 13 10 8 7 5 4 16 427 333 267 213 167 133 107 95 85 76 67 61 53 48 43 33 27 21 17 13 11 8 7 5 4 25 417 333 260 208 167 149 132 119 104 95 83 74 67 52 42 33 26 21 17 13 10 8 7 35 467 365 292 233 208 185 167 146 133 117 104 93 73 58 47 36 29 23 19 15 12 9 50 495 396 317 283 251 226 198 181 158 141 127 99 79 63 49 40 32 25 20 16 13 70 417 370 333 292 267 233 208 187 146 117 93 73 58 47 37 29 23 19 95 452 396 362 317 283 263 198 158 127 99 79 63 50 40 32 25 120 457 400 357 320 250 200 160 125 100 80 63 50 40 32 150 435 388 348 272 217 174 136 109 87 69 54 43 35 185 459 411 321 257 206 161 128 103 82 64 51 41 240 400 320 256 200 160 128 102 80 64 51
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)

 Sph Rated current (A) mm2 1 2 3 4 6 10 16 20 25 32 40 50 63 80 100 125 1.5 1200 600 400 300 200 120 75 60 48 37 30 24 19 15 12 10 2.5 1000 666 500 333 200 125 100 80 62 50 40 32 25 20 16 4 1066 800 533 320 200 160 128 100 80 64 51 40 32 26 6 1200 800 480 300 240 192 150 120 96 76 60 48 38 10 800 500 400 320 250 200 160 127 100 80 64 16 800 640 512 400 320 256 203 160 128 102 25 800 625 500 400 317 250 200 160 35 875 700 560 444 350 280 224 50 760 603 475 380 304
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

 Sph Rated current (A) mm2 1 2 3 4 6 10 16 20 25 32 40 50 63 80 100 125 1.5 600 300 200 150 100 60 37 30 24 18 15 12 9 7 6 5 2.5 500 333 250 167 100 62 50 40 31 25 20 16 12 10 8 4 533 400 267 160 100 80 64 50 40 32 25 20 16 13 6 600 400 240 150 120 96 75 60 48 38 30 24 19 10 677 400 250 200 160 125 100 80 63 50 40 32 16 640 400 320 256 200 160 128 101 80 64 51 25 625 500 400 312 250 200 159 125 100 80 35 875 700 560 437 350 280 222 175 140 112 50 760 594 475 380 301 237 190 152
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

 Sph Rated current (A) mm2 1 2 3 4 6 10 16 20 25 32 40 50 63 80 100 125 1.5 429 214 143 107 71 43 27 21 17 13 11 9 7 5 4 3 2.5 714 357 238 179 119 71 45 36 29 22 18 14 11 9 7 6 4 571 381 286 190 114 71 80 46 36 29 23 18 14 11 9 6 857 571 429 286 171 107 120 69 54 43 34 27 21 17 14 10 952 714 476 284 179 200 114 89 71 57 45 36 29 23 16 762 457 286 320 183 143 114 91 73 57 46 37 25 714 446 500 286 223 179 143 113 89 71 57 35 625 700 400 313 250 200 159 125 80 100 50 848 543 424 339 271 215 170 136 109
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.

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

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).

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)

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)

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.

Fig. F50: Improved equipotential bonding