Sizing of protective earthing conductor: Difference between revisions
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{{Menu_Sizing_and_protection_of_conductors}} | {{Menu_Sizing_and_protection_of_conductors}}__TOC__ | ||
{{FigureRef|G59}} below is based on IEC 60364-5-54. This table provides two methods of determining the appropriate c.s.a. for both PE or PEN conductors. | |||
{{tb-start|id=Tab1219|num=G59|title=Minimum cross section area of protective conductors|cols=4}} | |||
{| class="wikitable" | |||
{{ | |||
{ | |||
|- | |- | ||
! rowspan="2" | | ! rowspan="2" | Method | ||
! rowspan="2" | c.s.a. of phase | ! rowspan="2" | c.s.a. of phase<br>conductors Sph (mm<sup>2</sup>) | ||
conductors Sph (mm<sup>2</sup>) | ! rowspan="2" | Minimum c.s.a. of<br>PE conductor (mm<sup>2</sup>) | ||
! rowspan="2" | Minimum c.s.a. of | ! colspan="2" | Minimum c.s.a. of<br>PEN conductor (mm<sup>2</sup>) | ||
PE conductor (mm<sup>2</sup>) | |||
! colspan="2" | Minimum c.s.a. of | |||
PEN conductor (mm<sup>2</sup>) | |||
|- | |- | ||
! Cu | ! Cu | ||
! Al | ! Al | ||
|- | |- | ||
| rowspan="5" | '''Simplified method'''{{ | | rowspan="5" | '''Simplified method'''{{tn|A}} | ||
| S<sub>ph</sub> ≤ 16 | | S<sub>ph</sub> ≤ 16 | ||
| height="30" | S<sub>ph</sub>{{ | | height="30" | S<sub>ph</sub>{{tn|B}} | ||
| S<sub>ph</sub>{{ | | S<sub>ph</sub>{{tn|C}} | ||
| rowspan="2" | S<sub>ph</sub>{{ | | rowspan="2" | S<sub>ph</sub>{{tn|C}} | ||
|- | |- | ||
| 16 < S<sub>ph</sub> ≤ 25 | | 16 < S<sub>ph</sub> ≤ 25 | ||
| Line 40: | Line 35: | ||
| '''Adiabatic method''' | | '''Adiabatic method''' | ||
| Any size | | Any size | ||
| colspan="3" | | | colspan="3" | <math>S_{PE/PEN}=\frac {\sqrt {I^2 . t} }{k}</math> {{tn|C}}{{tn|D}} | ||
| | |} | ||
{{ | {{tb-notes | ||
| | |A= Data valid if the prospective conductor is of the same material as the line conductor. Otherwise, a correction factor must be applied. | ||
| | |B= When the PE conductor is separated from the circuit phase conductors, the following minimum values must be respected:<br> | ||
- 2.5 mm<sup>2</sup> if the PE is mechanically protected<br> | |||
- 4 mm<sup>2</sup> if the PE is not mechanically protected | |||
| | |C= For mechanical reasons, a PEN conductor, shall have a cross-sectional area not less than 10 mm<sup>2</sup> in copper or 16 mm<sup>2</sup> in aluminium. | ||
| | |D= Refer to table A.54 of IEC60364-4-54 or {{FigureRef|G60}} to get values of k factor. }} | ||
The two methods are: | The two methods are: | ||
*'''Adiabatic (which corresponds with that described in IEC 60724) | *'''Adiabatic''' (which corresponds with that described in IEC 60724) | ||
: This method, while being economical and assuring protection of the conductor against overheating, leads to small c.s.a.’s compared to those of the corresponding circuit phase conductors. The result is sometimes incompatible with the necessity in IT and TN schemes to minimize the impedance of the circuit earth-fault loop, to ensure positive operation by instantaneous overcurrent tripping devices. This method is used in practice, therefore, for TT installations, and for dimensioning an earthing conductor{{fn|1}} | : This method, while being economical and assuring protection of the conductor against overheating, leads to small c.s.a.’s compared to those of the corresponding circuit phase conductors. The result is sometimes incompatible with the necessity in IT and TN schemes to minimize the impedance of the circuit earth-fault loop, to ensure positive operation by instantaneous overcurrent tripping devices. This method is used in practice, therefore, for TT installations, and for dimensioning an earthing conductor{{fn|1}} | ||
* '''Simplified''' | * '''Simplified''' | ||
: This method is based on PE conductor sizes being related to those of the corresponding circuit phase conductors, assuming that the same conductor material is used in each case. | : This method is based on PE conductor sizes being related to those of the corresponding circuit phase conductors, assuming that the same conductor material is used in each case. | ||
: Thus, in {{FigRef|G58}} for: | |||
: Thus, in {{ | : Sph ≤ 16 mm<sup>2</sup> : S<sub>PE</sub> = S<sub>ph</sub> | ||
: 16 < Sph ≤ 35 mm<sup>2</sup> : S<sub>PE</sub> = 16 mm<sup>2</sup> | |||
: Sph ≤ 16 mm<sup>2</sup> : | : Sph > 35 mm<sup>2</sup> : S<sub>PE</sub> = S<sub>ph</sub> / 2 | ||
: 16 < Sph ≤ 35 mm<sup>2</sup> : | |||
: Sph > 35 mm<sup>2</sup> : < | |||
'''Note''': when, in a TT scheme, the installation earth electrode is beyond the zone of influence of the source earthing electrode, the c.s.a. of the PE conductor can be limited to 25 mm<sup>2</sup> (for copper) or 35 mm<sup>2</sup> (for aluminium). | '''Note''': when, in a TT scheme, the installation earth electrode is beyond the zone of influence of the source earthing electrode, the c.s.a. of the PE conductor can be limited to 25 mm<sup>2</sup> (for copper) or 35 mm<sup>2</sup> (for aluminium). | ||
| Line 68: | Line 59: | ||
The neutral cannot be used as a PEN conductor unless its c.s.a. is equal to or larger than 10 mm<sup>2</sup> (copper) or 16 mm<sup>2</sup> (aluminium). | The neutral cannot be used as a PEN conductor unless its c.s.a. is equal to or larger than 10 mm<sup>2</sup> (copper) or 16 mm<sup>2</sup> (aluminium). | ||
Moreover, a PEN conductor is not allowed in a flexible cable. Since a PEN conductor functions also as a neutral conductor, its c.s.a. cannot, in any case, be less than that necessary for the neutral. | Moreover, a PEN conductor is not allowed in a flexible cable. Since a PEN conductor functions also as a neutral conductor, its c.s.a. cannot, in any case, be less than that necessary for the neutral, as discussed in [[Sizing the neutral conductor]]. | ||
This c.s.a. cannot be less than that of the phase conductors unless: | This c.s.a. cannot be less than that of the phase conductors unless: | ||
| Line 74: | Line 65: | ||
*Imax likely to pass through the neutral in normal circumstances, is less than the current permitted for the selected cable size. | *Imax likely to pass through the neutral in normal circumstances, is less than the current permitted for the selected cable size. | ||
Furthermore, protection of the neutral conductor must be assured by the protective devices provided for phase-conductor protection | Furthermore, protection of the neutral conductor must be assured by the protective devices provided for phase-conductor protection (described in [[Protection of the neutral conductor]]) | ||
=== Values of factor k to be used in the formulae === | === Values of factor k to be used in the formulae === | ||
These values are identical in several national standards, and the temperature rise ranges, together with factor k values and the upper temperature limits for the different classes of insulation, correspond with those published in IEC60364-5-54, Annex A. | These values are identical in several national standards, and the temperature rise ranges, together with factor k values and the upper temperature limits for the different classes of insulation, correspond with those published in IEC60364-5-54, Annex A. | ||
The data presented in {{FigureRef| | The data presented in {{FigureRef|G60}} are those most commonly needed for LV installation design. | ||
{{ | {{tb-start|id=Tab1220|num=G60|title=k factor values for LV PE conductors, commonly used in national standards and complying with IEC60364-5-54 Annex A|cols=3}} | ||
{| class="wikitable" | |||
|- | |- | ||
! colspan="2" rowspan="2" | k values | ! colspan="2" rowspan="2" | k values | ||
| Line 98: | Line 90: | ||
| 30 | | 30 | ||
|- | |- | ||
| rowspan="3" | Insulated conductors not incoporated in cables or bare conductors in contact with cable jackets | | rowspan="3" style="width: 100px" | Insulated conductors not incoporated in cables or bare conductors in contact with cable jackets | ||
| Copper | | Copper | ||
| 143 | | 143 | ||
| Line 119: | Line 111: | ||
| 76 | | 76 | ||
| 94 | | 94 | ||
| | |} | ||
{{footnotes}} | {{footnotes}} | ||
| Line 126: | Line 117: | ||
{{fn-detail|1| Grounding electrode conductor}} | {{fn-detail|1| Grounding electrode conductor}} | ||
</references> | </references> | ||
Latest revision as of 09:48, 22 June 2022
Figure G59 below is based on IEC 60364-5-54. This table provides two methods of determining the appropriate c.s.a. for both PE or PEN conductors.
Fig. G59 – Minimum cross section area of protective conductors
| Method | c.s.a. of phase conductors Sph (mm2) |
Minimum c.s.a. of PE conductor (mm2) |
Minimum c.s.a. of PEN conductor (mm2) | |
|---|---|---|---|---|
| Cu | Al | |||
| Simplified method[a] | Sph ≤ 16 | Sph[b] | Sph[c] | Sph[c] |
| 16 < Sph ≤ 25 | 16 | 16 | ||
| 25 < Sph ≤ 35 | 25 | |||
| 35 < Sph ≤ 50 | Sph/2 | Sph/2 | ||
| Sph > 50 | Sph/2 | |||
| Adiabatic method | Any size | [math]\displaystyle{ S_{PE/PEN}=\frac {\sqrt {I^2 . t} }{k} }[/math] [c][d] | ||
- ^ Data valid if the prospective conductor is of the same material as the line conductor. Otherwise, a correction factor must be applied.
- ^ When the PE conductor is separated from the circuit phase conductors, the following minimum values must be respected:
- 2.5 mm2 if the PE is mechanically protected
- 4 mm2 if the PE is not mechanically protected - ^ 1 2 3 For mechanical reasons, a PEN conductor, shall have a cross-sectional area not less than 10 mm2 in copper or 16 mm2 in aluminium.
- ^ Refer to table A.54 of IEC60364-4-54 or Figure G60 to get values of k factor.
The two methods are:
- Adiabatic (which corresponds with that described in IEC 60724)
- This method, while being economical and assuring protection of the conductor against overheating, leads to small c.s.a.’s compared to those of the corresponding circuit phase conductors. The result is sometimes incompatible with the necessity in IT and TN schemes to minimize the impedance of the circuit earth-fault loop, to ensure positive operation by instantaneous overcurrent tripping devices. This method is used in practice, therefore, for TT installations, and for dimensioning an earthing conductor[1]
- Simplified
- This method is based on PE conductor sizes being related to those of the corresponding circuit phase conductors, assuming that the same conductor material is used in each case.
- Thus, in Fig. G58 for:
- Sph ≤ 16 mm2 : SPE = Sph
- 16 < Sph ≤ 35 mm2 : SPE = 16 mm2
- Sph > 35 mm2 : SPE = Sph / 2
Note: when, in a TT scheme, the installation earth electrode is beyond the zone of influence of the source earthing electrode, the c.s.a. of the PE conductor can be limited to 25 mm2 (for copper) or 35 mm2 (for aluminium).
The neutral cannot be used as a PEN conductor unless its c.s.a. is equal to or larger than 10 mm2 (copper) or 16 mm2 (aluminium).
Moreover, a PEN conductor is not allowed in a flexible cable. Since a PEN conductor functions also as a neutral conductor, its c.s.a. cannot, in any case, be less than that necessary for the neutral, as discussed in Sizing the neutral conductor.
This c.s.a. cannot be less than that of the phase conductors unless:
- The kVA rating of single-phase loads is less than 10% of the total kVA load, and
- Imax likely to pass through the neutral in normal circumstances, is less than the current permitted for the selected cable size.
Furthermore, protection of the neutral conductor must be assured by the protective devices provided for phase-conductor protection (described in Protection of the neutral conductor)
Values of factor k to be used in the formulae
These values are identical in several national standards, and the temperature rise ranges, together with factor k values and the upper temperature limits for the different classes of insulation, correspond with those published in IEC60364-5-54, Annex A.
The data presented in Figure G60 are those most commonly needed for LV installation design.
Fig. G60 – k factor values for LV PE conductors, commonly used in national standards and complying with IEC60364-5-54 Annex A
| k values | Nature of insulation | |||
|---|---|---|---|---|
| Polyvinylchloride (PVC) | Cross-linked-polyethylene (XLPE)
Ethylene-propylene-rubber (EPR) | |||
| Final temperature (°C) | 160 | 250 | ||
| Initial temperature (°C) | 30 | 30 | ||
| Insulated conductors not incoporated in cables or bare conductors in contact with cable jackets | Copper | 143 | 176 | |
| Aluminium | 95 | 116 | ||
| Steel | 52 | 64 | ||
| Conductors of a multi-core-cable | Copper | 115 | 143 | |
| Aluminium | 76 | 94 | ||
Notes
- ^ Grounding electrode conductor
