Sizing of protective earthing conductor
From Electrical Installation Guide
Figure G58 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.
| c.s.a. of phase|
conductors Sph (mm2)
| Minimum c.s.a. of|
PE conductor (mm2)
| Minimum c.s.a. of|
PEN conductor (mm2)
|16 < Sph ≤ 25||16||16|
|25 < Sph ≤ 35||25|
|35 < Sph ≤ 50||Sph/2||Sph/2|
|Sph > 50||Sph/2|
|Adiabatic method||Any size||(3) (4)|
(1) Data valid if the prospective conductor is of the same material as the line conductor. Otherwise, a correction factor must be applied.
(2) 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
(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.
(4) Refer to table G53 for the application of this formula.
Fig. G58: Minimum cross section area of protective conductors
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).
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 Figure G58 for:
Sph ≤ 16 mm2 SPE = Sph
16 < Sph ≤ 35 mm2 SPE = 16 mm2
Sph > 35 mm2
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.
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.
|(1) Grounding electrode 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 IEC 60724 (1984).
The data presented in Figure G59 are those most commonly needed for LV installation design.
|k values||Nature of insulation|
|Polyvinylchloride (PVC)|| Cross-linked-polyethylene (XLPE)|
|Final temperature (°C)||160||250|
|Initial temperature (°C)||30||30|
| Insulated conductors not incoporated in cables or bare conductors in contact|
with cable jackets
|Conductors of a multi-core-cable||Copper||115||143|
Fig. G59: k factor values for LV PE conductors, commonly used in national standards and complying with IEC 60724