Sizing of protective earthing conductor

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	Sph ≤ 16	Sph	Sph	Sph
	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]

•      - 2.5 mm² if the PE is mechanically protected
•      - 4 mm² if the PE is not mechanically protected

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

• 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 : S_PE = S_ph

16 < Sph ≤ 35 mm² : S_PE = 16 mm²

Sph > 35 mm² : S_PE = S_ph / 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