Protective Earth Conductor

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The protective earth (PE) conductor is defined as a conductor that is provided for safety purposes (e.g. against the risk of electric shock) and which also provides a conductive path to earth. The PE conductor can be integrated inside a multi-core cable (e.g. 3C+E cable) or can be a separate cable.

Minimum Protective Earth Conductor Size

The protective earth conductor should be sized so that during a fault, it will be able to withstand the prospective fault current. IEC 60364-5-54 provides two options for the sizing of protective earth conductors.

Sizing from a Table

For protective earth conductors constructed of the same material as the phase conductors, the minimum size for protective earth conductors are as follows:

Phase Conductor (mm2) Min. PE Conductor (mm2)
0.5 0.5
0.75 0.75
1 1
1.5 1.5
2.5 2.5
4 4
6 6
10 10
16 16
26 16
35 16
50 25
70 35
95 50
120 70
150 95
185 95
240 120
300 150
400 240
500 300
630 400
800 400
1000 500

For protective earth conductors that are not of the same material as the phase conductors, a factor [math]\frac{k_1}{k_2} [/math] must be applied to the minimum size PE conductor in the table above.

The numerator [math]k_{1} [/math] is the k value for the phase conductors and the denominator [math]k_{2} [/math] is the k value for the PE conductor. Calculation of the k values are described on this page.

Sizing by Calculation

As per Clause 543.1.2 of IEC 60463-5-54, the size of protective earth conductors can be calculated by the adiabatic short circuit temperature rise equation (for disconnection times <5s):

[math] A = \frac{\sqrt{i^{2}t}}{k} \, [/math]

Where [math]A \,[/math] is the minimum cross-sectional area of the PE conductor ([math]mm^{2}[/math])

[math]i^{2}t \,[/math] is the energy of the short circuit ([math]A^{2}s[/math])
[math]k \,[/math] is a constant term (this article has guidance for selecting the constant term)