Earth Electrodes — Foundation Earth Electrode, Rod and Ring Electrodes (§542)

An RCD and a correct loop impedance (see the loop impedance measurement Zs guide) are only as good as the earth electrode on which the entire protection system ultimately relies. NEN 1010 §542.2.1 lists the permitted types of earth electrodes and sets requirements for their construction.

Permitted types of earth electrodes

According to §542.2.1, the following are among those permitted:

Rod electrodes and earth rods — electrodes driven vertically into the ground.
Strip- and wire-type earth conductors — laid horizontally in the ground.
Earth plates.
Foundation earth electrodes — an electrode incorporated into a building's foundation (usually a closed loop of copper wire or strip in the concrete foundation body).
Reinforcement of concrete structures in the ground (under conditions).
Metal water pipe networks — only with written permission from the network operator, and not as the sole earthing provision.

Material and construction

The standard requires that an earth electrode keeps its earth spreading resistance sufficiently low, even when the ground dries out or freezes, and that the material and construction are sufficiently corrosion resistant. Common designs: solid copper (15 mm diameter, or 12 mm if used solely for protection against electric shock) or hot-dip galvanised steel (16 mm diameter).

Note: the exact minimum dimensions and material combinations are table-specific in the standard and depend on the application (shock protection only versus a full earthing function) — this article provides an overview of electrode types, not a complete material table.

Foundation earth electrode as the standard choice for new construction

For new construction, the foundation earth electrode is nowadays the most common solution: a closed loop that is installed before the foundation is poured and is subsequently enclosed in the concrete. Advantages compared with a standalone rod electrode:

A large contact surface with the ground around the full perimeter of the building, which generally gives a lower and more stable earth spreading resistance than a single rod electrode.
Less sensitive to drying out of the top soil layer, because the loop is located beneath the foundation.
No extra space needed outside the building, unlike a rod or ring electrode around the building.

A ring electrode (a loop in the ground around the building, outside the foundation) is the alternative when a foundation earth electrode is no longer possible after the fact — for example in an existing installation where the original earthing provision turns out to be inadequate.

Practical relevance

At a horticultural business or industrial hall with a TT system (see the TN/TT/IT comparison in the earthing guide), the quality of the site's own earth electrode directly determines the RCD's tripping current: an earth spreading resistance that is too high can cause the calculated touch voltage during an earth fault to exceed the permitted limit, even if the RCD itself functions correctly. The earth spreading resistance is checked periodically — see the earth electrode resistance measurement guide for the measurement method.

Common mistakes

Using a metal water pipe as the sole earth electrode without written permission from the network operator — not permitted under §542.2.1, and also unreliable if pipe sections are later replaced with plastic.
Installing a single short rod electrode in dry or sandy soil without measuring the earth spreading resistance — this can result in a resistance well above the required value, especially in dry summers.
Not accounting for corrosion when combining different metals in the earthing provision (see also the cable lug guide on galvanic corrosion in copper-aluminium connections) — the same principle applies to earth electrodes.