Earthing: loop, rods and equipotential bonding

Earthing is the invisible part of the installation: buried, concealed, never spectacular. Yet it carries real weight at inspection: the dispersion resistance of the earth electrode is measured there, and its value plays a part in the choice of RCDs. Loop or rods, earth disconnector, equipotential bonding: here is what Book 1 sets out for a domestic installation, from the foundation trench to the distribution board.

What earthing is for

In the event of an insulation fault, an exposed metal part can become live: the casing of the washing machine, the body of the oven. Book 1 deals with earthing as part of protection against electric shock by indirect contact. Protective measures with an automatic disconnection device, such as the RCD, require the exposed conductive parts to be connected to a protective conductor (subsection 4.2.3.1). The earthing installation comprises the earth electrodes, the earthing conductors, the protective conductors and any equipotential bonding, main and supplementary (subsection 4.2.3.2).

Earth loop or earth rods

For domestic installations, subsection 5.4.2.1 provides two families of earth electrodes:

• The earth loop, for any new building where the foundation trench for all or part of the foundations is at least 60 cm deep: at least one loop at the bottom of the trench, vertically below the outer walls. The copper conductor has a geometric cross-section of at least 35 mm². It is laid against the bare ground, covered with good soil, with no contact with the materials of the foundation walls, and its ends remain accessible for inspection.
• Additional earth electrodes, when a loop cannot be installed (for example if the trench is not deep enough) or when the resistance obtained with the loop is not low enough: a conductor buried horizontally (at least 35 mm², at least 0.80 m deep, at least 15 m long), or bars, rods and conductors driven into the ground. Bars and rods are at least 1.50 m long, and the length buried below the −0.60 m level is at least 1.50 m.

The loop goes in at the bottom of the trench, before the foundations. Keep it in mind when planning the structural work, even if the electrical job only starts months later. For existing domestic installations, chapter 8.2 of Book 1 provides derogations.

The earth resistance measured at inspection

During the conformity inspection before putting into use, the dispersion resistance of the earth electrodes is measured (subsection 6.4.5.2), and its value appears in the report. For domestic installations, subsection 4.2.3.2 requires a dispersion resistance as low as possible and below 100 ohms.

The 30 ohm threshold plays a part in the choice of RCDs (subsection 4.2.4.3). If the dispersion resistance exceeds 30 ohms, at least two high or very high sensitivity RCDs are placed immediately downstream of the RCD at the origin of the installation, each covering at most sixteen single or multiple socket-outlets, a fixed appliance or a set of fixed appliances controlled by a common switching device counting as one socket-outlet. Circuits connected directly to the additional RCDs at the origin, such as those for fixed appliances, also receive at least one RCD of 100 mA at most per circuit or group of circuits. Figures 4.10 and 4.11 illustrate the two configurations. The report then certifies that the installed RCDs match the measured resistance (subsection 6.4.6.4).

The earth disconnector

Between the earth electrode and the rest of the installation sits the earth disconnector. In principle, no switching device is inserted in the protective conductor circuit. But to allow the dispersion resistance of the earth electrode to be measured, Book 1 makes this disconnecting device indispensable, and it can only be removed with a tool (subsection 5.4.3.5).

For an earth electrode shared by several domestic installations, the text adds that a single disconnector is installed, that it remains accessible, and that the presence of a shared earth electrode and the location of the common earth disconnector are shown on the single-line diagram and the position plan of each installation concerned (point c. of subsection 5.4.2.1).

Equipotential bonding: main and supplementary

Earthing does not stop at the earth electrode. Two types of bonding extend it through the building:

• Main equipotential bonding is compulsory in every building (subsection 4.2.3.4). It connects to the main earthing terminal, among other things, the main metal water and gas pipes, the main metal risers of central heating and air conditioning, and the fixed and accessible metal parts forming part of the building structure that can propagate a potential and are regarded as exposed conductive parts (subsection 4.2.3.2). Its conductor has a cross-section of at least half that of the largest protective conductor (earthing conductor excluded), with a minimum of 6 mm² copper, and may be limited to 25 mm² copper (subsection 5.4.4.1).
• Supplementary equipotential bonding locally connects the exposed conductive parts and the simultaneously accessible extraneous conductive parts. In a location containing a bath or shower, subsection 7.1.4.4 requires it, with exceptions, including non-conductive pipes such as plastic. Its conductors have a cross-section of at least half that of the protective conductor connected to the exposed conductive part (earthing conductor excluded) when the bond connects that part to an extraneous conductive part, or of the smallest cross-section of the protective conductors connected when it connects exposed conductive parts of different appliances; in that second case, it must be ensured that a fault current cannot cause, in the protective conductor with the smaller cross-section, a thermal stress greater than it can withstand. In any case, they are no smaller than 2.5 mm² when mechanically protected, and 4 mm² otherwise (subsection 5.4.4.2). The other rules are covered in our article on the bathroom volumes.

At inspection, the report also covers the continuity of the equipotential bonding, main and supplementary, and of the protective conductors (subsection 6.4.6.4). After plumbing work, check that the existing bonds have been restored.

The points to check

• The loop in the schedule. On a new building concerned, it goes in at the bottom of the trench: that is the moment not to miss.
• The resistance against the RCDs. Above 30 ohms, the RCD configuration changes (subsection 4.2.4.3).
• The earth disconnector present, and removable with a tool for the measurement.
• Equipotential bonding restored after a renovation: new plumbing, old bond to check.
• Earthing in the documents. The inspection report describes the type of earth electrode installed and states the value of its dispersion resistance (subsection 6.4.6.4).

In Amperio, a task such as fitting an equipotential bond is tracked alongside the other job tasks, and the electrical file (single-line diagram, position plan and installation data) is exported as a PDF for the inspection body. For the course of the visit, see our article on the accredited inspection body’s visit.

This article is informative. It replaces neither the RGIE (Book 1) nor the advice of your inspection body.