Earthing electrode: permitted values and standard resistance by installation type
The earthing electrode is one of the most important components of a safe electrical installation, yet also one of the most frequently neglected. A correctly installed earthing electrode protects people against electrocution and equipment against overvoltages, but its effectiveness is expressed through a single measurable quantity: the earth resistance (resistance to earth), expressed in ohms. The lower this value, the better the electrode conducts the fault current into the ground. In what follows we explain which values are permitted by installation type, how to interpret them and how to verify them in practice.
What “permitted value” means for an earthing electrode
When we talk about “permitted values” we are referring to the earth resistance (also called resistance to earth) — that is, the resistance offered by the assembly made up of the electrodes, conductors and soil to the passage of electric current towards the mass of the earth. This is not a single, universally valid figure. It depends on:
- the role of the electrode (protection against electrocution, lightning protection, combined earthing electrode, or a functional earth for sensitive equipment);
- the type and sensitivity of the protective devices in the consumer unit (in particular the residual current device – RCD);
- the maximum permitted touch voltage that the installation tolerates in the event of a fault;
- the nature of the soil (sand, gravel, clay, wet or dry soil).
The basic idea is simple: in the event of a fault, the touch voltage to which a person may be exposed must not exceed the threshold considered dangerous (typically 50 V AC for dry spaces, lower in damp environments). The resistance of the earthing electrode is chosen so that, together with the residual current protection, this condition is met.
Indicative resistance values by installation type
The table below summarises the values usually targeted in design and execution. These are indicative values; the actual permitted value for a specific project is established through the design documentation and confirmed by measurements.
| Type of earthing electrode / role | Value usually targeted | Notes |
|---|---|---|
| Combined earthing electrode (protection + lightning protection) | ≤ 1 Ω | The most demanding requirement; typical for buildings with a lightning protection system |
| Earthing electrode for a low-voltage installation with a 30 mA RCD | ≤ 4 Ω | Value frequently accepted for residentially protected (RCD) installations |
| Protective earthing electrode (simple earth connection) | ≤ 10 Ω | Acceptable when protection is also ensured through other measures |
| Earthing electrode for sensitive / IT equipment | as low as possible, established through design documentation | Correlated with the equipment manufacturer’s requirements |
| Lightning protection earthing electrode (PTSA) | as per specialised design | Designed separately, depending on the protection level |
Two essential points are worth remembering. First: a low value (for example under 1 Ω) is always better, but is not mandatory for every installation — it depends on the functional requirement. Second: the resistance of the earthing electrode works “as a team” with the RCD. A correctly fitted 30 mA RCD ensures rapid disconnection even when the resistance is not extremely low, which explains why the 4 Ω threshold is so commonly encountered in homes.
The relationship between resistance and the RCD
The safety condition can be expressed in simplified form as follows: the product of the electrode resistance and the tripping current of the RCD must remain below the permitted touch voltage. For example, with a 30 mA RCD and a permitted touch voltage of 50 V, the theoretical maximum resistance would be of the order of hundreds of ohms — but in practice much lower values are targeted (under 4–10 Ω) to provide a comfortable safety margin and to cover situations without a sensitive RCD as well. This is why a good earthing electrode plus a functional RCD represents the standard combination in a modern installation.
Why the soil type matters
Soil resistivity directly influences the result. Clayey, wet soil conducts far better than sandy, dry soil. In difficult conditions (stony, dry or rocky soil), achieving the desired value may require:
- longer electrodes or more electrodes connected in parallel;
- a “loop”-type (ring) earthing electrode around the foundation;
- soil treatment or the use of materials that reduce resistivity around the electrodes;
- combining the natural earthing electrode (the foundation reinforcement) with artificial electrodes.
Precisely because there are so many variables, a correct earthing electrode is not “estimated” but designed and verified. You can find details about the actual execution on the dedicated earthing electrode page.
How the value is verified: PRAM measurements
The only way to confirm that the earthing electrode meets the permitted value is to measure it with specialised equipment. These checks are known as PRAM measurements (Prize, Rezistențe, Aparate de Măsură — earthing electrodes, resistances, measuring instruments) and include:
- measuring the earth resistance of the earthing electrode;
- checking the continuity of the protective and earthing conductors;
- checking the insulation resistance of the installation, where applicable.
The result is recorded in a verification report (buletin de verificare), a document attesting that the installation complies with the standard values. This report is often required at the acceptance of works, at grid connection or during periodic inspections. Scheduling and carrying out these tests are described on the PRAM measurements page.
When measurements are repeated
- when commissioning a new installation;
- after modifications to the electrical installation;
- periodically, in accordance with standards and recommendations (the interval depends on the type of installation and the environment);
- when there is suspicion of deterioration (corrosion of the electrodes, unexplained tripping of the RCD).
Common mistakes that lead to values that are too high
In practice, an earthing electrode “fails” the measurements for recurring reasons:
- insufficient electrodes for the soil type;
- corroded or loose connections between the electrodes and the connecting conductors;
- an undersized or interrupted protective conductor;
- the absence of an earthing ring in buildings where the soil has high resistivity;
- an earthing electrode installed “on a trial basis”, without design documentation and without subsequent verification.
All of these can be corrected, but they cost more than doing the job correctly the first time. That is why we recommend that the earthing electrode be part of the overall design of the electrical installation, rather than being treated as a last-minute add-on. For complete installation works, see the electrical installations page.
Conclusion: a low value, confirmed in writing
There is no single magic number valid for every earthing electrode. The permitted value is chosen according to the role and the protective devices of the installation — from under 1 Ω for combined earthing electrodes with lightning protection, to a maximum of 4 Ω for low-voltage installations with a 30 mA RCD, and up to 10 Ω for simple protective earthing electrodes. Whatever the target, the rule remains the same: the earthing electrode must be designed to suit the soil type and confirmed through PRAM measurements, set out in a verification report.
SUN WATT SRL (bransam.ro), a company certified by ANRE for execution and design, carries out earthing electrodes, PRAM measurements and verification reports in Bucharest, Ilfov and Giurgiu. For an assessment of your situation, you can contact us on 0733 097 440 or through the contact page.