Earth resistance testing: which method and which target values?
Earth resistance shows how well an electrode conducts fault and lightning currents into the ground. It is measured classically with the fall-of-potential method using auxiliary probes, or without probes using the clamp method. Target values depend on the application, and soil resistivity strongly affects the result.
View test instrumentsWhich methods measure earth electrode resistance?
There are two basic methods: the fall-of-potential method with auxiliary probes (as a 3-terminal or 4-terminal setup) and the clamp or loop method without probes. The probe method is the reference for single electrodes, while the clamp method suits meshed, in-service earthing systems.
In the fall-of-potential method the instrument drives a current between the electrode under test and a distant current probe (auxiliary earth). A second potential probe measures the voltage drop in the soil. Resistance is calculated from current and voltage. The probes are placed in a line, with the potential probe typically at about 62 percent of the distance to the current probe, where the potential forms a plateau.
The 4-terminal setup fully separates the current and voltage paths and so removes the resistance of the test leads and clamps from the result. It is the more accurate choice for very low earth resistances and for measuring soil resistivity (Wenner array with four spikes). The 3-terminal setup is simpler but includes the lead resistance in the reading.
The clamp or loop method needs no auxiliary probes. A measuring clamp induces a voltage in the electrode loop and measures the resulting current; this gives the loop resistance. It requires a meshed system with several parallel electrodes forming the return path. The electrode does not need to be disconnected - the installation stays in service.
Which target values apply to earth resistance?
Fixed limits exist only per application. For lightning protection systems an earth resistance below 10 Ω is commonly aimed for, while functional and protective earths follow their own values depending on the system type and protection concept. What matters is always that the required disconnection condition is met under fault.
In TT systems the permissible earth resistance follows from the requirement that the touch voltage stays below the limit under fault and that the upstream protective device trips. With a residual current device rated at 30 mA, the calculated permissible resistance is high, but in practice much lower values are targeted to keep reserves and ensure reliable tripping.
| Method | Principle | When to use |
|---|---|---|
| Fall-of-potential 3-terminal | Current fed via current probe, potential read via one probe | Single electrode, disconnected, medium accuracy |
| Fall-of-potential 4-terminal | Separate current and voltage paths, lead resistance removed | Very low resistances, soil resistivity measurement |
| Clamp/loop method | Clamp induces voltage, measures loop current, no probes | Meshed systems in service, no disconnection possible |
Because requirements vary with system type, protective device and local rules, the specific target values should always be derived from the relevant installation standard and the system concept. The guide values given here are for orientation and do not replace the normative requirement.
How does the soil affect the result?
Soil resistivity is the most important factor: it ranges from a few ohm-metres in moist clay to several thousand ohm-metres in rock or dry sand. Moisture, temperature, salt content and season change it considerably - a value measured in summer can be far higher in frost.
The earth resistance is therefore ideally assessed under unfavourable conditions, such as dry or frozen soil, and the electrode is sized accordingly. Deeper rod electrodes reach more stable, less seasonal layers and give more consistent values than shallow buried tape conductors.
Place the current probe far enough away so the resistance zones of the test object and probe do not overlap.
Move the potential probe and check that the reading forms a plateau - only then is it reliable.
Mains-frequency stray currents in the soil can distort the result; instruments to IEC 61557-5 suppress them.
Record the reading with the soil condition and season so values remain comparable later.
Measurements are taken at new installation, after changes to the earthing system and as part of periodic testing. Alongside electrode testing, insulation resistance belongs to electrical safety testing: read more in the guide at /en/ratgeber/insulation-resistance-test.
Frequently asked questions
When do I use the clamp method instead of the probe method?
The clamp method is the right choice for meshed earthing systems with several parallel electrodes that should stay in service. It needs no auxiliary probes and no disconnection. For a single, standalone electrode only the fall-of-potential method gives a correct result.
Which earth resistance is required for lightning protection?
As a guide value, lightning protection earthing systems commonly aim for below 10 Ω. The specific required value follows from the lightning protection class and the installation standard; a low-resistance, well-meshed design of the whole system is decisive too.
Why does the measured value change over the year?
Soil resistivity depends strongly on moisture and temperature. Dryness and frost raise it, moist soil lowers it. The earth resistance is therefore ideally assessed under unfavourable conditions and the measurement date is documented.
The right earth tester for your installation
From the fall-of-potential test set to the earth clamp meter to IEC 61557 - matched to electrode type and application.
Standards
Earth testers to IEC 61557-5 for reliable results.
Neutral advice
Recommendation by electrode type and application, brand-independent.
Matching methods
Instruments for probe and clamp methods and soil resistivity.
Expertise
Related reading: our insulation testing guide at /en/ratgeber/insulation-resistance-test.