Testing and verification
How is earth electrode resistance measured in Spain?
Earth electrode resistance is preferably measured with an earth tester using the fall-of-potential method, disconnecting the electrode where this can be done safely and placing auxiliary stakes outside its influence area. Alternative methods can be valid where stakes are impractical, but their limitations matter.
Why there is no universal 10-ohm rule
For a TT system with RCD protection, the equation relates earth-plus-protective-conductor resistance (RA), operating current (Ia) and touch-voltage limit. With 30 mA and 50 V, the mathematical limit is 1,667 Ω; that is not a design target and does not by itself make an installation satisfactory. Margin, stability and any specific ITC rules are still needed.
Test methods
Fall-of-potential testing with stakes measures the electrode directly. A clamp earth tester needs a suitable return loop and can measure a different quantity from an isolated electrode. A loop-impedance test at the board is also a different test, though it informs fault protection.
For the next part of the decision, see the guides to Insulation resistance and Protective continuity.
What can distort an earth resistance test?
An earth electrode may appear better than it is if the reading includes unidentified parallel electrodes or metallic paths. In a stake test, auxiliary electrodes placed too close also distort the result. The technician checks different probe positions and reading stability: a displayed number is not enough without understanding the circuit measured.
Soil moisture affects earth resistance. Testing immediately after heavy rain may not represent the worst annual condition. ITC-BT-18 requires a check at least annually during the driest soil conditions. Where soil is unfavourable to electrode preservation, it additionally requires exposing electrodes for examination at least every five years. That particular requirement should not be generalised to every earthing system.
The earthing report should identify the electrode, method, ground conditions, instrument and connections included. Protective continuity and coordination with the RCD also need checking. Replacing an electrode alone will not repair a broken protective connection between the board and exposed metalwork, even if soil resistance is low.
Fall-of-potential test procedure
- 01
Identify the earthing arrangement and associated protection.
- 02
Open the test link only under safe, controlled conditions.
- 03
Place auxiliary electrodes and confirm reading stability.
- 04
Restore and confirm continuity of the earth conductor.
- 05
Check RA × Ia ≤ U and any installation-specific rule.
Frequently asked questions
How many ohms should an earth electrode have?
It depends on protection and installation type. TT systems use RA × Ia ≤ U and other ITCs may add requirements. A recommended figure is not automatically the legal limit.
Can a clamp earth tester be used?
Yes where a suitable return loop exists and the instrument is appropriate, but its result and limitations differ from stake testing.
Regulatory basis and official sources
- Spanish Low Voltage Regulation (REBT) and technical instructions — BOE
- Installation verification — official Guía-BT Annex 4
Always check the current version of the rule and the administrative procedure that applies to the specific case.