Frequently asked questions

The maximum grounding resistance depends on the grid system (TT or TN), the type of RCD and the touch voltage limit (UL). The formula is Ra ≤ UL / Ia. The EarthGND Resistance Calculator computes this instantly per NEN 1010, NEN 62305 and NEN 50522.

Required rod depth depends on soil resistivity (ρ), groundwater depth and target resistance. In dry sandy soil this may be 6–15 m; in clay or wet soil often 3–6 m. The Rod Depth Calculator computes this precisely using BRO soil data for your Dutch postal code.

NEN 1010 states Ra ≤ UL / Ia. For a 30 mA RCD: Ra ≤ 1667 Ω. For a B16 circuit breaker without RCD: Ra ≤ 0.63 Ω. EarthGND calculates the exact limit for each installation scenario.

BRO (Basis Registratie Ondergrond) is the Dutch national soil registration database. EarthGND connects to it automatically: enter a postal code and the calculator retrieves soil type, groundwater depth and pH from the nearest measurement point.

Parallel rods are needed when a single rod cannot achieve the target resistance — typically for low Ra requirements (< 1 Ω) or unfavourable soil. EarthGND automatically advises on the number of rods and minimum spacing.

In a TT system the installation earthing is independent and relies on a local earth rod. In a TN system the supply neutral is already earthed. Maximum grounding resistance requirements differ significantly: TT systems with RCDs impose stricter Ra limits.

Risk classes (per NEN 62305 / EN 50522) indicate lightning protection requirements: class I is the highest risk (buildings with hazardous content), class IV the lowest. Soil resistivity affects the classification. EarthGND displays the risk class after each calculation.

Dwight's formula and NEN/EN standards are internationally applicable. However, BRO soil data is only available for Dutch postal codes. For locations outside the Netherlands, you can manually enter soil resistivity based on a local soil survey.