Jordmotståndet

Jordmotståndet, or earth resistance, is the electrical resistance between a point on a grounding system and the surrounding soil. It is a property of the soil and the grounding arrangement and is central to the performance of protective grounding in electrical installations.

The value of jordmotståndet affects safety: lower resistance enables fault currents to flow quickly, enabling protective devices to trip promptly and limiting touch voltages during faults. If the earth resistance is too high, fault currents may be insufficient for reliable protection, increasing the risk of electric shock and equipment damage.

Measurement and methods: It is measured in ohms. Common methods include the fall-of-potential (three-point) method and the four-point method, often using dedicated earth resistance testers. Clamp-on meters may be used in some contexts, but they typically measure different quantities related to the grounding network.

Influencing factors: Soil resistivity, moisture content, temperature, salinity, and depth of grounding electrodes strongly affect jordmotståndet. The geometry of the grounding system (single rod, grid, or combined networks) and nearby metallic structures also play significant roles.

Standards and practical considerations: National and international standards guide measurement methods and target values for protective grounding. In Sweden and many other countries, standards base targets on the installation’s fault-current requirements and protective device characteristics. Designers seek a balance between safety, reliability, and cost, often aiming for a low enough earth resistance to ensure proper operation of protection devices.