Published 13 July 2026 at 23:14
Electrical safety in low-voltage networks relies on fault currents finding a safe path back to the transformer without passing through a human body. This is ensured by a GROUNDING ELECTRODE — a metal rod, ring, or network that is physically connected to the earth and electrically connected to the installation’s protective conductor.
If the grounding electrode has high resistance (e.g., dry soil around the rod, corroded connection), the fault current circuit will not close quickly enough during a cable fault. Consequence: the residual current device (RCD) does not trip, voltage remains present on the building’s metal surfaces, and the next person who touches a tap or a washing machine receives the full mains voltage through their body.
The ground resistance tester measures this resistance and verifies that it is below the applicable standard requirements.
The 3-Electrode Method (Fall of Potential)
Measuring ground resistance is physically different from measuring resistance between two visible terminals. “Ground” is not a point — it is a volume into which the current spreads. The standard method is the 3-electrode technique (also known as Fall of Potential):
- Electrode E: Connected to the grounding electrode being tested.
- Electrode H (helper electrode): Driven into the ground 20-30 meters from E. The instrument injects a known AC test current between E and H.
- Electrode S (measuring stake): Driven into the ground 10-15 meters from E, midway between E and H. The instrument measures the voltage drop between E and S.
Ohm’s law gives: R = V(E-S) / I(E-H).
Two additional grounding rods are included in this product set (H and S). Cable lengths are typically 20-40 meters for field measurements.
Why AC Test Current Instead of DC?
Soil is not a passive resistor — it has natural electrochemical processes (galvanic cells between different minerals in the ground), induced voltage from the power grid, and capacitances from layer structures.
If the instrument used DC test current, the measurement would include these natural ground voltages and yield false values. AC test current (typically 128 Hz or 823 Hz — outside the power grid’s 50/60 Hz and its harmonics) allows the instrument to filter out natural ground signals and isolate only its own test signal response.
Automatic Zero Adjustment
Test leads between the instrument and the electrodes have their own resistances (typically 0.1-0.5 Ω per 10 meters of cable). When measuring a ground electrode with a target value of 2-5 Ω, the lead resistance can constitute a significant portion of the measured value.
Automatic zero adjustment: the instrument first measures its own lead circuit resistance without the object under test connected, stores this value, and automatically subtracts it from the next measurement. This means the operator does NOT need to manually “zero” the instrument at every test instance.
Standard Requirements in Sweden
According to SEK Handbook 444 and SS-EN 61936-1:
- TN-C-S system (detached house, terraced house, industry): Ground resistance ≤ 5 Ω at the main grounding point. For higher values, supplementary equipotential bonding or an RCD with a lower tripping current is required.
- TT system (e.g., transformer stations without direct ground connection): R × I_ΔN ≤ 50 V for RCD-based protection. With a 30 mA RCD, R ≤ 1667 Ω is allowed. With a 300 mA RCD, R ≤ 167 Ω is allowed. Practically: max 200 Ω is recommended.
- Lightning protection: ≤ 10 Ω for normal buildings, ≤ 4 Ω for buildings with explosion risk.
- Power stations and power plants: Depending on the system, typically ≤ 1 Ω.
Factors Affecting Ground Resistance
Soil Type:
- Clay soil: 10-100 Ωm — excellent ground for electrode.
- Sandy soil: 100-1000 Ωm — average.
- Peat bog/marsh: 100-500 Ωm — good but variable.
- Bedrock: 5000-50000 Ωm — very poor, requires large electrode surface area.
- Sea sand (dry): 20000-50000 Ωm — challenging.
Moisture Content: Resistance can change by a factor of 10-100 between a dry summer and a wet autumn. Verification should be performed during the driest season (July-August in Sweden) to capture the worst-case scenario.
Temperature: Frozen ground has 10-100 times higher resistance. Winter measurements in northern Sweden are unreliable if the ground is frozen solid.
Electrode Geometry: Deeper electrodes provide better contact with deeper, moister soil layers. A depth of 1.5 m is standard; 3-5 m yields better results in dry soil.
Additional Features in This Instrument
In addition to ground resistance measurement, the instrument also measures:
- AC/DC Voltage: Verification that the grounding network does not have induced voltage from the power grid (if the instrument shows >10 V at startup, the equipment must be secured before measurement begins).
- Resistance (general ohmmeter function): For checking the resistance of conductors from the ground bar to the electrode.
- Continuity: Beeps at low resistance (typically <30 Ω) for quick continuity testing of ground conductors.
This makes the instrument a combined ground verification multimeter, not just a pure ground resistance tester.
Test Hold and Dual Display
Test Hold Function: During one-handed measurement, the operator can press HOLD before pulling up the stake electrodes. The last measured value is frozen on the display so that the operator can read it after the electrodes have been removed.
Dual Display: Simultaneously displays both test current and calculated resistance, or the current value and the HOLD-stored value. Facilitates troubleshooting when the measurement is unstable (fluctuating values indicate poor electrode contact or induced voltage interference).
Practical Measurement Procedure
- Ensure the installation is energized but NO loads are active (to minimize induced voltage noise).
- Disconnect the main ground bar from the transformer neutral conductor (to isolate the grounding electrode from the network’s other ground paths).
- Place helper electrode H at a straight-line distance of 20-30 m from the grounding electrode.
- Place measuring stake S midway — typically 10-15 m from E.
- Connect the instrument with its leads between the three electrodes.
- Press TEST. The instrument injects AC test current and displays the resistance value.
- Move S 1-2 meters closer or further away — the value should be stable (±10 %). Instability indicates that S is within H’s influence zone and distances must be increased.
- Note the value. Check against standard requirements.
- Reconnect the main ground bar.
Applications
New installation of detached/terraced houses: Electrician verifies the grounding electrode’s resistance before inspection.
Industrial installation: New transformer station, new production line with high current.
Annual periodic inspection: Power stations, hospitals, data centers require periodic ground verification as part of electrical safety work.
Lightning protection verification: Churches, masts, telecommunications towers.
After work in the ground: If ground conductors are damaged during excavation, ground verification must be repeated after repair.
After lightning strike: A direct lightning strike can melt the ground conductor or degrade the electrode. Verification is required to confirm functional status.
Electrical failure diagnosis: When an RCD does not trip correctly, the cause may be a poor grounding electrode.
What You Get for Your Money
Ground resistance tester with 3-electrode method, large backlit dual display, Test Hold function, automatic zero adjustment, AC/DC voltage and resistance measurement, continuity beeper, automatic shutdown, limit range and battery indication. Delivered complete with measurement cables, TWO EXTRA ground resistance rods, case, protective rubber holster, and six 1.5V AA batteries. 4,320 kr.
The standard tool for electricians, electrical installation companies, power station maintenance, and all businesses that must verify the functionality of grounding electrodes according to SEK Handbook 444 or other electrical safety requirements. The premium over a standard multimeter (200-800 kr) is justified because a multimeter CANNOT measure ground resistance — it requires dedicated 3-electrode measurement that only a ground resistance tester can perform.
Read more: Ground resistance tester in the shop →
