Grounding Resistance Measurement — Briefly
Grounding resistance measurement is the process of measuring, in Ohms, the electrical resistance between a facility’s grounding electrode (ground rod, plate, or strip) and the earth. In Turkey, pursuant to TS 540, TS HD 60364 and the Electrical Strong Current Installations Regulation, grounding resistance has limit values between 0.5 Ω and 50 Ω depending on the type of installation. Measurement is performed using the 3-point (Fall of Potential) method, clamp-on measurement, or soil resistivity methods. Incorrect grounding can lead to serious risks such as electric shock, fire, equipment failure, and TEDAŞ violations.
Grounding Resistance Limit Values (TS HD 60364)
| Facility Type | Maximum Grounding Resistance |
|---|---|
| Residence | 5 Ω |
| Apartment Building / Housing Site | 4 Ω |
| Commercial workplace | 4 Ω |
| Industrial facility | 2 Ω |
| Hospital / Healthcare facility | 2 Ω |
| MV distribution center | 1 Ω |
| Substation | 1 Ω |
| Lightning protection system (lightning rod) | 10 Ω (single electrode), 1 Ω (system) |
| Telecommunications | 0.5 Ω |
| Computer system room | 1 Ω |
Required Equipment
- Grounding resistance tester (Earth Tester):
- For the 3-point method: Fluke 1623, Megger DET2/3, Kyoritsu KEW 4105
- For clamp-on measurement: Fluke 1630, Megger DCM, Hioki FT6380
- 2 grounding rods (probe rod, 60 cm)
- Cable set (manufacturer-specific, 20 m and 5 m lengths)
- Hammer (to drive the ground rods)
- Multimeter (pre-check for voltage)
- PPE: Insulating gloves, face shield, antistatic boots
1. 3-Point (Fall of Potential) Method
It is the most common and most accurate method. Resistance is calculated by measuring the voltage difference between three points.
Setup
- E (Earth): The grounding electrode to be measured
- P (Potential probe): Auxiliary potential probe placed away from E (10–20 m)
- C (Current probe): Auxiliary current probe placed farther than P (15–30 m)
- The three points must be on a straight line
Step-by-Step Procedure
Step 1: Preparation
- Disconnect the grounding electrode from the installation it is connected to (open the temporary shunt)
- Measure residual voltage on the electrode with a multimeter (must be ≤ 1 V)
- Check the device battery
Step 2: Driving the Probes
- Drive the P probe 10–20 m from E (60 cm below the ground surface)
- Drive the C probe 5–15 m beyond P
- The soil between probes should be moist (pour water if necessary)
Step 3: Cable Connections
- Connect the E clip from the test device to the grounding electrode
- Connect the P clip to the P probe
- Connect the C clip to the C probe
- Ensure there is no cable looseness
Step 4: First Measurement
- Start the tester → “Range” is selected automatically
- Wait a few seconds → read the Ohm value on the screen
- Record the result as R1
Step 5: 62% Method Verification
- Move the P probe to 62% of the distance between C and E
- Measure again → R2
- If R2 is close to R1 (±5%) → measurement is correct
- If there is a large difference between R2 and R1 → P probe position is wrong; repeat
Step 6: Fall-of-Potential Curve Analysis
- Measure 5–6 times with the P probe at different positions
- Plot a graph with resistance on the Y-axis and distance on the X-axis
- The value of the flat (plateau) section = true grounding resistance
2. Clamp-On Measurement Method
Advantages
- Measurement without disconnecting the grounding electrode connection
- Fast (results within seconds)
- Practical for multi-electrode systems (such as a lightning ring)
Limitations
- CANNOT BE USED in a single-electrode system (no reference)
- Soil resistivity cannot be calculated accurately
- Not as sensitive as the 3-point method near limit resistance values
Procedure
- Place the clamp around the grounding conductor
- Start the device → automatic measurement
- If there are multiple electrodes, measure the resistance of each branch separately
3. Soil Resistivity Measurement
Performed when designing a new grounding installation. The Wenner 4-electrode method is used.
- 4 probes are driven in a straight line at equal spacing (a)
- The device applies current and measures voltage
- Resistivity is calculated with ρ = 2πa × R (Ω·m)
- Typical values:
- Clay soil: 10–100 Ω·m (good)
- Sandy soil: 100–1000 Ω·m (moderate)
- Dry rocky ground: 1000–10,000 Ω·m (poor)
Step 7: Grounding Improvement Practices
If resistance is high, the following methods are applied:
- Driving deeper electrodes: 6–8 m instead of 4 m
- Adding parallel electrodes: 2–3 rods connected in parallel
- Chemical grounding (BSV-compound): Bentonite + salt mixture reduces soil resistivity
- Ground mesh (mesh): Copper strip embedded in the foundation concrete for large facilities
- Star configuration: Multiple electrodes arranged at a 60° angle
- After implementation, measurement is repeated and the new resistance value is recorded
Step 8: Reporting and Annual Repetition
- Measurement date, serial number of the device used, calibration certificate
- Measurement method (3-point or clamp-on)
- Measured resistance value and comparison with the limit value
- Improvement recommendations (if any)
- Next inspection date (1 year later)
- Signature of the EMO-registered engineer
- It must be added to the facility’s annual occupational safety file
Grounding Improvement Methods
If resistance is high:
- Driving deeper electrodes (6–8 m instead of 4 m)
- Adding parallel electrodes (2–3 electrodes)
- Chemical grounding (bentonite + salt mixture, BSV-compound)
- Ground mesh (mesh) — for large facilities
- Star configuration electrode arrangement
Periodic Inspection Frequency
| Facility | Frequency |
|---|---|
| Residence / Apartment Building | Once every 5 years |
| Commercial workplace | Once every 2 years |
| Industrial | Once a year |
| Hospital | Twice a year |
| MV / Substation | Once a year |
| Lightning rod | Once a year (end of rainy season) |
Common Mistakes
| Mistake | Result | Solution |
|---|---|---|
| Measuring without disconnecting the electrode | Installation circuit resistance included | Open the connection shunt |
| Probes driven too close | Reads high resistance | Set C to 30 m, P to 18 m |
| Dry soil | High resistance | Pour water around the probes |
| Measuring while phase current is present | Incorrect result | Shut down the installation |
| Aged device | Incorrect result | Annual calibration |
DOA Enerji Grounding Service
DOA Enerji, with its EMO-registered engineers, provides new grounding installation design, measurement of existing installations, and improvement services (including chemical grounding). Measurement reports compliant with TS 540 and TS HD 60364 standards, an annual periodic inspection agreement, and TEDAŞ compliance are provided.
Frequently Asked Questions
Is grounding resistance affected by rain?
Yes. In the rainy season, since the soil is moist, resistance can be read 20–50% lower. Standard measurement should be performed in the dry season (summer months).
The resistance is above the limit value—what should I do?
- First check the connection quality at the electrode
- Add a second, deeper (6–8 m) electrode (parallel connection reduces resistance)
- If soil resistivity is high, apply chemical grounding (bentonite + salt)
- In rocky areas with very high resistivity, consider a ground mesh system
Why is there a difference between clamp-on measurement and the 3-point method?
Clamp-on measurement measures the equivalent resistance of the entire parallel-branched system. The 3-point method measures the independent resistance of each electrode. In multi-electrode systems, clamp-on gives a lower value.
How can grounding resistance be reduced below 0.5 Ω?
This level is required for telecommunications and sensitive IT systems. It is generally achieved with a ground mesh system + chemical grounding + 3–4 parallel electrodes. Achieving 0.5 Ω with a single electrode is very difficult.
Where should the grounding electrode be placed in a new reinforced-concrete building?
When the foundation is poured, the foundation grounding electrode (galvanized or copper strip) should be embedded in the concrete. This is the most stable method with the lowest resistance. Probes driven afterward are less effective.