Published on July 12, 2026 at 6:36 AM
An electrical engineer needs to check the ground resistance of a telecommunications base station. The requirement: below 10 Ω for effective lightning protection. Traditionally, they would use the 3-point method (fall-of-potential): disconnect the station’s grounding connection from the system, drive two auxiliary ground stakes 20-40 meters away, run test leads, and take the measurement. Time required: 30-45 minutes per grounding point. And the station has to be TAKEN OUT OF SERVICE to disconnect the ground, which is impossible for 24/7 communications infrastructure.
With a clamp-on ground resistance tester like the Extech 382357, the clamp is placed around the grounding conductor, a button is pressed, and 5 seconds later the resistance value appears on the display. No ground stakes. No cable runs. No shutdown.
Why the Traditional 3-Point Method Exists
To measure resistance, you have to drive a known CURRENT through the resistance under test and measure the resulting VOLTAGE. R = V / I. In a classic electrical resistance measurement with an ohmmeter, the two test probes deliver current and voltage to the resistive element.
Ground measurement is more complicated. Ground resistance exists between the grounding conductor and the „rest of the earth“ — there is no second test probe you can simply connect. The classic solution (fall-of-potential, in use since around 1900):
- Disconnect the installation’s grounding conductor from the system (E1).
- Drive auxiliary ground stake H 20-40 m away in one direction.
- Drive auxiliary ground stake S 10-25 m away in the same direction, between E1 and H.
- Pass alternating current (AC — not DC, to avoid polarization effects in the soil) through the E1-H circuit.
- Measure the voltage between E1 and S.
- R = V / I.
This method is precise (0.01 Ω resolution possible) but logistically demanding. Driving ground stakes is nearly impossible on asphalt or concrete surfaces.
The Clamp-On Principle: Inductive Current Measurement
The Extech 382357 uses a fundamental trick: the instrument creates its own test circuit by INDUCING a known voltage in the grounding conductor (transformer principle) and MEASURING the resulting current with a second winding.
Specifically, the clamp has TWO concentric ferrite cores:
- Core 1 (transmitter): Generates a sinusoidal 128 Hz AC signal (the frequency matters — not 50 Hz, to avoid interference from the power grid).
- Core 2 (receiver): Measures the induced current in the grounding conductor that flows in response to the transmitter field.
From the known transmitter voltage and the measured receiver current, the instrument calculates the ground resistance using Ohm’s law.
Important prerequisite: the object under test must be part of a CLOSED LOOP with the grounding system. This is always the case for telecommunications installations with multiple grounding points. The clamp-on method does not work for isolated single ground electrodes (e.g., on a single lightning rod with no other connection).
Technical Specifications
- Resistance range: 0.025 to 1,500 Ω, resolution 0.002 Ω (at the low end).
- Auto-ranging: automatic range switching.
- True-RMS AC leakage current range: 0.2 mA to 30 A, resolution 1 mA.
- Clamp opening: 23 mm — sufficient for thick grounding conductors (Cu 35 mm² and larger).
- Programmable data logging: 116 data points stored internally.
- Hi/Lo alarms: user-definable limits for quick pass/fail indication.
Limits for Different Applications
Different installations have different ground resistance limits:
- Lightning protection systems (DIN EN 62305): ≤ 10 Ω for individual lightning protection grounds.
- Industrial electrical systems (VDE 0100): ≤ 1-2 Ω for installations with RCD protection.
- Telecommunications installations: ≤ 5 Ω for base stations.
- Substations: ≤ 1 Ω for high-voltage grounds.
- Residential buildings: ≤ 100 Ω for single-family home grounds (rarely critical).
The 382357, with its 0.025-1,500 Ω range, covers all of these applications.
True-RMS Leakage Current Measurement as a Bonus
In addition to resistance measurement, the instrument can also measure AC leakage current (0.2 mA to 30 A). This is an independent function — instead of measuring resistance, it simply measures the current flowing through the grounding conductor.
Why is this important? Leakage currents above 3-30 mA are typically a symptom of:
- An insulation fault in a connected device.
- Capacitor moisture or aging.
- An electrolytic fault in motors.
A grounding conductor leaking 500 mA points to a serious insulation problem somewhere in the installation. An RCD would trip at 30-300 mA depending on type. Detecting leakage currents preventively, before the RCD trips, saves repair costs.
Areas of Application
Lightning protection maintenance: Annual inspection of lightning protection grounds on buildings, towers, and wind turbines.
Telecommunications infrastructure: Base stations, data centers, server rooms.
Industrial plant maintenance: Factories, power plants, chemical plants.
Substations and distribution stations: Electrical power infrastructure.
Railway infrastructure: Overhead line grounds, signaling system grounds.
Harbors and marinas: Boat connection grounds (protection against electric shock in the water).
Renewable energy: PV systems, wind turbines.
Hospital electrical maintenance: Medical rooms with IT system requirements.
Limitations of the Clamp-On Method
- Requires a closed grounding loop: The method does not work for isolated single ground electrodes with no connection to other grounding points.
- Lower precision than fall-of-potential: ±1.5% typical vs. ±0.5% for the 3-point method.
- Not for very low resistances (below 0.025 Ω): High-voltage substations with sub-mΩ grounds require other methods.
- Not for initial commissioning tests: The ground must already be connected to the system and have a return current path.
For these edge cases, the traditional fall-of-potential method is still used. For the majority of maintenance applications, however, the clamp-on method is the clear choice.
What You Get for Your Money
Extech 382357 clamp-on ground resistance tester with True-RMS AC leakage current function, resistance range 0.025-1,500 Ω with 0.002 Ω resolution, leakage current range 0.2 mA to 30 A, 23 mm clamp opening for thick grounding conductors, programmable data logging of 116 data points, programmable Hi/Lo alarms, auto-ranging, LCD display, battery-powered, hard carrying case. EUR 2,266.
A standard instrument for electrical engineers who perform routine ground maintenance without shutting down installations and without the mechanical effort of auxiliary ground stakes. With 20+ grounding points per maintenance round, the clamp-on method saves 5-10 hours of labor — the investment pays for itself quickly in commercial use.
Read more: Extech 382357 Clamp-on Ground Resistance Tester in the Shop →
