Clamp On Earth Tester vs Fall of Potential Method: Which Earth Resistance Test Should You Use?

Earth resistance testing is an essential part of electrical safety, preventive maintenance and commissioning. However, one common question continues to create confusion:

Should we use a clamp-on earth tester or the fall of potential method to test an earth pit?

The correct answer depends on the earthing arrangement.

A clamp-on earth tester is useful for fast testing of a multi grounded system that has a closed return path. The fall of potential method is generally preferred for a single isolated earth electrode, new installation, commissioning test or detailed verification of an individual electrode.

Neither method is universally better. Each method measures the earthing system in a different way and must be applied under the correct conditions.

Quick Answer

Use a clamp on earth tester when:

• Several earth electrodes are interconnected.

• A complete return loop exists.

• The individual earth conductor is accessible.

• The earthing system should remain connected during testing.

• A quick maintenance or comparative check is required.

Use the fall of potential method when:

• The earth pit is isolated or has only one path to earth.

• A new earth electrode is being commissioned.

• An individual electrode resistance must be verified more directly.

• A clamp reading is unusually low, high or doubtful.

• The total resistance of an earthing system to remote earth must be evaluated using a properly designed test arrangement.

Clamp on testing is therefore not a complete replacement for the fall of potential method.

What is Earth Resistance?

Earth resistance describes the opposition offered by an earthing electrode or grounding system to the flow of test current into the surrounding earth.

The result is affected by several factors, including:

• Soil resistivity

• Soil moisture and temperature

• Electrode dimensions and depth

• Number and spacing of electrodes

• Condition of joints and conductors

• Parallel metallic paths

• The design and size of the overall earth grid

There is no single resistance value that is automatically acceptable for every electrical installation. The required performance depends on the system earthing arrangement, protective-device operation, fault-current level, touch and step voltage limits, project specifications and statutory requirements.

IEEE 81-2025 covers the measurement of earth resistivity, grounding system impedance, touch and step voltages, grounding integrity, measurement limitations and factors that can distort test results. It is the current active edition and supersedes IEEE 81-2012.

What Is a Clamp On Earth Tester?

A clamp on earth tester, also called a stakeless earth tester, measures the resistance of a closed earthing loop without using auxiliary test spikes.

The instrument normally contains two functional sections:

1. A voltage injection coil that induces a known test voltage into the conductor.

2. A current sensing coil that measures the current flowing through the completed loop.

The instrument then applies Ohm’s law:

R=V/I

IEC 61557-5:2019 specifies requirements for equipment used to measure resistance to earth using AC voltage. The current edition includes requirements for clamps and an annex covering measurements using loop clamps. This confirms that clamp based testing is a recognised measurement technique when the equipment and application conditions are appropriate.

How Does a Clamp On Earth Tester Reading Work?

Consider one earth electrode under test, identified as Rx​, connected to several other earth electrodes.

The displayed result can be simplified as:

Rdisplay=Rx+Rp​

Where:

• Rx​ is the resistance of the selected earth electrode.

• Rp​ is the equivalent parallel resistance of all the other connected earth paths.

The parallel return resistance is:

A clamp tester therefore does not automatically measure Rx​ alone. It measures the resistance of the complete loop formed by the selected electrode and the other available return paths.

When many good earth electrodes are connected in parallel, Rp​ may be much smaller than Rx. In that situation:

Rdisplay≈Rx

Hioki’s official measurement explanation uses the same loop equation and confirms that clamp on earth testers are intended for multiple grounded systems, not single or independent residential earth electrodes.

Example 1: Two Similar Earth Pits

Assume:

Rx=5 Ω

The only other earth pit is also:

R1=5 Ω

The displayed loop resistance will be approximately:

Rdisplay=5+5=10 Ω

The selected pit is actually 5 Ω, but the meter displays approximately 10 Ω.

Therefore, in a two pit system, the clamp result should not be treated as the exact individual resistance of the selected pit.

Example 2: Ten Similar Earth Pits

Assume:

• Selected pit Rx=5 Ω

• Nine other pits, each equal to 5 Ω

The equivalent resistance of the other nine electrodes is:

Rp=5/9=0.56 Ω

Therefore:

Rdisplay = 5+0.56

Rdisplay ≈ 5.56 Ω

The displayed result is now much closer to the selected electrode’s actual resistance.

This explains why clamp on testers are often effective for routine checks in large multi-grounded facilities. However, this is a simplified resistance model. Large grids may also involve conductor impedance, mutual resistance, current division, connected neutrals, structural steel, cable screens and other influences that require a more detailed measurement approach.

Where Should a Clamp On Earth Tester Be Clamped?

For an individual electrode assessment in a multi grounded system, place the clamp around:

The single conductor connecting the selected earth electrode to the common earth bar or earth grid, before that conductor joins other earthing conductors.

The following conditions should be satisfied:

1. Only the selected earth conductor should pass through the clamp jaws.

2. The conductor should belong to one identifiable earth electrode.

3. The clamp should be positioned before the conductor reaches the common bonding point.

4. The clamp jaws must close completely.

5. The jaw surfaces must be clean.

6. A closed return path through the remaining earthing system must exist.

7. There should be no separate metallic connection bypassing the clamp position.

Clamp manufacturers also advise placing the clamp at the narrowest accessible point of the individual earth conductor.

Where Should You Not Clamp?

1. Do not clamp the common earth bar for an individual pit reading

If several earth conductors have already joined a common earth bar, earth ring or grid conductor, placing the clamp around that common section will not isolate one electrode.

The result may represent a local loop within:

• The common GI strip

• A copper earth ring

• Structural steel

• Cable armour

• Parallel bonding conductors

• Other interconnected metallic paths

It should not be recorded as the resistance of one earth pit.

2. Do not use clamp only testing on a single isolated earth pit

A clamp on tester requires a complete return loop.

A single independent electrode has no alternative earth path for the injected test current to return through. The tester may show:

• OL

• Open loop

• An unstable result

• An over range indication

• No valid reading

Fluke states that when there is only one path to earth, the stakeless method does not provide an acceptable value and the fall of potential method should be used.

3. Do not clamp around multiple conductors together

If the outgoing and return paths are both enclosed by the clamp, their magnetic effects may cancel or produce an incorrect result.

The jaw should enclose only the intended individual conductor.

4. Do not assume every low reading is good earthing

An unusually low result can occur when the test current returns through a metallic path instead of travelling through the soil and the connected electrodes.

Megger explains that suspiciously low clamp on readings may indicate that the instrument is measuring a metallic loop or continuity path rather than earth resistance.

What Does a Reading of 0.04 Ω Mean?

A reading such as 0.04 Ω should not automatically be accepted as the resistance of an individual earth pit.

It may indicate:

• The clamp is placed on a common earth strip.

• A parallel GI or copper strip completes the loop.

• Structural steel creates a metallic bypass.

• Cable armour or metallic piping provides another return path.

• The tester is measuring local metallic continuity.

• The selected point is part of a very large low impedance grid.

• The reading is close to the instrument’s lower measurement capability.

A genuinely very low grid resistance may be possible in a large and well designed substation or extensive grounding installation. However, system topology, instrument range, measurement uncertainty and the location of the clamp must be reviewed before accepting the result.

A suspicious value should be verified using:

• Fall of potential testing

• Selective testing with stakes and a current clamp

• A properly engineered large grid measurement method

  
  

Does a Low Clamp Reading Confirm Good Earth Continuity?

A low clamp reading confirms only that a low resistance closed loop exists around the measurement point.

It does not independently prove that:

• Every joint is mechanically secure.

• The protective conductor has sufficient fault current capacity.

• The equipment body is correctly bonded throughout the complete path.

• The soil to electrode resistance is acceptable.

• The protective device will disconnect within the required time.

• Touch and step voltages are within safe limits.

Clamp on testing is useful as a maintenance and troubleshooting tool, but formal protective conductor verification may require visual inspection, low resistance continuity testing, earth fault loop testing and other installation specific tests.

What Does “OL” Mean on a Clamp On Earth Tester?

OL should first be interpreted as:

The instrument could not identify a measurable closed loop within its operating range.

Possible causes include:

• A single isolated electrode

• No closed return path

• A broken conductor

• A loose or corroded joint

• Excessively high loop resistance

• Incorrect clamp position

• Jaws not fully closed

• Dirt or damage on the clamp surfaces

• Electrical interference

• Instrument or battery condition

OL does not automatically prove that an earth conductor is broken. The complete circuit must be inspected and tested before reaching that conclusion.

What Is the Fall of Potential Method?

The fall of potential method is a stake based earth resistance test. It is commonly called:

• The three point method

• The three pole method

• The spike method

• The traditional earth resistance test

It normally uses:

1. The earth electrode under test

2. A temporary potential probe

3. A temporary current probe

The tester injects a known current between the electrode under test and the remote current probe. It measures the voltage between the electrode and the potential probe.

The instrument calculates:

R=V/R

When correctly arranged, this method provides a more direct and verifiable measurement of the individual electrode or grounding system under test.

Basic Fall of Potential Test Procedure

The test should be performed by a competent electrical professional using an approved safe-work procedure.

Step 1: Identify the electrode under test

Confirm the earth-pit identification, conductor route, system connection and whether the electrode can be safely isolated.

Step 2: Apply the required safety controls

Do not disconnect an operational earthing conductor without an approved shutdown, risk assessment and any necessary temporary earthing arrangement.

Step 3: Connect the earth tester

Connect the tester to:

• The electrode under test

• The potential probe

• The current probe

Follow the instrument manufacturer’s terminal identification and instructions.

Step 4: Position the probes

Place the probes in a straight line away from the electrode under test.

The probes must be sufficiently far away to avoid overlapping resistance areas around the electrode and the current probe.

Step 5: Take multiple readings

Move the potential probe slightly forward and backward and repeat the measurement.

A reasonably stable group of readings indicates that the potential probe is in the flatter portion of the resistance curve. If the readings change significantly, increase the test distance and repeat the test.

Fluke’s fall of potential guidance recommends repositioning the inner probe and increasing the spacing when the readings change significantly.

Step 6: Record the complete test conditions

The report should include:

• Electrode identification

• Test method

• Instrument make, model and serial number

• Calibration status

• Probe distances

• Test direction

• Individual readings

• Soil and weather conditions

• Date and time

• Isolation status

• Photographs

• Name and qualification of the person conducting the test

  
  

Is the 62% Rule Always Accurate?

The 62% method places the potential probe at approximately 61.8% of the distance between the electrode under test and the current probe.

It can be a useful shortcut for a simple electrode under suitable soil and spacing conditions. However, it should not be treated as universally accurate.

Its reliability can be affected by:

• Non uniform soil

• Large earth grids

• Nearby buried metal

• Underground cables or pipelines

• Insufficient current-probe distance

• Overlapping resistance areas

• Multiple connected electrodes

Megger notes that the 62% method relies on ideal conditions and that a full or simplified fall of potential verification is more defensible when site conditions are complex.

What Is the Selective Earth Testing Method?

Selective testing combines:

• Fall of potential test leads and probes

• A current measuring clamp around the selected electrode conductor

It allows the tester to measure the current flowing through the selected electrode while reducing the effect of parallel earth paths.

This method is useful when:

• Several earth electrodes are interconnected.

• The selected electrode cannot be safely disconnected.

• An individual electrode result is required.

• Adequate space for auxiliary probes is available.

Fluke describes selective testing as a fall of potential based method that measures a selected electrode without disconnecting it from the site. Hioki provides a similar clamp assisted measurement function for excluding the influence of other grounds.

Clamp-On Tester vs Fall of Potential Method

Test condition	Clamp on method	Fall of potential method
Multiple interconnected earths	Suitable when a closed loop exists	Suitable with the correct test arrangement
Single isolated earth pit	Not suitable	Recommended
Auxiliary test spikes	Not required	Required
Earth conductor disconnection	Normally not required	Often required for an individual electrode test
Measurement obtained	Ground-loop resistance	Electrode or system resistance to remote earth
Testing speed	Fast	Slower
Routine maintenance	Very useful	Useful but more time-consuming
New installation commissioning	Should not normally be the only method	Preferred
Exact individual electrode assessment	Approximate under defined conditions	More direct and verifiable
Risk of metallic bypass error	Significant	Lower when the setup is correct
Space required	Minimal	Adequate probe distance required

Which Method Is Better?

There is no universal winner.

Choose a clamp-on earth tester when:

• The system has several interconnected earth electrodes.

• A valid earth return loop exists.

• The individual electrode conductor can be accessed.

• Disconnecting the earth conductor is unsafe or impractical.

• The purpose is periodic maintenance or comparison with previous readings.

• Auxiliary probes cannot be installed.

Choose fall of potential testing when:

• Testing a single isolated electrode

• Commissioning a new earth pit

• Verifying an individual electrode

• Investigating an abnormal clamp result

• Preparing a detailed acceptance or engineering report

• Measuring system resistance to remote earth using an appropriate test layout

Choose selective testing when:

• The system has parallel electrodes.

• The selected electrode cannot be disconnected.

• An individual electrode result is required.

• There is sufficient space for test probes.

  
  

Common Earth Testing Mistakes

Recording loop resistance as individual earth pit resistance

A clamp result is not automatically the selected electrode resistance. The return-path resistance is also included.

Clamping a common earth strip

This can produce a very low metallic loop reading instead of an individual electrode measurement.

Testing a single isolated pit with a clamp meter

Without a closed loop, clamp only testing is invalid.

Assuming OL always means a broken earth conductor

OL may simply mean that no return loop exists.

Assuming the lowest value is always the best value

An unrealistically low value may be caused by a metallic bypass or an incorrect measurement point.

Using one fixed resistance limit for every installation

Acceptance criteria must be based on the earthing design, protection system, statutory requirements and project specifications.

Using unverified equipment

The tester should be within calibration, visually inspected and checked with the manufacturer’s verification loop where supplied.

Kerala Electrical Inspectorate’s Meter Testing and Standards Laboratory is a state government electrotechnical calibration facility with NABL accreditation.

Standards and Regulatory References

This article has been prepared with reference to the following current technical and regulatory framework as checked in August 2026:

IS 3043:2018

Code of Practice for Earthing — Second Revision

This is the current BIS-listed Indian standard for earthing practice.

IEC 61557-5:2019

Equipment for Testing, Measuring or Monitoring Protective Measures — Resistance to Earth

This edition includes requirements for clamp-based measuring equipment and test measurements using loop clamps.

IEEE 81-2025

Guide for Measuring Earth Resistivity, Ground Impedance and Earth Surface Potentials of a Grounding System

This is the active IEEE edition and supersedes IEEE 81-2012.

Central Electricity Authority Regulations

The CEA currently lists the Measures Relating to Safety and Electric Supply Regulations, 2023, together with the 2026 Amendment Regulations. The applicable provisions and later amendments should be reviewed for each project.

Kerala Electrical Inspectorate

Kerala Electrical Inspectorate states that approvals for applicable electrical installations are processed with reference to the CEA safety regulations.

Final Engineering Recommendation

A clamp on earth tester is a valid and highly useful professional instrument. The problem is not the instrument; the problem is using it without understanding the earthing topology.

Remember these three rules:

Clamp on one identifiable earth conductor in a multi grounded system.

Do not treat a common strip or metallic loop reading as an individual earth pit resistance.

Use fall of potential or selective testing when an individual, commissioning or technically defensible verification result is required.

For critical installations, earth resistance should never be evaluated from one unexplained number alone. The test method, clamp location, system arrangement, parallel paths, instrument accuracy, soil conditions and protective-system requirements must all be documented.

Prepared by: Ensemble Electric Technical Team

Technical Review: John Anto, B.Tech Electrical & Electronics Engineering, Kerala State Electrical A-Grade Supervisor Licence Holder

Last technically reviewed: August 2026

Editorial note: This article is intended for professional education. Site testing, isolation and acceptance decisions must be undertaken by competent electrical personnel using calibrated instruments and installation specific safety procedures.