Quick Summary
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Measure without shutdown: Clamp-on (earth resistance clamp) works on multiple-earthed systems.
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Proof & single electrodes: 3-/4-wire is mandatory when no closed loop exists (new/single electrodes, acceptance tests).
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Practical focus: Decide by use case, accuracy, documentation—not just device price.
Introduction: The problem behind measurement requests
“We can’t shut down,” “Everything’s paved,” “The auditor wants solid proof”—this is exactly where the wheat is separated from the chaff. Grounding measurements rarely fail because of physics but because of constraints: plant downtime, construction-site conditions, time pressure, documentation duties.
The good news: With the right choice of method (clamp-on vs. 3-/4-wire) almost any scenario can be handled cleanly—either quickly during live operation or with high precision using auxiliary electrodes. Below you’ll find the quick comparison and the matching product group from your shop.
Quick comparison at a glance
| Criterion | Clamp-on (earth resistance clamp) | 3-wire | 4-wire |
|---|---|---|---|
| Shutdown required | No | Usually yes | Usually yes |
| Prerequisite | Multiple earthing / closed loop | Auxiliary electrodes (P/C) | Auxiliary electrodes (P/C), higher precision |
| Typical use cases | Lighting poles, fences, large plant networks, maintenance | Single electrodes, new/retrofit, high-resistance soil | Long runs, acceptance tests, high accuracy requirement |
| Strengths | Very fast, probe-less, often incl. leakage-current measurement | Broadly applicable, solid baseline proof | Best results with lead/contact influences |
| Matching products | Earth resistance clamps in the category → | Ground & soil resistance meters → | Ground & soil resistance meters → |
| Link | Go to product category | Go to product category | Go to product category |
Rule of thumb: No parallel path? → No clamp-on. In that case, use 3-/4-wire.
The methods in 90 seconds: What measures what?
Clamp-on (earth resistance clamp)
Measures the loop resistance via induction—without disconnecting the conductor. Works only when a closed loop / parallel paths exist (multiple earthing, meshed network). Ideal for maintenance without plant shutdown.
→ Find suitable instruments here: Ground & soil resistance meters (incl. clamps)
3-wire measurement
Classic earth resistance measurement with two auxiliary electrodes (P & C). Universally applicable—even for single electrodes or new/retrofit jobs. Requires a field setup.
4-wire measurement
Like 3-wire, plus separate sense leads to compensate lead/contact resistances. More precise on long runs/unfavorable geometry; preferred for acceptance tests.
Technical quick summary (table)
| Method | Measurement principle | Prerequisite | Typical points | Strength | Limitation |
|---|---|---|---|---|---|
| Clamp-on | Induced test current, loop evaluation | Multiple earthing/closed loop | Poles, fences, large plants | Fast, probe-less, live | Unsuitable for single electrodes |
| 3-wire | Current through electrode, voltage at P electrode | Aux electrodes P & C | Single electrodes, new/retrofit | Widely applicable | Setup required, usually shutdown |
| 4-wire | Like 3-wire, but sense leads separate load/voltage | Aux electrodes P & C, 4 terminals | Long runs, acceptance | Highest accuracy | More effort |
Rule of thumb: No parallel path → no clamp-on. Then use 3-/4-wire.
Decision in 60 seconds (flow)
Quick check (yes/no table)
| Question | Yes → | No → |
|---|---|---|
| Are there multiple earths/parallel paths? | Clamp-on (no shutdown) | 3-/4-wire |
| Must the installation stay in operation (no downtime possible)? | Clamp-on | 3-/4-wire (plan a time window) |
| Is it a single/new electrode? | 3-/4-wire | Clamp-on possible |
| Is highest accuracy/audit-proof evidence required (acceptance, long runs)? | 4-wire (or 3-wire + plausibility) | Clamp-on/3-wire sufficient |
| Many measurement points in the field (poles/fences, urban/asphalt)? | Clamp-on (efficiency) | 3-/4-wire selectively |
Practical procedure (checklist)
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Identify the scenario: Multiple earthing present? Single electrode? Downtime possible?
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Choose the method:
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Multiple earthing & operation running → Clamp-on.
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Single/new electrode or acceptance → 3-/4-wire.
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Secure measurement quality: Take multiple readings (2–3×), avoid EMC interference; with 3/4-wire check distance variation.
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Document: Location, conductor/position, weather/soil, photos, measurement series (x3), average, comment.
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Trend over single value: Compare with previous years; remeasure conspicuous deviations (switch method if needed).
Straight to the right instrument
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Category with clamps and 3/4-wire meters:
Ground & soil resistance meters (ICS-Schneider)
Typical use cases (and which method fits)
| Scenario | Recommended method | Why | Measurement notes | Instruments* |
|---|---|---|---|---|
| Lighting poles / fences / large meshed site | Clamp-on | Multiple earths ⇒ closed loop present | Select a conductor with a reliable return path, 2–3 repeats, avoid EMC interferers | Ground & soil resistance meters |
| Newly installed electrodes / single electrodes | 3-/4-wire | No parallel path ⇒ clamp unusable | Place auxiliary electrodes with sufficient spacing (distance variation!), good ground-rod coupling | see category |
| PV system / EV charging infrastructure (maintenance in operation) | Clamp-on, optionally 3-/4-wire as supplement | Quick check without downtime; acceptance/troubleshooting with probes | Compare trends (previous years), verify outliers with 3-/4-wire | see category |
| Industrial plant with multiple earths | Clamp-on for routine, 3-/4-wire for evidence | Efficiency + documentable spot checks with probes | Clearly document the measuring point (conductor, position, photo) | see category |
| Lightning protection/equipotential bonding (design/acceptance) | 3-/4-wire | Higher evidential certainty/accuracy | Prefer 4-wire for long runs/unfavorable geometry | see category |
| Asphalt/concrete/rock – probes difficult or impossible | Clamp-on | Probe-free | Ensure a return path exists | see category |
* The linked category contains both clamp-on meters and 2/3/4-wire instruments.
Step-by-step: Measuring correctly
A) Clamp-on (earth ground clamp meter) – no downtime
Preparation
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Check return path: Are there parallel paths/multiple earths? (No return path, no clamp measurement.)
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Select measurement conductor: Earth strap / equipotential bonding (PA) conductor, not PE busbars with strong interference fields.
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Prepare contact point: Remove paint/rust/dirt (if needed).
Measurement
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Close the clamp centrally around the conductor (no tilting).
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Minimize interference fields: Keep distance to live conductors/transformers.
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Repeat: Take 2–3 readings and calculate the average.
Documentation
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Location, conductor/position, date/time, weather/soil, measurement series (×3), average, comment, photo.
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Trend assessment: Compare with values from previous years.
Typical errors & quick fixes
| Symptom | Possible cause | Fix |
|---|---|---|
| Implausibly low resistance | Many parallel paths understate the value | Choose another measuring point; additionally use 3-/4-wire |
| Strongly fluctuating readings | EMC interference, clamp not firmly centered | Increase distance to interferers, close clamp calmly/centrally, measure multiple times |
| No reading / error code | No closed loop | Switch method: 3-/4-wire |
Suitable instruments (clamps & probe-based meters):
ics-schneider.de – Ground & soil resistance meters
B) 3-/4-wire – with auxiliary electrodes (precise verification)
Preparation
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Choose measuring point: Access to the earth electrode/connection lug.
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Place auxiliary electrodes: C (current) & P (potential) with sufficient spacing in different directions; route cables neatly.
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Ensure contact quality: Drive ground stakes deep enough; moist soil improves coupling.
Measurement (3-wire)
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Connect the instrument according to the terminal plan (E–P–C).
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Start the measurement; perform a distance variation (slightly move C) as a plausibility check.
Measurement (4-wire)
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Additionally use separate sense leads (E–ES / C–CS) to compensate lead/contact resistances.
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Measure as above and record the results.
Documentation
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Sketch/photo of stake positions, distances, soil condition, measurement series (×3), average, comment.
Typical errors & quick fixes
| Symptom | Possible cause | Fix |
|---|---|---|
| Values vary strongly with stake position | Spacing too small / poor coupling | Increase distances; choose another direction; moisten soil (where permitted) |
| Unexpectedly high resistance | Poor contact at electrode/stake, corroded clamps | Clean contact surfaces, check clamps, inspect cables/connectors |
| Result “jumps” | Shunt paths / parallel paths | Check cable routing, try an alternative direction, use 4-wire |
All variants (2/3/4-wire) & accessories:
ics-schneider.de – Ground & soil resistance meters
Pocket decision table
| Goal | Recommendation | Note |
|---|---|---|
| Maintenance without downtime | Clamp-on | Only with multiple earthing/parallel paths |
| Acceptance / single electrode | 3-/4-wire | Prefer 4-wire for long runs |
| Many test points (urban/asphalt) | Clamp-on | Track trends; verify a sample with 3-/4-wire |
Accuracy, limits & interpretation
Why do the two methods read differently?
Clamp-on captures the loop resistance of the entire earthing network — parallel paths (multiple earthing points) can make the value appear lower. 3-/4-wire measures the earthing resistance of a defined electrode — more accurate, but requires a test setup. Therefore: trend & context beat a single reading.
Quick overview: strengths, limits, practice
| Criterion | Clamp-on (clamp) | 3-wire | 4-wire |
|---|---|---|---|
| Downtime required | No | Usually yes | Usually yes |
| Measurement object | Loop / network | Single/defined electrode | Like 3-wire, but cable/contact influence compensated |
| Accuracy (practical) | Good in meshed networks | Good | Very good (long runs) |
| Typical error source | Value “improved” by parallel paths | Stakes too close together | Contact/cable resistances |
| Best use cases | Maintenance, many test points, asphalt/concrete | Single electrodes, new build/modification | Acceptance tests, long cable runs |
| Link to instruments | Go to category | Go to category | Go to category |
Interpretation guide (what to do with the result?)
| Finding | Possible cause | Recommended action |
|---|---|---|
| Value significantly lower than expected (clamp-on) | Many parallel paths in the network | Change measurement point; 3-/4-wire for verification |
| Value higher than before (all methods) | Corrosion/contact resistance, loose connection | Visual inspection, clean/tighten terminals; repeat measurement |
| Strong fluctuation | EMI sources, poor contact, clamp misalignment | Increase distance to interferers; close clamp centered; measure 2–3× |
| Measurement aborts / no reading (clamp-on) | No closed loop (single electrode) | Switch to 3-/4-wire |
| 3-wire varies with stake position | Distances too small / poor coupling | Increase distances, choose another direction, moisten soil (if permitted) |
Trend over single value (audit-ready)
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Series measurement: Always take 2–3 repeats, document the mean.
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Time series: Compare with previous years/months (consider seasonal effects of moisture/temperature).
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Plausibility check: Confirm conspicuous clamp-on values with 3-/4-wire.
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Documentation: Location, conductor/position, weather/soil, photos, sketch, measurement series, mean, comment, instrument/calibration details.
When to switch method?
| Starting situation | Switch to … | Why |
|---|---|---|
| Clamp-on, result conspicuous | 3-/4-wire | Check single electrode in isolation, rule out parallel paths |
| 3-wire, long conductors/high requirements | 4-wire | Compensate lead/contact resistances |
| 3-/4-wire difficult on asphalt/concrete | Clamp-on | Measure without probes – only with multiple earths |
Mini checklist “measurement quality”
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Before: Method suited to the scenario? Ground path/parallel paths present?
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Setup: Clean contacts (clamp centered, stakes firm & deep), cable routing without loops.
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Environment: Keep distance from strong EMC sources, keep measurement short & steady.
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After: Double measurement, mean value, photo/sketch, compare with previous values.
Suitable instruments (clamps & 2/3/4-wire meters) including accessories:
ics-schneider.de – Ground & soil resistance meters
FAQ – Grounding measurements at a glance
1) Does the clamp-on method work with single electrodes?
Short answer: No. The clamp needs a closed current loop (parallel paths/multiple earths). For single or newly installed electrodes always use 3-/4-wire.
2) How can I tell that a clamp-on measurement is possible?
There are several return paths to earth: meshed bonding network, multiple electrodes, interconnected plant parts/masts. Practical check: If you were to disconnect the conductor, would there be other connections to earth? If yes → clamp-on is usually possible.
3) 3-/4-wire: How large should the stake spacing be?
It depends on soil and geometry. Procedure: start with a sufficiently large spacing, then apply distance variation (move the C stake further away) until the reading stabilizes on a plateau. That plateau is the reliable value. (Rule of thumb to start: a multiple of the electrode depth; then verify with variation.)
4) For 3-/4-wire, do I have to shut down?
Often yes, at least isolate the conductor under test. Plan a time slot or, if shutdown is impossible, use clamp-on for the in-operation check and confirm anomalies later with 3-/4-wire.
5) How often should I measure and how should I document?
Follow standards/manufacturer/company policies. Maintenance practice: annually or after intervention/incidents. Always document measurement location, photo/sketch, conditions (weather/soil), measurement series (≥2–3), mean value, instrument & calibration status – that way you’re audit-ready.
Mini product recommendations by use case
| Use case & goal | Method | Why this choice | Device group / Link |
|---|---|---|---|
| Many points in the field (masts, fences), no shutdown | Clamp-on (earth resistance clamp) | Fast, probe-free, return path present | Earth resistance clamps → Earth & soil resistance meters |
| Single electrode / new build or retrofit / acceptance test | 3-/4-wire | Defined electrode, high evidential certainty | Earth & soil resistance meters (3/4-wire) → Go to category |
| Long leads / highest accuracy required | 4-wire | Compensation of lead/contact resistances | 4-wire-capable models → Go to category |
| Asphalt/concrete – probes hard to place | Clamp-on | No auxiliary stakes needed, operation continues | Earth resistance clamps → Go to category |
Tip: In the category, additionally highlight clamps with leakage-current function (useful for trend/fault diagnosis) and sets with stakes & leads for 3-/4-wire.
Conclusion & CTA
Your next steps:
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Use the 60-second decision flow above to choose method & measurement point.
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Create a standardized record (measurement series, mean value, photos, sketch, instrument/calibration).
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Select your instrument – clamp for in-operation checks or 3/4-wire for robust proof:
To the product overview (clamps, 3-/4-wire, accessories):
ics-schneider.de – Earth & soil resistance meters