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PIS-CS-001/Measurement

Soil resistivity & rod spacing

Never Trust One Soil Resistivity Test.

New CairoHealthcare3 min read
Survey zone, installed soil and electrode interactionEngineered approach for PIS-CS-001. Characterise the actual soil around the installed electrodes. Coordinate spacing using the site soil model and electrode geometry. Validate probe geometry and repeatability before accepting the reading. Conceptual teaching detail, not an as-built drawing or construction instruction. © PIS Engineering Academy · All rights reserved.PIS ENGINEERING / CASE 001Survey zone, installed soil and electrode interactionENGINEERED APPROACHSURVEYED SOILBACKFILLED INSTALLATIONSOIL SURVEYSOIL RESISTIVITYC1P1P2C2COORDINATED SPACINGSoil changes across the site. Rods interact below ground.1231Characterise the actual soilaround the installedelectrodes.2Coordinate spacing using thesite soil model and electrodegeometry.3Validate probe geometry andrepeatability beforeaccepting the reading.READ THE DETAILEquipment, joints and current-path interfaces are shown in context.Conceptual teaching plate · no project dimensions or acceptance limits impliedPIS / REV 03© PIS Engineering Academy · All rights reserved

Engineering concept / vector field note

01

The field problem

Hospital - New Cairo: Soil Resistivity, Rod Spacing & Unstable Measurements

project / field problem

Hospital - New Cairo. The grounding design relied on soil-resistivity measurements taken in one part of the project, while the actual earthing installation was located in another area that had undergone backfilling/replacement. The measured soil therefore did not necessarily represent the soil surrounding the installed grounding system.

root cause 1 - non-representative soil resistivity

A soil resistivity report is valid for the investigated location and conditions. In large projects, backfilling and soil replacement can materially change the electrical characteristics of the ground between one area and another.

02

Why it failed

Root Cause 1 - Non-Representative Soil Resistivity

A soil resistivity report is valid for the investigated location and conditions. In large projects, backfilling and soil replacement can materially change the electrical characteristics of the ground between one area and another.

Root Cause 2 - Ground-Rod Spacing

Ground rods were installed too close to one another, causing overlapping resistance areas and mutual resistance effects. The additional rods therefore provided much less benefit than expected.

Root Cause 3 - Unstable Earth-Resistance Measurements

Measurements reportedly varied from about 0.9 ohm to 3.7 ohm even after changing the test instrument. The supplied case identifies possible contributors including mutual coupling, insufficient Fall-of-Potential test distance, nearby metallic networks, parallel metallic paths and non-uniform soil/current distribution.

03

Follow the engineering

Switch between the field condition and the engineering concept. Trace the path and examine what changes.

Inspect the engineering detail

Survey zone, installed soil and electrode interactionEngineered approach for PIS-CS-001. Characterise the actual soil around the installed electrodes. Coordinate spacing using the site soil model and electrode geometry. Validate probe geometry and repeatability before accepting the reading. Conceptual teaching detail, not an as-built drawing or construction instruction. © PIS Engineering Academy · All rights reserved.PIS ENGINEERING / CASE 001Survey zone, installed soil and electrode interactionENGINEERED APPROACHSURVEYED SOILBACKFILLED INSTALLATIONSOIL SURVEYSOIL RESISTIVITYC1P1P2C2COORDINATED SPACINGSoil changes across the site. Rods interact below ground.1231Characterise the actual soilaround the installedelectrodes.2Coordinate spacing using thesite soil model and electrodegeometry.3Validate probe geometry andrepeatability beforeaccepting the reading.READ THE DETAILEquipment, joints and current-path interfaces are shown in context.Conceptual teaching plate · no project dimensions or acceptance limits impliedPIS / REV 03© PIS Engineering Academy · All rights reserved

Case 001 · Engineered approach

100%
Survey zone, installed soil and electrode interactionEngineered approach for PIS-CS-001. Characterise the actual soil around the installed electrodes. Coordinate spacing using the site soil model and electrode geometry. Validate probe geometry and repeatability before accepting the reading. Conceptual teaching detail, not an as-built drawing or construction instruction. © PIS Engineering Academy · All rights reserved.PIS ENGINEERING / CASE 001Survey zone, installed soil and electrode interactionENGINEERED APPROACHSURVEYED SOILBACKFILLED INSTALLATIONSOIL SURVEYSOIL RESISTIVITYC1P1P2C2COORDINATED SPACINGSoil changes across the site. Rods interact below ground.1231Characterise the actual soilaround the installedelectrodes.2Coordinate spacing using thesite soil model and electrodegeometry.3Validate probe geometry andrepeatability beforeaccepting the reading.READ THE DETAILEquipment, joints and current-path interfaces are shown in context.Conceptual teaching plate · no project dimensions or acceptance limits impliedPIS / REV 03© PIS Engineering Academy · All rights reserved

Scroll to inspect. Numbered details match the notes below the drawing. Conceptual geometry; use the case text for the engineering requirements.

What changes

Characterize the actual installation zone, account for backfill/soil changes, coordinate electrode spacing, and validate the measurement method and test geometry.

Teaching schematic · not to scale© PIS Engineering Academy · All rights reserved
04

Correction & verification

Ground rods were installed too close to one another, causing overlapping resistance areas and mutual resistance effects. The additional rods therefore provided much less benefit than expected.

Measurements reportedly varied from about 0.9 ohm to 3.7 ohm even after changing the test instrument. The supplied case identifies possible contributors including mutual coupling, insufficient Fall-of-Potential test distance, nearby metallic networks, parallel metallic paths and non-uniform soil/current distribution.

Verification recorded in the case

  • Record as-built topology and connection points.
  • Repeat the measurement or functional test under a defined condition.
  • Confirm continuity and mechanical integrity of the corrected path.
  • Document instrument/setup, environmental condition and test method.
  • Close the case only when evidence supports the conclusion.
References & project context

References are reproduced from the PIS casebook. Select the governing edition and project acceptance criteria before using them for a design. The diagrams explain the concept, rather than define construction dimensions.

Apply the contract-specified IEC/IEEE/NFPA/local framework relevant to the actual project scope. Verify exact editions and clauses before external publication.

PIS reviewed field handbook · PIS-CS-001

View the original social card
05

The engineering lesson

A soil resistivity report is valid only for the investigated location - not necessarily for the entire project.

Rod spacing can matter more than simply increasing rod quantity.

Changing the tester does not solve a measurement problem caused by the test arrangement or surrounding network.

Take the lesson into your next design review.

Keep exploring

Connected lessons.

All cases
Fall-of-Potential test geometryEngineered approach for PIS-CS-004. Extend the line outside the relevant influence zone. Move the potential probe around the justified test position. Look for a defensible stable region; extend or change method if absent. Conceptual teaching detail, not an as-built drawing or construction instruction. © PIS Engineering Academy · All rights reserved.PIS ENGINEERING / CASE 004Fall-of-Potential test geometryENGINEERED APPROACHVERIFYEARTH TESTEREPCPCEXTEND BEYOND INFLUENCEConfirm the curve around the justified potential-probe position.SCHEMATIC TEST CURVE1231Extend the line outside therelevant influence zone.2Move the potential probearound the justified testposition.3Look for a defensible stableregion; extend or changemethod if absent.READ THE DETAILEquipment, joints and current-path interfaces are shown in context.Conceptual teaching plate · no project dimensions or acceptance limits impliedPIS / REV 03© PIS Engineering Academy · All rights reserved
04

Measurement

The grid was blamed. The test was wrong.

The earth grid was blamed. The test geometry was wrong.

Upper Egypt3 min
A rock mass interrupts the test circuitEngineered approach for PIS-CS-005. Change test direction and place auxiliary probes in suitable ground. Increase separation as required to establish the injection circuit. Confirm injection and repeatability before interpreting resistance. Conceptual teaching detail, not an as-built drawing or construction instruction. © PIS Engineering Academy · All rights reserved.PIS ENGINEERING / CASE 005A rock mass interrupts the test circuitENGINEERED APPROACHVERIFYEARTH TESTEREPCPCBURIED ROCK MASSChange the test direction to establish an injection circuit.1231Change test direction andplace auxiliary probes insuitable ground.2Increase separation asrequired to establish theinjection circuit.3Confirm injection andrepeatability beforeinterpreting resistance.READ THE DETAILEquipment, joints and current-path interfaces are shown in context.Conceptual teaching plate · no project dimensions or acceptance limits impliedPIS / REV 03© PIS Engineering Academy · All rights reserved
05

Measurement

An open loop in rocky ground

OPEN LOOP did not mean an open earthing system.

New Minya3 min
Solar trackers, geology and the measurement objectiveEngineered approach for PIS-CS-006. Profile soil depth and select electrode arrangements from evidence. Use the test method appropriate to isolated or interconnected topology. Verify tracker bonding and correlate measurements with control behaviour. Conceptual teaching detail, not an as-built drawing or construction instruction. © PIS Engineering Academy · All rights reserved.PIS ENGINEERING / CASE 006Solar trackers, geology and the measurement objectiveENGINEERED APPROACHTRACKERARRAY STRUCTURE / TRACKER DRIVE / CONTROL INTERFACESSoil model, test topology and control behaviour are separate checks.1231Profile soil depth and selectelectrode arrangements fromevidence.2Use the test methodappropriate to isolated orinterconnected topology.3Verify tracker bonding andcorrelate measurements withcontrol behaviour.READ THE DETAILEquipment, joints and current-path interfaces are shown in context.Conceptual teaching plate · no project dimensions or acceptance limits impliedPIS / REV 03© PIS Engineering Academy · All rights reserved
06

Measurement

Solar earthing & tracker control

One test method cannot describe every part of a solar plant.

Benban, Aswan3 min
Discuss a similar challenge Next field note: 02