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PIS-CS-007/Measurement

A low reading can hide a poor earth

The meter showed 1 Ω. The isolated grid was actually far worse.

ObourInfrastructure3 min read
The meter sees every connected parallel pathEngineered approach for PIS-CS-007. Define the test objective and safely isolate only when appropriate. Test the standalone electrode with a suitable independent arrangement. Record topology; label interconnected results as system measurements. Conceptual teaching detail, not an as-built drawing or construction instruction. © PIS Engineering Academy · All rights reserved.PIS ENGINEERING / CASE 007The meter sees every connected parallel pathENGINEERED APPROACHISOLATEDEARTH TESTEREPCSCREENPE / NREMOTESafe isolation is a test procedure, not an operating arrangement.1231Define the test objective andsafely isolate only whenappropriate.2Test the standalone electrodewith a suitable independentarrangement.3Record topology; labelinterconnected results assystem measurements.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

Distribution Network Audit — Obour

Earth resistance was being measured while the main earthing bar remained connected to equipment, cable screens, neutrals and other earth paths.

why the obvious answer can be misleading

The tester was seeing multiple parallel return paths. The low reading therefore represented the interconnected installation as seen from the test point, not the standalone electrode/grid performance required by the audit.

02

Why it failed

The tester was seeing multiple parallel return paths.

The low reading therefore represented the interconnected installation as seen from the test point, not the standalone electrode/grid performance required by the audit.

03

Follow the engineering

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

Inspect the engineering detail

The meter sees every connected parallel pathEngineered approach for PIS-CS-007. Define the test objective and safely isolate only when appropriate. Test the standalone electrode with a suitable independent arrangement. Record topology; label interconnected results as system measurements. Conceptual teaching detail, not an as-built drawing or construction instruction. © PIS Engineering Academy · All rights reserved.PIS ENGINEERING / CASE 007The meter sees every connected parallel pathENGINEERED APPROACHISOLATEDEARTH TESTEREPCSCREENPE / NREMOTESafe isolation is a test procedure, not an operating arrangement.1231Define the test objective andsafely isolate only whenappropriate.2Test the standalone electrodewith a suitable independentarrangement.3Record topology; labelinterconnected results assystem measurements.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 007 · Engineered approach

100%
The meter sees every connected parallel pathEngineered approach for PIS-CS-007. Define the test objective and safely isolate only when appropriate. Test the standalone electrode with a suitable independent arrangement. Record topology; label interconnected results as system measurements. Conceptual teaching detail, not an as-built drawing or construction instruction. © PIS Engineering Academy · All rights reserved.PIS ENGINEERING / CASE 007The meter sees every connected parallel pathENGINEERED APPROACHISOLATEDEARTH TESTEREPCSCREENPE / NREMOTESafe isolation is a test procedure, not an operating arrangement.1231Define the test objective andsafely isolate only whenappropriate.2Test the standalone electrodewith a suitable independentarrangement.3Record topology; labelinterconnected results assystem measurements.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

Define the measurement objective first.

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

Correction & verification

Define the measurement objective first.

If standalone electrode performance is required, isolate it safely and test it independently.

If operational system performance is required, document all parallel paths and do not label the result as individual electrode resistance.

Verification recorded in the case

  • Compare isolated and interconnected readings, document the topology and verify the corrected system after improvements.
  • 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 NCR/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.

IEC 60364-6 verification; IEC 61557-5.

PIS reviewed field handbook · PIS-CS-007

View the original social card
05

The engineering lesson

A low number can be the most misleading result on the test sheet.

Take the lesson into your next design review.

Keep exploring

Connected lessons.

All cases
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
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
01

Measurement

Soil resistivity & rod spacing

Never Trust One Soil Resistivity Test.

New Cairo3 min
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
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