AGL-CS-020 — Different IR Results From Two Laboratories
The AGL Knowledge Hub | Engineering Case Study Series. A diagnostic interpretation case where two independent laboratories produced conflicting IR results for the same AGL transformer — from test methodology review through time-dependent analysis to standardized procedures.
Important: This is an engineering simulation created for training. It is not presented as an actual airport incident. All readings, values, dates and outcomes are illustrative, and no airport, organisation, person or manufacturer is identified or implied.
1Case Study Overview
An airport authority submitted a suspected defective AGL isolating transformer and associated cable sample to two independent electrical testing laboratories for insulation resistance evaluation.
The results were unexpectedly different.
- Laboratory A: Greater than 1000 MΩ
- Laboratory B: 95 MΩ
Both laboratories used calibrated insulation resistance testers, and both reports were technically valid.
The maintenance team questioned whether one laboratory had made an error or whether the equipment condition had changed.
A detailed investigation revealed that test methodology, temperature, humidity, stabilization time, and measurement procedures significantly influenced the reported IR values.
This case demonstrates that different IR results do not necessarily mean that one laboratory is incorrect.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand why different laboratories may obtain different IR results.
- Evaluate laboratory test methodology.
- Interpret temperature and humidity effects.
- Understand polarization and absorption behavior.
- Compare short-duration and stabilized IR measurements.
- Assess the reliability of laboratory reports.
- Make engineering decisions using multiple test results.
3System Background
Insulation resistance testing evaluates the dielectric integrity of:
- AGL primary cables.
- Cable joints.
- Isolating transformers.
- Secondary circuits.
- Insulation systems.
The measured value is affected by:
- Test voltage.
- Temperature.
- Humidity.
- Surface contamination.
- Moisture content.
- Test duration.
- Instrument characteristics.
- Sample conditioning.
Therefore, IR is not a perfectly fixed value.
4Problem Statement
The reported laboratory results were:
| Laboratory A | Greater than 1000 MΩ |
| Laboratory B | 95 MΩ |
Both reports indicated:
- Calibrated instruments.
- Standard test procedures.
- No obvious measurement error.
How can the same AGL equipment produce two substantially different insulation resistance values?
5Initial Observations
The equipment under investigation was:
- One 6.6 A isolating transformer.
- One associated cable section.
- Removed from service after monsoon-related maintenance.
The laboratories tested the equipment one week apart.
6Initial Engineering Hypotheses
The maintenance team considered:
One laboratory instrument was inaccurate.
The equipment dried between tests.
Different test voltages were used.
Different test durations were used.
Temperature and humidity influenced the measurements.
7Review of Laboratory Reports
Laboratory A
- Test voltage: 5 kV
- Temperature: 32°C
- Relative humidity: 38%
- Stabilization time: 10 minutes
- Sample dried before testing
Result: Greater than 1000 MΩ
Laboratory B
- Test voltage: 5 kV
- Temperature: 21°C
- Relative humidity: 72%
- Stabilization time: 1 minute
- Sample tested immediately after unpacking
Result: 95 MΩ
8Why IR Values Can Differ
Insulation resistance is affected by several components:
A short-duration measurement may differ substantially from a stabilized measurement.
9Test Equipment
The investigation reviewed:
- Laboratory calibration certificates.
- Test procedures.
- Environmental conditions.
- Sample preparation records.
- Test duration records.
- Instrument specifications.
10Investigation
Step 1: Instrument Verification
Both laboratories had:
- Valid calibration certificates.
- Appropriate test voltage.
- Acceptable instrument accuracy.
Instrument error was unlikely.
Step 2: Temperature Analysis
Insulation resistance generally changes with temperature.
The transformer tested at:
- 21°C
- 32°C
Temperature correction alone could not explain the entire difference.
Step 3: Humidity Analysis
Laboratory B tested the sample at 72% relative humidity.
Surface leakage was significantly higher.
Moisture on insulation surfaces reduced the measured IR.
Step 4: Stabilization Time
This became the most important finding.
Laboratory B recorded the value after 1 minute.
Laboratory A recorded the value after 10 minutes.
The transformer exhibited dielectric absorption and polarization.
The measured resistance increased significantly with time.
Step 5: Repeat Controlled Testing
The same transformer was retested under identical conditions.
Results:
| 30 seconds | 82 MΩ |
| 1 minute | 95 MΩ |
| 5 minutes | 420 MΩ |
| 10 minutes | Greater than 1000 MΩ |
The transformer was not changing rapidly. The measurement was time-dependent.
11Root Cause
The different laboratory results were caused primarily by:
- Different stabilization times.
- Different humidity conditions.
- Different sample conditioning procedures.
Both laboratories produced technically valid measurements, but they were measuring different stages of insulation polarization behavior.
12Engineering Interpretation
Laboratory A represented:
- Dried insulation.
- Stabilized measurement.
- Maximum insulation performance.
Laboratory B represented:
- Field-like moisture condition.
- Early measurement.
- Initial leakage behavior.
For maintenance decision-making, both results were valuable.
13Corrective Action
The engineering team:
- Standardized the IR test procedure.
- Defined a minimum stabilization time.
- Required temperature recording.
- Required humidity recording.
- Implemented temperature correction where appropriate.
- Introduced PI (Polarization Index) testing.
- Established laboratory comparison procedures.
- Updated transformer acceptance criteria.
- Trained maintenance staff on interpreting time-based IR trends rather than single-point readings.
14Frequently Asked Questions
Quick answers about conflicting insulation resistance results between laboratories, based on this case study.
Q1What is AGL-CS-020?
AGL-CS-020 is an engineering training case study on why two independent laboratories produced conflicting insulation resistance results for the same AGL transformer and cable sample. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What were the two laboratory results?
Laboratory A reported greater than 1000 MΩ and Laboratory B reported 95 MΩ. Both used calibrated insulation resistance testers, both reports were technically valid, and the tests were performed one week apart.
Q3Was one of the laboratories wrong?
No. Instrument error was unlikely, since both laboratories had valid calibration certificates, appropriate test voltage and acceptable accuracy. The results differed because of test conditions and methodology.
Q4What caused the different results?
The main causes were different stabilization times, different humidity conditions and different sample conditioning procedures. The laboratories were measuring different stages of insulation polarization behaviour.
Q5Why did stabilization time matter so much?
Laboratory B recorded its value after 1 minute and Laboratory A after 10 minutes. Because the transformer showed dielectric absorption and polarization, measured resistance increased significantly with time.
Q6Did temperature and humidity play a part?
Yes. Insulation resistance changes with temperature, and the samples were tested at 21°C and 32°C, but temperature correction alone could not explain the gap. Laboratory B tested at 72% relative humidity, where higher surface leakage reduced the measured IR.
Q7What corrective actions did the engineering team take?
They standardized the IR test procedure, defined a minimum stabilization time and required temperature and humidity recording. They also applied temperature correction where appropriate, introduced Polarization Index (PI) testing, set up laboratory comparison procedures, updated transformer acceptance criteria and trained staff to read time-based IR trends.
Q8What is the key lesson for maintenance decisions?
A conflicting insulation resistance reading is not automatically a faulty test. Check that stabilization time, temperature and humidity were controlled and recorded consistently, and read IR trends, including Polarization Index, rather than a single snapshot value.
Engineering & Expert Notes
Standardise Test Duration and Reading Time
MethodDifferent stabilisation times give different values. Define the time at which the reading is taken and record it.
Control Humidity and Sample Conditioning
ConditionsMoisture content changes insulation behaviour. Agree conditioning procedures before comparing laboratories.
Align the Laboratories Before Judging the Result
CoordinationDifferences between labs usually come from method. Compare procedures and instruments first.
Report Method With the Number
ReportingA figure without its method is hard to interpret. Include voltage, time, temperature and instrument details.
15Case Study Conclusion
A conflicting insulation resistance reading is not automatically a faulty test. Before assuming an instrument or laboratory error, check whether stabilization time, temperature, and humidity were controlled and recorded consistently between the two measurements.
Standardizing test procedure and duration, and reading IR trends (including Polarization Index) rather than a single snapshot value, prevents unnecessary transformer replacement and improves confidence in insulation diagnostics.
Turn Case Studies Into Engineering Capability
A case study shows you how a problem is investigated. Precision AGL Academy helps you build the knowledge to understand why it happens, diagnose it systematically, and apply the right engineering approach.
Study real-world-style engineering scenarios, identify failure patterns, understand root causes, and develop a systematic approach to AGL troubleshooting.
Study the case. Understand the system. Build the skill.
Follow the Academy
Important Notice, Disclaimer & Limitation of Liability
Please read before relying on any content on this page or on any record downloaded from it.
- Purpose and intended users. This page is an independent educational and informational reference for competent, suitably trained and authorised airfield ground lighting (AGL) and aerodrome electrical personnel. It is not professional engineering, legal or regulatory advice, and using it does not create any professional or client relationship.
- Fictional training simulation. This case study is an engineering simulation created for training. It is not a report of an actual airport incident. Scenarios, sequences, dates, readings, measurements, outcomes and numerical values are illustrative and invented for teaching. No fault, failure, cause or liability is attributed to any person, company, airport, regulator, manufacturer or product, and any resemblance to a real event is coincidental. The content is not a forensic, legal, insurance, regulatory or accident-investigation finding and must not be cited as evidence, as a record of an event, or as a basis for disciplinary, contractual, insurance or legal action.
- Safety. Work on series circuits, constant current regulators (CCRs), isolating transformers, cables, connectors and control equipment can be fatal. Always apply the aerodrome’s permit-to-work and isolation/LOTO procedures, prove dead, use correctly rated instruments and PPE, and follow the manufacturer’s instructions. Nothing here authorises work on live equipment or the bypassing of any safety device or interlock. Where a case narrates an inspection, measurement or test, that narrative is not an instruction or an endorsement of the method.
- Values, test results and acceptance. Test voltages, insulation-resistance and current values, tolerances, thresholds, photometric and angular figures and pass/fail or “acceptable” wording in a case are specific to its invented scenario. They are not acceptance criteria, limits or targets. Use the project specification, the applicable standard and the manufacturer’s data for real work. Completing or following a case produces no test certificate, calibration record, acceptance or evidence of compliance.
- Aviation operations. Nothing here authorises return to service, a change of operating category (CAT I/II/III), a NOTAM, a flight inspection outcome or any aerodrome certification decision. These rest with the aerodrome operator, the air navigation service provider and the competent regulator.
- Standards and regulations. References to ICAO, FAA, IEC, IEEE, ISO and other standards are for orientation only. They are summarised, may be incomplete or out of date, and standards are revised from time to time. Consult the current official editions. Following this content does not guarantee compliance with any law, standard or regulatory approval. Official documents, regulations and manufacturers’ instructions always prevail.
- Learning aids. Knowledge checks, decision points, interview questions, FAQs and suggested answers are teaching aids with illustrative answers. They are not an assessment, certification, accreditation or competency or authorisation to work on any aerodrome, and they do not predict or guarantee the outcome of any job interview or application.
- Generic content; no warranty. Diagnostic sequences, cause rankings, tolerances and values are generic and indicative; real installations differ in design, equipment make and model, age and condition. Content is provided “as is” and “as available”, without warranty of any kind, express or implied, including accuracy, completeness, currency or fitness for a particular purpose.
- Limitation of liability. To the fullest extent permitted by applicable law, Precision AGL Academy and its owners, authors and contributors are not liable for any injury, death, loss, damage, downtime, penalty or regulatory consequence, whether direct or indirect, arising from the use of, reliance on, or inability to use this content. Nothing in this notice excludes or limits any liability that cannot lawfully be excluded or limited.
- Independence and trademarks. This is an independent publication, not affiliated with or endorsed by ICAO, the FAA, the IEC, DGCA, any airport, regulator or equipment manufacturer. Third-party names, standard designations and trademarks belong to their respective owners and are used for identification only.
- Copyright and use. © 2026 Precision AGL Academy. All rights reserved. You may read this page and use it for personal reference and internal training, keeping this notice attached. Any other copying, republication, redistribution, scraping or commercial use requires our prior written permission.
- Corrections. Content is reviewed periodically, with no obligation to update. To report an error, safety concern or rights issue, write to precisionaglsolutions@gmail.com.