View Latest Blog Entries
Testing & Assessment Certification Aging Wires & Systems Management Standard & Regulation Conference & Report Research Maintenance & Sustainment Protection & Prevention Arcing Miscellaneous
Popular Tags
Visual Inspection MIL-HDBK MIL-HDBK-525 FAR AS50881 FAR 25.1707 Electromagnetic Interference (EMI) Wire System High Voltage Arcing Damage FAR 25.1709 Degradation
All Tags in Alphabetical Order
25.1703 Accelerated Aging ADMT Aging Systems Aircraft Power System Aircraft Service Life Extension Program (SLEP) arc damage Arc Fault (AF) Arc Fault Circuit Breaker (AFCB) Arc Track Resistance Arcing Damage AS22759/87 AS4373 AS4373 Method 704 AS50881 AS5692 AS6019 AS85485 AS85485 Wire Standard ASTM F2799 ATSRAC Attenuation Automated Wire Testing System (AWTS) Bent Pin Analysis Best of Lectromec Best Practice Cable cable testing Carbon Nanotube (CNT) Certification Chafing Chemical Testing Circuit Breaker Circuit Protection Coaxial cable cold bend comparative analysis Compliance Component Selection Condition Based Maintenance Conductor Connector connectors contacts Corona Corrosion Corrosion Preventing Compound (CPC) Cracking D-sub data analysis data cables degradat Degradation Delamination Derating dielectric constant Distributed Power System DO-160 Electrical Aircraft Electrical Component Electrical Testing Electromagnetic Interference (EMI) Electromagnetic Vulnerability (EMV) EMC EMF EN3197 EN3475 EN6059 End of Service Life End of Year Energy Storage Environmental Environmental Cycling ethernet EWIS Component EWIS Design EWIS Failure EWIS Thermal Management EZAP FAA AC 25.27 FAA AC 25.981-1C Failure Database Failure Modes and Effects Analysis (FMEA) FAQs FAR FAR 25.1703 FAR 25.1707 FAR 25.1709 fault tree Fixturing Flammability fleet reliability Flex Testing Forced Hydrolysis fuel system fuel tank ignition functional testing Fundamental Articles Future Tech Green Taxiing Grounding Harness Design Hazard Analysis health monitoring heat shrink tubing high current high Frequency high speed data cable High Voltage History Hot Stamping Humidity Variation ICAs Instructions for Continued Airworthiness Insulation insulation resistance IPC-D-620 ISO 17025 Certified Lab Laser Marking life limited parts life projection Maintenance Maintenance costs Mandrel Mechanical Testing MECSIP MIL-C-38999 MIL-C-85485 MIL-DTL-17 MIL-DTL-3885G MIL-DTL-38999 MIL-E-25499 MIL-HDBK MIL-HDBK-1646 MIL-HDBK-217 MIL-HDBK-454 MIL-HDBK-516 MIL-HDBK-522 MIL-HDBK-525 MIL-HDBK-683 MIL-STD-1560 MIL-STD-1798 MIL-STD-464 MIL-T-7928 MIL-T-81490 MIL-W-22759/87 MIL-W-5088 Military 5088 modeling MS3320 NASA NEMA27500 No Fault Found off gassing Outgassing Overheating of Wire Harness Parallel Arcing part selection Performance Physical Testing Polyimide-PTFE Power over Ethernet Power systems predictive maintenance Probability of Failure Product Quality Radiation Red Plague Corrosion Reduction of Hazardous Substances (RoHS) relays Reliability Research Rewiring Project Risk Assessment SAE Secondary Harness Protection Separation Requirements Series Arcing Service Life Extension Severe Wind and Moisture-Prone (SWAMP) Severity of Failure Shield Shielding signal cable silver plated wire smoke Solid State Circuit Breaker Space Certified Wires stored energy supportability Sustainment Temperature Rating Temperature Variation Test methods Test Pricing Testing Thermal Circuit Breaker Thermal Endurance Thermal Index Thermal Shock Thermal Testing Tin plated conductors Troubleshooting TWA800 UAVs verification Visual Inspection white paper whitelisting Wire Ampacity Wire Certification Wire Comparison wire failure wire properties Wire System wire testing Wire Verification work unit code

Insulation Resistance: Measuring Leaky Wires

Maintenance & Sustainment Testing & Assessment

Wire insulation integrity is a fundamental part of wire performance. Without it, the safety and reliability of a wire is significantly reduced. Over the years, the industry has developed dozens of insulation integrity assessment methods.

Among the range of tests that exist in the world of wiring, one of the tests that is often misunderstood or misapplied is the insulation resistance test. Although part of the qualification testing for nearly every wire on the market, the insulation resistance test can be misapplied and misunderstood.

This article provides a review of this test, how different values can impact system performance, and what to look for in aging systems.


At a fundamental level, a wire is the combination of a conductive medium protected by a resistive medium. The performance of this resistor, or insulator, vary significantly between different materials, thickness, and the operating conditions. In most applications, it is advantageous for the insulator to have a high resistance; this provides safety for those handling the wires while they are powered. Further, this ensures that any signal or power transmitted down the wire does not take an unintended path such as another wire or conductive target (e.g. structure).

Insulation Resistance Test Animation
Wire insulation is not a perfect insulator. When at high voltage is placed on the conductor, an electrical current will flow through the insulation. The amount of current is dependent on wire construction, materials, existing damage, degradation, humidity, and voltage.

The insulation resistance test provides a means of assessing the resistance of the wire insulation. When performed in a lab setting, the wire is submerged in a water bath with the ends above the water. A high voltage is placed on the conductor and the electrical ground is placed in the water bath. While the setup for the test is straight forward, gathering useful data requires care.

One of the difficulties with performing the insulation resistance (IR) test is that it does require specialized test equipment and a noticeable length of wire. To give an example, the AS4373 test method suggests the use of a wire that is at least 26 feet long, and there is a reason for this: modern wire insulation types are very good resistors.

The way to determine the resistance of a component requires one of two methods: comparative voltage drop or precision electrical current measurements. The difficulty with performing the comparative voltage drop measurements is that most voltmeters using internal 10 megaohms resistor for measurements. Measurements performed on resistors above 10MOhm are inaccurate.

To address this, the method typically employed requires a precision electrical current measurement or a pico ammeter. In this configuration, the leakage current through the insulator is directly measured. Given that most wire types have an insulation resistance in the giga ohms per thousand feet, the electrical current flow through the insulation, even with 100ft of wire, is often measured in nano amps.

Proper Power Supply

To perform the IR test correctly, a DC power supply must be used. The DC power supply is preferred because this avoids repeated charge and discharge of the insulation. With the in-lab test setup, the insulation acts as a capacitor dielectric. If the power supply does not deliver clean, ripple free power, an inconsistent and unreliable insulation resistance measurement will be made.

Implications for High Voltage

It is important to note that insulation resistance measurements do not provide any indication of an insulation’s high-voltage performance. High resistance insulation types can still have relatively low partial discharge inception voltages. Other tests are better for determining high voltage performance and longevity.

Performing IR in the Field

The insulation resistance of wires does reduce with age. This may be from electrical stresses on the insulation, elevated temperature exposure causing polymer degradation, thermal cycling to introduce cracks, mechanical damage, or variety of other sources of degradation. For some wire types, the insulation resistance can be used as an indicator of wire health; certainly, those wires that had a significant reduction (i.e. 90%) should immediately be considered for replacement. However, an insulation resistance degradation does not directly mean that the wire should be replaced. Numerous studies have found that in application, insulation resistance is just one value that should be considered.

Best Aircraft Wire Test Lab

As an example, those using harness testers on aircraft will often find different (low) insulation resistance values during humid mornings than during dry afternoons. Also, temperature has an important role in IR. Some insulations will have a 50% decrease in IR with a 10oC temperature increase. Because of this variability, it is important that comparative testing or health monitoring assessments be performed under similar conditions; failure to do so can lead to incorrect conclusions.


The insulation resistance test is a great means of evaluating a wire/cable performance and insulation integrity. It is also important to know that how the test is performed is just as important as the results themselves; without a clear understanding of the equipment, voltages, and system to be evaluated, the results can be meaningless.

To get the most out of your wire/cable assessment tests, contact Lectromec.

Michael Traskos

Michael Traskos

President, Lectromec

Michael has been involved in wire degradation and failure assessments for more than a decade. He has worked on dozens of projects assessing the reliability and qualification of EWIS components. Michael is an FAA DER with a delegated authority covering EWIS certification and the chairman of the SAE AE-8A EWIS installation committee.