View Latest Blog Entries
Testing & Assessment Certification Standard & Regulation Aging Wires & Systems Maintenance & Sustainment Management Conference & Report Protection & Prevention Research Miscellaneous Arcing
Popular Tags
Visual Inspection High Voltage AS50881 MIL-HDBK MIL-HDBK-525 FAR AS4373 Maintenance Electromagnetic Interference (EMI) FAR 25.1707 Wire System Arcing Damage
All Tags in Alphabetical Order
2021 25.1701 25.1703 abrasion AC 33.4-3 AC 43 Accelerated Aging accessibility ADMT Aging Systems AIR6808 AIR7502 Aircraft Power System aircraft safety Aircraft Service Life Extension Program (SLEP) altitude arc damage Arc Damage Modeling Tool Arc Fault (AF) Arc Fault Circuit Breaker (AFCB) Arc Track Resistance Arcing Arcing Damage AS22759 AS22759/87 AS23053 AS29606 AS4373 AS4373 Method 704 AS50881 AS5692 AS6019 AS6324 AS81824 AS83519 AS85049 AS85485 AS85485 Wire Standard ASTM B355 ASTM B470 ASTM D150 ASTM D2671 ASTM D8355 ASTM D876 ASTM F2639 ASTM F2696 ASTM F2799 ASTM F3230 ASTM F3309 ATSRAC Attenuation Automated Wire Testing System (AWTS) Automotive Avionics backshell batteries bend radius Bent Pin Analysis Best of Lectromec Best Practice bonding Cable Cable Bend cable testing Carbon Nanotube (CNT) Certification cfr 25.1717 Chafing Chemical Testing Circuit Breaker circuit design Circuit Protection cleaning clearance Coaxial cable cold bend collision comparative analysis Compliance Component Selection Condition Based Maintenance Conductor Conductor Testing conductors conduit Connector Connector rating connector selection connector testing connectors contacts Corona Corrosion Corrosion Preventing Compound (CPC) corrosion prevention Cracking creepage D-sub data analysis data cables degradat Degradation Delamination Derating design safety development diagnostic Dielectric breakdown dielectric constant Dimensional Life disinfectant Distributed Power System DO-160 dry arc dynamic cut through E-CFR electric aircraft Electrical Aircraft Electrical Component Electrical Power Electrical Testing Electrified Vehicles Electromagnetic Interference (EMI) Electromagnetic Vulnerability (EMV) Electrostatic Discharge EMC EMF EN2235 EN3197 EN3475 EN6059 End of Service Life End of Year Energy Storage engines Environmental Environmental Cycling environmental stress ethernet eVTOL EWIS certification EWIS Component EWIS Design EWIS Failure EWIS sustainment EWIS Thermal Management EZAP FAA FAA AC 25.27 FAA AC 25.981-1C FAA Meeting failure conditions Failure Database Failure Modes and Effects Analysis (FMEA) FAQs FAR FAR 25.1703 FAR 25.1707 FAR 25.1709 Fault fault tree Fixturing Flammability fleet reliability Flex Testing fluid exposure Fluid Immersion Forced Hydrolysis fuel system fuel tank ignition Functional Hazard Assessment functional testing Fundamental Articles Fuse Future Tech galvanic corrosion Glycol Gold Gold plating Green Taxiing Grounding hand sanitizer handbook Harness Design harness protection hazard Hazard Analysis health monitoring heat shrink heat shrink tubing high current high Frequency high speed data cable High Voltage High Voltage Degradation HIRF History Hot Stamping Humidity Variation HV connector HV system ICAs IEC 60851 IEC60172 IEEE immersion insertion loss Inspection installation installation safety Instructions for Continued Airworthiness insulating material insulating tape Insulation insulation breakdown insulation resistance insulation testing interchangeability IPC-D-620 ISO 17025 Certified Lab ISO 9000 J1673 Kapton Laser Marking life limit life limited parts Life prediction life projection Lightning lightning protection liquid nitrogen lithium battery lunar Magnet wire maintainability Maintenance Maintenance costs Mandrel mean free path measurement mechanical stress Mechanical Testing MECSIP MIL-C-38999 MIL-C-85485 MIL-DTL-17 MIL-DTL-23053E 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-1353 MIL-STD-1560 MIL-STD-1798 MIL-STD-464 MIL-T-7928 MIL-T-7928/5 MIL-T-81490 MIL-W-22759/87 MIL-W-5088 MIL–STD–5088 Military 5088 modeling moon MS3320 NASA NEMA27500 Nickel nickel plating No Fault Found OEM off gassing Outgassing Over current Overheating of Wire Harness Parallel Arcing part selection Partial Discharge partial discharge at altitude Performance physical hazard assessment Physical Testing polyamide polyimdie Polyimide-PTFE Power over Ethernet power system Power systems predictive maintenance Presentation Preventative Maintenance Program Probability of Failure Product Quality PTFE pull through Radiation Red Plague Corrosion Reduction of Hazardous Substances (RoHS) regulations relays Reliability Research Resistance Revision C Rewiring Project Risk Assessment S&T Meeting SAE SAE Committee Sanitizing Fluids Secondary Harness Protection separation Separation Requirements Series Arcing Service Life Extension Severe Wind and Moisture-Prone (SWAMP) Severity of Failure shelf life Shield Shielding Shrinkage signal signal cable Silver silver plated wire silver-plating skin depth skin effect Small aircraft smoke Solid State Circuit Breaker Space Certified Wires Splice standards Storage stored energy superconductor supportability Sustainment System Voltage Temperature Rating Temperature Variation Test methods Test Pricing Testing testing standard Thermal Circuit Breaker Thermal Endurance Thermal Index Thermal Runaway Thermal Shock Thermal Testing tin Tin plated conductors tin plating tin solder tin whiskering tin whiskers top 5 Transient Troubleshooting TWA800 UAVs UL94 USAF validation verification video Visual Inspection voltage voltage differential Voltage Tolerance volume resistivity vw-1 wet arc white paper whitelisting Winding wire Wire Ampacity Wire Bend Wire Certification Wire Comparison wire damage wire failure wire performance wire properties Wire System wire testing Wire Verification wiring components work unit code

Partial Discharge at Altitude

Protection & Prevention

Key Takeaways
  • At higher altitudes, a lower applied voltage is required for partial discharge to occur.
  • The altitude-dependent air density directly affects the mean-free path of air molecules present within an insulation gap.
  • Molecules with a longer mean free path are able to collide at higher energy than those with a short mean free path.


Partial Discharge (PD) is a phenomenon that causes long-term degradation over time within the imperfections of a wire/ cable insulation. A recent Lectromec article provides valuable information about the phenomenon and the impact it can have on aerospace Electrical Wiring Interconnection Systems (EWIS).

PD is not a simple phenomenon to model as there are many environmental factors that have influence. Factors such as heat, humidity, pressure, are some that can change the voltages required to initiate and extinguish PD within a wire/cable’s insulation. This article specifically discusses the influence of altitude on partial discharge events. The consideration of the operational altitude effect on partial discharge is particularly necessary for aerospace applications as aircraft tend to spend a lot of operating time at high altitudes.

Years of partial discharge testing (at Lectromec) have shown that PD inception voltage and extinction voltage (PDIV and PDEV) of any wire/cable’s construction decrease at higher altitudes. But why and how does the altitude affect the partial discharge voltages of wire/ cable?

Paschen's Curve Graph
Paschen’s curve: a graph describing breakdown voltage as a function of pd (p=pressure and d=distance between charged surfaces). Source

What’s Happening Inside the Gap?

As discussed in a previous article, partial discharge occurs when a low current “jumps” the gap between two points with a voltage differential across or through an insulation. These gaps, or voids, are typically air bubbles within the wire’s insulation that occur during wire/cable fabrication. The voltage differential between the two points creates an electric field inside the gap polarizing the air molecules within. Naturally, the electric field strength increases as greater voltage is applied to the wire, further polarizing the air molecules in the gap.

As the electric field grows stronger, the molecules within the insulation gap are more apt to collide and ionize, that is, to become either positively or negatively charged due to the loss or gain of an electron, respectively. The ionized particles are then accelerated by the surrounding electric field, further colliding and ionizing the gas within. Thus, the ionized particles form a conductive path by which current can “jump” across the gap resulting in partial discharge. This mechanism remains the same regardless of altitude, so why are the extinction and inception voltages lower at higher altitudes? The change in PDIV with altitude is accounted for by the air pressure/density and the resulting mean free path of air particles within the gap.

Mean Free Path

Mean free path is a fancy term for the average distance a particle travels before experiencing a collision (i.e., the average distance any air molecule in the insulation gap will travel before hitting another.) As altitude increases, air pressure decreases, meaning that air density is reduced at high altitudes and increased at low altitudes. This is also true of the air present in the gap(s) within a wire’s insulation.

At low altitudes, the higher density of the air within an insulation gap results in more frequent collisions, meaning that the mean free path is shorter; following the same logic, there are fewer collisions at high altitudes where the air density is reduced resulting in a longer mean free path.

Putting it All Together

At first glance, one may infer that more frequent collisions within a gap of greater air density lead to a higher rate of ionization and that a partial discharge inception voltage should be easier to reach at low altitudes and high pressures. In fact, the opposite is true.

The collisions of molecules in a high-pressure (low altitude) environment occur more frequently (because of the shorter mean free path) but due to the high density of air molecules and the shorter mean free path, each collision occurs with relatively low energy. For ionization to occur, the energy of a collision must surpass a minimum threshold so that electrons may be transferred. Without sufficient energy in collisions, the air molecules are unable to ionize and do not create a conductive path across the gap.

At high altitudes (low-pressure), despite fewer collisions, the longer mean free path allows the particles to accelerate over a greater distance and collide with significantly greater kinetic energy. These higher energy collisions are more likely to facilitate ionization. So, in reality, the rate of ionization at higher altitudes is higher than the rate near sea level, thus a lower applied voltage can create an electrical path within the air gap.

A Simple Analogy

A useful way to picture this is to consider a group of 100 blindfolded people trying to run around a gymnasium. Due to the density of people, each person is not able to build up much speed (or in this case, kinetic energy) without bumping into someone else. Because of the low speed of collision, the people are unlikely to lose their balance and fall during a collision.

However, if the number of people in the gymnasium is reduced to 10, the blindfolded runners can accelerate to a higher speed without bumping into one another. The runners will bump into each other less frequently, but when they do, it is with significantly greater energy and a higher likelihood of both falling.

Thus, as the density of people in the gymnasium decreases, the probability of falling increases. Similarly, as the density of air molecules in the insulation gap decreases, the probability of ionization increases. The probably of ionization increases until the the air pressure is sufficiently low to reduce the collision frequency. This is shown in the graph above where low pressure-distance values correspond to very high breakdown voltages.


The mean free path of air molecules in the induced electric field within an insulation gap has a significant impact on the rate of ionization within. Therefore, the voltage required to initiate (and subsequently extinguish) partial discharge within the insulation of a wire/cable decreases with an increase of altitude (to a point). This concept is clearly an important consideration to make when choosing EWIS components for an aircraft as each component must be fully functional at high altitudes where partial discharge is more likely to occur. Contact Lectromec today for help evaluating the partial discharge performance of your wire/ cable construction.

Laura Wishart

Laura Wishart

Engineer, Lectromec

Laura has been with Lectromec since 2019 and has been a key contributor on projects involving testing of EWIS/fuel system failure modes, the impact of poor installation practices on EWIS longevity, and wire/cable certification testing. Her knowledge and attention to detail ensure consistent delivery of accurate test results from Lectromec’s lab.