View Latest Blog Entries
Testing & Assessment Certification Aging Wires & Systems Management Standard & Regulation Research Protection & Prevention Conference & Report Maintenance & Sustainment Arcing Miscellaneous
Popular Tags
Visual Inspection MIL-HDBK MIL-HDBK-525 FAR AS50881 FAR 25.1707 Electromagnetic Interference (EMI) Wire System High Voltage FAR 25.1709 Degradation Circuit Protection
All Tags in Alphabetical Order
25.1703 Accelerated Aging Aging Systems Aircraft Power System Aircraft Service Life Extension Program (SLEP) 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 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 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 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 speed data cable High Voltage Hot Stamping Humidity Variation 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-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 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 properties Wire System wire testing Wire Verification work unit code

Circuit Protection Selection Guidance


Circuit protection devices have existed since 1864; one would think that selection of circuit protection would be a straight forward task. However, it is not. NASA developed a seven-step process for circuit protection selection and both EN3179 and AS50881 provide some guidance.

In a past article, Lectromec began to consider the differences between two major EWIS standards in aerospace; AS50881 and EN3197. This article reviews these standards and how they address circuit protection selection.


EN3197 contains good guidance on selection criteria and principles to achieve a safe and properly functioning system.


The following are important criteria to live by when selecting the correct part for circuit protection:

  • The circuit protection chosen should prevent overload of the most sensitive Electrical Wiring Interconnection System (EWIS) component within the circuitry.
  • The circuit protection should protect the EWIS rather than the equipment, the equipment should be protected separately.
  • Any protection device must be “trip free” so that there is no override facility that allows a circuit overload to be maintained.
  • The part should be “fail safe” so that hidden defects do not affect performance.

System Level Guidance

For a circuit protection device to work effectively, it must trip prior to reaching the maximum component current rating. For example, if it is determined that the maximum current allowed on a cable for a given time duration is given by the curve in red in the figure, then the circuit breaker tripping curve should be at a lower current threshold everywhere along the curve. The most sensitive (lowest maximum current rating) EWIS component in the system should set the expected circuit protection performance. To do this analysis, one needs to determine the wire’s/cable’s current carrying capacity that can be carried through different wire and cable harness designs.

Circuit breaker trip curves in relation to maximum cable carrying capacity.

Specific Failure Mode Guidance

There are several failure modes defined and considered in EN3197 whose effect on the system may be mitigated by appropriate circuit protection. Possible failure phenomena are defined in EN3197 are shown in the table below. Mitigation of many of these failure modes are handled by wire selection and wires are required to be specifically tested. The test methods generally call for thermal breaker type circuit protection, but other circuit protection technologies are acceptable [link].

Circuit protection selection and wire selection should go hand in hand to mitigate risk. System level testing is invaluable in determining the system failure event behavior. The test methods associated on the arcing phenomena are generally focused on wire selection and not on circuit protection selection; this could be an area of improvement in both standards. In addition to aiding in circuit protection selection, the test standard could be established to certify performance.

Failure Mode

Relation to Circuit Protection

Short Circuit

Circuit protection should trip in the event of a hard short.

Wet Short Circuit:

Wire selection and testing is used generally to mitigate the effect of this type of failure in the circuit. Thermal breakers are used as the circuit protection device.

Wet Arc Tracking:

If a breach in the insulation is near a breach on an adjacent line, moisture can be a means of conduction between the lines. This can occur for long periods of time without tripping the circuit protection and can also develop into a more violent arcing scenario.

Dry Arc Tracking

This can be caused by vibration of an exposed wire conductor with a conducting structure. It may not trip a conventional circuit breaker or may take a long time to trip causing significant damage.

Circuit Protection Selection Guidance

The EN3197 test method also provides guidance on many of the possible circuit protection technologies that may be used in the system. These include the following: circuit breakers, arc fault circuit interrupters, arc fault circuit breakers, remote control circuit breakers, solid state circuit breakers, residual current circuit breakers, and fuses [described here link].

These technologies have been developed to mitigate varying failure possibilities in the system. Arc fault circuit breakers, for example, can significantly decrease the duration of an electric arc compared to conventional circuit breakers thereby greatly limiting the arc damage risk [link].

AS50881 Circuit Protection Guidance

In contrast, AS50881 has sparse guidance on circuit protection that may be summed up as follows:

  • Wires shall be sized so that they have capacity which is equal or greater than the circuit protection rating under all operating conditions.
  • Circuits shall be powered with a single operating current interrupt system. Circuit protection shall not be paralleled or ganged together to achieve a specific current rating.

AS50881 could benefit substantially from guidance found in EN3197.


Modern circuit protection technologies can provide considerable benefits including mitigation of potential failure risk. Those seeking guidance on the best practices in selection of circuit protection should consider reviewing the applicable sections in the EN 3197 standard. Further, the AS50881 could benefit from incorporation of guidance included in the European standard.

Tristan Epp Schmidt

Tristan Epp Schmidt

Engineer, Lectromec

Since starting at Lectromec in early 2015, Tristan has been key in many of test and assessment wire systems assessment projects wire systems assessment. His attention to detail has lead to several key insights in Lectromec’s research initiatives.