Improvement of the Wire Rating Standards based on TV Testing and Thermal Modeling

Due to an increasing complexity, mass and volume of electrical harnesses in aerospace systems, wiring harnesses can be considered as a subsystem in itself that requires dedicated analyses and optimization. Since the 50s, harness design is driven by (de)rating rules based on the current of a free wire in vacuum at its maximum temperature, an approach that is over-conservative for modern spacecraft’s. The ESA, NASA and JAXA standards are derived on the single wire current as defined in MIL- STD-975L, however the environment (40°C or 70°C) and maximum wire temperature (120°C or 200°C) are ambiguously defined in the various standards. This situation leads to different harness sizing and weights in Europe, the US and Japan for the same electrical functionality. For this reason ESA initiated a research project to conduct (de)rating tests on space wires and bundles which are now-a-days applied in the European Aerospace industry. The aim of the ESA project, which has been led by Airbus Defense & Space with the Netherlands Aerospace Centre as subcontractor, is to recommend improvements of the ECSS derating standard (ECSS-Q-ST-30-11C) and to encourage the use of validated thermal models for space harness design optimization. Single wire rating and bundle derating tests were conducted and the results were evaluated. This paper highlights the single wire test results by comparison with thermal analysis and proposes a refinement of the current rating standards using a validated formula rather than graphs or tables. The new formulation substantiates the current rating of single wires by taking out margins while addressing a larger range of environmental conditions and wire types. A 5 to 25% increase of the rated currents can be proposed. Additional improvements of the derating standards are expected after evaluation of the bundle tests results which will be reported in another paper.