There are several reasons to deploy secondary harness protection (SHP): chafe protection, maintenance protection, fire protection, harness cable management and, in some cases, arc protection. The EN6059-502 test examines secondary harness protections and the potential damage from arcing. This article runs through the test method and problems that could occur if the results are misinterpreted.
The EN6059-502 test starts with placing the sample secondary harness protection around a target wire harness. The protection is then closed by appropriate means (lacing tape or equivalent). A second wire harness that has a common configuration with the EN3475 wet arc short circuit test method is referred to as the aggressive wire bundle. The aggressive wire bundle is attached to the outside of the SHP; this is the group that initiates the acting event. The samples are placed into the test jig and, after the test power is applied, saline drops hit the active arcing harness until an arcing event occurs.
The EN6059-502 test method provides the parameters on how to perform on a variety of aggressive bundle conditions. If the SHP is thin, then it may only be rated to be tested with 20AWG wires and 7.5A thermal circuit breakers. Thicker or more stout protection types can be evaluated with heavier gauge wire. It is important to understand this distinction and arc damage resistance rating when selecting SHPs.
Pass Fail Criteria
The pass/fail criteria for SHP are specification/product dependent and include limitations on the SHP protected wire harness damage. For the interior SHP wires, superficial damage is permitted but breaches to the wire conductor are not. The presence of wire insulation breaches are determined by a posttest dielectric voltage withstand test.
How Results Can be Misinterpreted
The EN6059-502 SHP resistance to electrical arcs test examines what can happen when an arc occurs outside the harness protection, not inside. It is important to understand the differences that can occur when the arc is inside of secondary protection. When the arcing wire harness is placed on the outside of the secondary harness protection, the energy from the arcing event can escape and may not impact the secondary protection. Assuming an equal energy distribution from the electrical arc, it is likely that the secondary harness protection is exposed to less than 20% of the total arc plume energy. Of the total arc plume energy, most is deflected and not readily absorbed by the SHP. Under these conditions, it is possible for low quality materials to pass this test under low energy conditions.
If an arcing event were to occur inside of a wire harness protected by SHP (possible by crush damage or by insulation degradation), the energy is contained within the SHP without a means for escape. This trapped energy can cause damage to more wires in the harness or create a hole in the SHP creating a jet of arc energy. This means that a SHP that successfully passed a test with an arcing wire on the outside may not pass when the same arcing event occurs inside the SHP.
Beyond the Test
Just because a protection does not pass a particular configuration, it does not mean that it is a bad construction. This test evaluates intra-bundle protection; the SHP may still be viable as inter-bundle protection. Consider the following: an aggressive wire harness constructed with 16AWG wire is strapped to a SHP material fails when tested. If the separation distance between the wire harness is increased to 0.5 inches, the percentage of an arc energy incident to the SHP is reduced by more than 50%. Further separation provided greater reductions of incident arc energy.
In that sense, the secondary protection does offer a segregation material the might be necessary for system safety such as those areas where physical separation cannot be achieved. In the consideration of FAA Regulation 25.1709, the aircraft level impact of each wire harness loss must be considered. But the protection is only viable if the contained wires are signal wires. Inclusion of power wires inside will violate the separation offered by the protection.
The use of secondary harness protection material is a necessary part of EWIS design and protection. There are certain applications where it is a great solution and can benefit the overall EWIS reliability. But the secondary harness protection is not a blanket solution for all wire problems. Thoughtful consideration must be given to where this is used and how the other parts of the EWIS are impacted. Lectromec can help you identify the correct secondary harness protection solutions from the component to system level.