The development of a safe, reliable, and easy to maintain Electrical Wire Interconnection System (EWIS) is a challenging task. The considerations range from selecting each EWIS component, to matching the load and Electromagnetic Interference (EMI) requirements of the connected devices, to ensuring your design can maintain performance across multiple zone environments. EWIS design takes a lot of knowledge.
As a guide to help navigate some of these challenges, the following are seven considerations when looking at wire separation on aerospace platforms:
1. Defining separation distance
Defining separation distance starts with identifying the surrounding area by conceptualizing a cylinder around a wire harness. The cylinder radius should be based on conservative damage potential estimates using simulation or test data for a similar known configuration. Because there can be significant changes in the damage radius by changing a single circuit parameter, there is not a single accepted separation distance for systems (see Lectromec’s EWIS Regulatory Compliance with 25.1707–Part VII article). Also, the separation distance from a harness varies, depending on equipment material (in the “Wire Harness Visual Description” figure, the safe separation distance to prevent damage to another harness is noticeably larger than that required from a fuel tank).
By starting with identifying the surrounding area, those wire harnesses routed near critical components can be highlighted for a more focused analysis. Several of the other items in this list help to classify criticality.
Furthermore, in some cases it may also be necessary to extend the assessment downward. This is recommended for scenarios where damaged components may drop molted metal onto other objects.
2. Separation and/or protection from heat sources
System failure is not just about the damage wires can do to other systems, but also how much those systems can damage the EWIS. In the case of heat sources, it is important to protect the EWIS from overheating. Thus, it is necessary to ensure that the long term heat exposure of airflow or radiant heat does not degrade the physical or electrical performance of the wire insulation.
The EWIS Regulatory Compliance with 25.1707–Part V article provides a good source on how to determine the allowable temperature rise and a wire harness ampacity.
3. Failure is more than just voltage, but also current
One of the most important factors in determining the level of energy in an arcing event is the distance from the power source. If the power system is using a thermal protection scheme, then the arc event duration can be dramatically impacted. Consider the SAE 4373 test standard for wet arc track resistance. There are five resistance values between 0.0 – 2.0 Ohms tested in this method; in practice, this represents the resistance of a 20AWG wire at distances between 0 – 200ft from a power bus (note: this is the wire’s electrical resistance, not the overall system’s resistance). For a standard 7.5 A MS3320 circuit breaker, this can correspond to a trip in less than 0.1s or, in the case of a 2.0 Ohm resistance setting, a trip time of 0.2-0.8 seconds.
What does this mean? In practice, this means that as you move from the power bus or circuit protection, the arcing will progress through these three damage types:
- Too much current: the current is high and creates a short energetic arcing event. This will rapidly destroy the conductor, activate circuit protection, and result in a short arc event.
- Sweet spot: the current is high, but low enough to allow for a sustained arcing event. This region possesses the greatest risk to nearby components.
- Too little energy: In this region, the current is too low to create a sustained arcing event. Typically, the wire(s) will tack weld and circuit protection will activate.
4. Segregation materials
There are a number of aerospace-approved protective materials on the market. Each is designed to satisfy a particular need. Some are designed for chaffing protection, others to protect from heat, and others are designed to minimize arc damage. The selection of the protective sleeving can reduce the physical separation distance by several inches. Because of the limited space available on aircraft, a reduced separation distance can help with maintaining system separation.
However, there are penalties for using protective sleeving. Among these are the size and weight cost for these materials. Because of this, protective sleeving should not be considered a one-size fits all solution.
5. Separation from redundant systems
As with the examination of all faults on aircraft, the objective is to identify and eliminate (or make extremely remote) the likelihood of a hazardous or catastrophic failure. Failures of EWIS components must also be considered for their functional impact on aircraft airworthiness. Redundant systems (such as wheel break and thrust reverser) should not be collocated.
This type of analysis requires combining information from fault tree analysis with the physical aircraft model. Collocation with other electrical systems as well as physical systems should be done to ensure no single point of failure is possible, as required by the FAA EWIS regulation 25.1709.
6. Separation from control cables
Separation from control cables is directed to limiting the exposure to mechanical chaffing and pinching. Beyond the obvious damage that control cables can cause, one must consider the routing and protection of EWIS in zones with moveable parts. Proper clamping and slack will prevent equipment damage or loss.
7. Separation from fuel/hydraulic lines
Separation from flammable systems is an important factor for system separation. Because there is such a high failure severity from damage to these lines, it is important to understand that direct contact arcing is not the only risk, but also arc energy heating that can cause a rupture and ignition event. The short term localized hot spots can cause physical damage and reduce the mechanical strength; both should be considered for lines routed near EWIS components.
The most important issue to remember is that there is no rule of thumb for a safe separation distance. Different power systems, different current overload protection devices, and different materials—all play a role in a safe separation distance. The seven items above provide a starting point for ensuring the safe design and maintenance of your EWIS. Additional information on aircraft wire separation can be found in the Improving Wire Management Systems: The Entanglement of Separation Standards article.