This is the third article in this series that is looking at the individual components of Federal Aviation Administration (FAA)’s EWIS regulation 25.1707 and EWIS regulatory compliance. This is part of a series on Regulatory Compliance. If you missed the others in this series, they are listed below:
Continuing the examination of the FAA’s EWIS separation requirements detailed in 25.1707, this article examines the next section of the regulations that deals with the physical separation of heavy current components. The third paragraph from FAA’s regulation 25.1707 specifically identifies the need to assess the physical separation of heavy current cables from other components. The regulation states:
(c) Wires and cables carrying heavy current, and their associated EWIS components, must be designed and installed to ensure adequate physical separation and electrical isolation so that damage to circuits associated with essential functions will be minimized under fault conditions.
The supplemental documentation (AC 25.1701), specifically contains the requirements that were in §25.1353 as well as for the necessary supporting EWIS components.
Some of the considerations for compliance were discussed in the first article. To go beyond the concepts discussed in that article, we will focus on some of the finer points when considering developing data for these tests.
There are a couple of important considerations before starting testing:
Are you looking to solve for the particular case or the general?
Solving for the particular case can be advantageous for several reasons; the two most important are cost and compliance. On the other hand, solving for the general case can require a broad test matrix to address many configurations (physical, electrical, and environmental). In addition, when it comes to compliance, a general case may lead to recommendations for a boundary that are too large for a given area and are more severe than the particular case.
What is in the surrounding area?
As discussed in PART I, it is important to consider the damage to nearby equipment. Assessing its potential to nearby wiring components will likely result in separation distances greater than those identified for equipment.
In this example, we will focus on the particular case shown in PART II of this series. The minimum separation distance was two inches. The heavy power cables include three-4AWG wires with three different phases. In practice, it is most likely that the breach and arcing would start at the clamps (shown in the diagram).
While a general case would examine the 4AWG under various conditions, the particular case will examine the potential damage to the jacketed, shielded, twisted pair in the signal harness. The reason that this type of tube one was selected was because, based on past assessments, it was deemed to be the weakest (i.e. most susceptible) to arcing damage.
The results of the test should determine if the arc damage does not impact the EWIS airworthiness. In particular, the following questions must be addressed:
- Is the separation sufficient?
- If the separation is sufficient, what is the margin of safety?
- If the separation is not sufficient, what is/are the necessary change(s) to make the configuration safe?
Also, the Instructions for Continued Airworthiness (ICA) must also be considered if any additional segregation materials are used for protection.
In the next article, we will look at section ‘d’ of 25.1707—physical separation of power.
Michael Traskos
President, Lectromec
michael.traskos@lectromec.comMichael has been involved in wire degradation and failure assessments for more than a decade. He has worked on dozens of projects assessing the reliability and qualification of EWIS components.