A common question when it comes to designing an aircraft EWIS is which of the cables should be shielded? There are several standards, such as MIL-STD-461, “Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment” provide an excellent basis for topics and testing that should be considered. But for those that are not interested in reading a 280-page standard, this article reviews the several shielding considerations that should be made before making particular design choices.
The quick and simple answer to why use shielding is to limit the impact of Electromagnetic Interference (EMI) on system performance. But just because EMI is a concern does not mean that shielding should be used on wiring from every system. The consequences of selecting a shielded cable construction include increased bend radius, increased weight, and increased installation/repair time. For those interested, we have covered the common shielding types in another article.
There are a couple of options when protecting systems from EMI and implementing shielding. These options include: shielding only the signal wires, shielding only the power wires, and shielding both. Each of these is reviewed here.
Shield Signal Wires
The starting point for many engineers and EWIS designers is to limit the potential EMI impact on signal cables. After all, these cables, which send signals with low voltages (often less than 10V) at high data speeds, can be negatively impacted by the electrical noise of aircraft equipment. Protecting the integrity of signals thus improves the confidence in the data integrity.
Of course, once the signal wires are identified for EMI protection/shielding, the question then goes to whether the individual wires/pairs should also be shielded if inside a multiconductor cable. The selection is dependent on the data rate (high frequencies generate more EMI and are more susceptible to EMI). With shielding a cable, then the internal pairs, the outer shielding can be a generalized shield and the internal shield can be optimized to shield the frequencies likely to be carried by the internal wires.
The benefits of shielding the signal wires:
- Transmitted data for each system is isolated
- If improved EMI protection is needed, it can be implemented system by system
- Conceptually easy to address and plenty of available shielded cable options
Shield Power Wires
Another perspective is to shield the power wires. A typical aircraft design will have a larger percentage of signal wires than power wires, and thus it can be easier to shield the power wires.
When considering the latest trends with electrical power systems, there are several systems which gain equipment control with the use of pulse width modulation and wide frequency power generation. Each of these systems have benefits from the perspective of power and control, but create new challenges for EMI. Certainly, the use of pulse width modulated power with high-frequency current changes (dI/dt) can generate strong Electromagnetic Flux (EMF) and impact nearby signal wires.
The benefits of shielding the power wires:
- Reduce the EMI sources throughout the aircraft
- Possible overall aircraft EMI reduction thus reducing noise for sensitive electronic equipment
- Additional chafe protection for power cables.
Another option is to shield both which provides the benefits of both; however, it comes with significant weight penalty. There are solutions that exist for reducing the overall weight burden of shielding, but there will always be a financial and weight cost for shielding. The difficulty with implementing this solution is that many of the harnesses become Line Replaceable Units (LRUs) and not designed for field maintenance. Naturally, the argument here is the if you can increase the reliability of a wire harness by a factor of 2 (as an example, no data to support), would you be willing to remove the field serviceable capability?
The use of shielding to protect wiring and signals has been around for generations, but with the modern aircraft design, the importance of protecting signal integrity is more important. There are multiple strategies for accomplishing this, and the implementation of shielding is entirely dependent on the performance requirement and the operation frequencies of the signal equipment.
In the end, it comes down to what design choice can create the most safe EWIS design capable of completing the prescribed task. To find out more about how to improve your EWIS assessments and EWIS safety, contact Lectromec.