fault current to ground. This current return or ground path is crucial for managing EMI in EWIS and power-feeder cabling by directing excessive electri- cal energy away from sensitive components to the aircraft’s ground reference point, thus protecting the aircraft and its sensitive avionics—as well as its passengers and crew— from electrical hazards. The key test measurement is again resistance and conti- nuity. But now we are looking at the voltage drop across a complex multi-path network, alternatively called the “Elec- trical Structural Network.” Whatever the name, the point is that modern aircraft are equipped with a preferred, low-resistance ground path for lightning, EMI and other errant currents to follow, directing them away from critical systems and structures. All designed to mitigate potential damage to the aircraft and its passengers, and reduce the risk of elec- trical interference leading to system malfunction. The key requirement from FAR 25.581 (Lightning Protection) states, § 25.581 Lightning protection. (a) The airplane must be protected against catastrophic effects from lightning…by (2) Incorporating acceptable means of diverting the resulting electrical current so as not to endanger the airplane. While there is considerable functional and structural over- lap between grounding and bonding, we are fundamen- tally describing two distinct systems within the aircraft. Let’s look at both in greater detail.
The Electrical Structure Network and the
Metallic Bonding Network
Electrical Structure Network (ESN) Metallic Bonding Network (MBN)
Modern aircraft are equipped with two distinct but interconnected systems, the “Electrical Structure Network” and “Metallic Bonding Network” The Electrical Structure Network (ESN) or Current Return Path refers to interconnected electrical components (metallic frames, racks, ground cables, raceways, flexible joints, and so on) as well as the pressurized fuselage of the aircraft itself. The ESN serves multiple purposes, including • Lightning Strike Protection: The conductive nature of the fuselage allows it to act as a Faraday cage, directing the majority of the lightning current along the exterior of the aircraft, protecting the interior with its passengers and sensitive electronic systems. • Grounding: The ESN serves as the basis for establishing grounding connections throughout the aircraft, particularly for power transmission and EWIS cabling, ensuring that electromagnetic and radio frequency interference is controlled to manageable levels. The Metallic Bonding Network (MBN) refers to the intentional interconnection of various metallic components within the aircraft’s extremities (wings, empennage, vertical stabilizer, and so on) to establish low-resistance paths and equalize electrical potentials. The MBN ensures effective bonding between different parts of the aircraft’s structure and equipment, particularly for the purpose of ESD mitigation. The MBN helps to eliminate electrostatic discharge events by providing low-resistance paths for static charges to dissipate through the conductive structure via ionization into the environment, or directly to ground via landing gear and conductive rubber aircraft tires. Faying surface bonding and bond straps (also called “flexible joints”) are used to physically connect metallic and structural components, such as wings, control surfaces, antennas, and static wicks, to establish electrical continuity and equalize electrical potentials. These bonding connections help prevent any build-up of static electricity throughout the aircraft, a critical safety requirement and mitigating factor in the management of electromagnetic interference. The ESN and MBN are integral parts of the overall electrical grounding and protection strategy, working together to provide a reliable electrical environment, mitigate lightning-related risks, and manage EMI. Above all, the two systems are critical to meeting passenger and environmental safety requirements—a fundamental issue for all aircraft, but one of particular concern to eVTOL vehicles which must implement even more stringent grounding procedures to effectively ground HV batteries, controllers, and motors for passenger as well as ground crew protection.
QwikConnect • July 2023
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