While electrical arcing on aircraft is often associated with a dramatic failure event, a much more common occurrence on aircraft is series arcing. Series arcing can contribute to overheating and component damage, reduction in signal quality, and poor/sporadic performance of electrical equipment. In this article, we discuss the basics of series arcing, the failure effects, and mitigation techniques.
The basic process of a series arc is that the electrical energy still passes through the load as opposed to parallel arcing where the load is bypassed. As seen in the circuit diagram below, series arcing may occur at several locations within a circuit as long as there is a current limiting load. This may occur at any of the connection points including connectors (poor mating), terminal boards (not sufficiently tightened), and or ground studs (lock nut not in good contact).
Although there are many methods for the arcing to start, a simple example of series arcing is a wire terminated with a ring terminal on a terminal board with a loose terminal. The figure below is an example of series arcing. On the left side, a wire terminated with a ring terminal is connected to a circuit with a resistive load and left with a loose connection. Without mechanical stress, it is possible that this would go undetected with functional checks. To simulate an aircraft environment, this was placed on a vibration table and the power was applied. The figure on the right shows the series electrical arcing captured during this test.
As mentioned earlier, series arcing is not as dramatic as parallel arcing, but can impact airworthiness. The arcing at the loose connection creates a hot point in the circuit. Although the wire conductor acts as a heat sink, the heat can increase quickly depending on the circuit load (e.g. a circuit with a 10A load will cause more heating due to series arcing than a circuit with a 5A load).
This localized heating can damage the terminal, wire insulation, and the protective sleeving. If the wire is in a harness, this may also damage other wires and lead to a more dramatic failure event if different voltages are present. This is also a concern as it may create a scenario in which uncommanded activation of a circuit occurs. Lastly, this heating may cause the ignition of any flammable materials of fluids.
Diagnosing series arcing can be difficult as the issue will likely only occur when there is a vibration or other mechanical stress on the system. As such, a component may appear to be malfunctioning while in-flight, but operate normally when on the ground. Simple point-to-point checks will not be able to detect this as the ring-terminal connection will be made (albeit not with an ideal connection, but still good enough for an ohm check). As such, maintenance and troubleshooting will require shaking at or near the connection points while the system is under electrical stress. The breaks in the circuit are likely to be brief, so using the right equipment for the task is important.
Arc fault circuit protection devices that comply with SAE standard AS5692 are designed to identify series arcing and open the circuit once detected. The obvious difficulty is that some devices can generate current and voltage signatures that are very similar to those of series arcing. As such, care must be taken to check the circuit protection compatibility.
During the week of September 8th, at the Federal Aviation Administration (FAA) Tech Center in Atlantic City, Lectromec, Ametek, the Navy, and the Air Force came together to work on common research goals. In addition to examining the effects of electrical arcing damage on composite materials (to be discussed in a future article), the group evaluated the characteristics of 270VDC series arcing. Although the results are preliminary and we are still examining the data, there were some interesting discoveries that the preliminary results showed. As the data is examined and the implications better understood, we will post the results in future blogs.
If you are interested in contrasting parallel and serial arcing, you may want to read Understanding Series and Parallel Arcing for Aircraft Engineers.