The use of circuit protection devices is a fundamental part of circuit design; it is difficult to think of systems without them. For the aircraft industry, thermal circuit protection devices have been the primary circuit protection components for decades, and there is good reason for this: flight crew and maintenance personnel are familiar with their operation and they are reliable and relatively low cost.
With that said, there are areas that can be improved upon with advanced devices. In a previous article, the pros and cons of using various aircraft circuit protection devices was discussed. This article goes into more depth with the understanding of the Arc Fault Circuit Breakers (AFCBs) as protective devices, whether as individual components or integrated as part of a power system. The understanding and use of these components is a critical part of moving the industry forward. How these devices work and how they can be implemented in an aircraft design is the next step to improving system design.
What are AFCBs?
In particular, the AFCBs work by one of two methods:
- they respond to the voltage change on the system, or
- they evaluate the frequency signal on the line.
In the example shown to the right, there is an arcing waveform. The first three half cycles show a steady state for the system and no electrical arcing. After 1.4 milliseconds, an arc fault is introduced to the system. This is a typical single-phase electrical arc signature. The arc has a steady voltage during the event, then turns off as the voltage decreases. After 15ms, the voltage has increased to a sufficiently high level that the arc re-ignites. The cycle continues until circuit protection opens and/or the separation between the electrodes is too great.
The goal of AFCBs is to significantly reduce the duration of the event. The AS5692 standard requires that the circuit be opened within 8 half-cycles of event initiation. With a quick calculation, this means about 100J of energy with a 20AWG arcing wire. For those that are looking to protect critical systems such as required by regulation 25.981 (fuel tank), you need to ask the question: is this energy sufficiently low enough to prevent damage to my system?
What are some tests that must be performed on AC circuit protection?
As with any electrical device, the specification that covers the performance of the device includes a variety of tests. For the AFCBs, in addition to the standard circuit breaker testing that is performed on the AFCBs (dielectric testing, vibration endurance), there is a number of tests that are particular to AFCB testing. There are several unique test methods and we will go over a couple of them below.
Among the unique tests for the AFCB performance is ensuring that these devices activate in response to an electrical arcing event. As discussed above, there are requirements for how quickly the device must respond. For parallel arcing, the assessment is covered in testing that closely mirrors the AS4373 Wet/Dry arc track resistance test configuration.
Beyond this, there are other tests that must be performed to determine product performance. One such test examines the ability of the circuit protection device to detect series arcing (see Lectromec’s The Impact of Series Arcing article on the dangers of series arcing). In this test, several wires are connected through a terminal strip mounted on a shaker table. The terminal strip is configured such that the terminal is loose and free to shake in order to allow for the electrical connection to be broken. If the circuit protection device is able to detect the series arcing, then the test is halted. If the arcing is not detected, the test is terminated after a defined time period.
The last assessment discussed here for the arc fault circuit protection devices is the potential for cross-talk between circuits. This assessment method seeks to determine if an arc fault in circuit A will cause the activation of the AFCB protecting circuit B. The concern with cross-talk activation is that, if there is a single arc fault detected, it does not remove power to several aircraft systems.
Are AFCBs the answer for every application? No, but there are significant advantages to using some of these advanced technologies (watch the video in the Aircraft Wire Degradation and Looking in the Arc article).
If you are looking for the testing of an arc fault circuit breaker or a similar technology, contact Lectromec to find out more about our testing capabilities.