Lectromec has published several articles and a white paper on our web site that discuss the hazards of electrical arcing and how they can be mitigated. In order to broaden the discussion, this article will try to give our readers some insight on systems that offer solid state protection, generally called arc fault circuit protection devices.
SAE standards AS5692 (for 115VAC systems) and AS6019 (for 28VDC systems) define the performance requirements of arc fault protection devices. Please note that within each of these performance requirements there are different classifications. Not all devices that are said to be arc fault protection devices work the same way. Below are some of the differences between devices as well as a few issues to be considered before implementation.
Integrated or Discrete Components for Arc Fault Protection
Arc fault protection devices can be standalone devices or can be integrated with power distribution systems that, in addition to detecting electrical shorts, detect arc faults (or other circuit anomalies) via software and rapidly respond and remove power to the circuit. When operating correctly, these devices can significantly reduce the impact and necessary separation distance between wire harnesses and nearby systems (see video). When integrated into circuit breakers, these devices come with an indicator light or colored ring that is visible when the circuit it tripped when an arc fault is detected.
What do AFCDs look for? And what are they able to detect?
The means and technology used to detect an arc fault vary from vendor to vendor, but, in most cases, they will look at the current and voltage signature on the circuit. The following figure shows the voltage and current waveforms for a 20AWG, 115VAC, 400Hz configuration with the wire arcing to a grounded aluminum tube. For a laboratory setup, this is an easy arcing signature to detect; in application, this becomes significantly more complicated due to the variety of electrical signals on the system and variability in the power system quality.
Most arc fault (AF) devices are able to detect parallel arcing, which is the most dramatic failure event. However, there are others factors that need to be considered.
1. Series Arcing
Series arcing is among the more difficult to detect failure conditions. This is because there is often a negligible difference between the normal operational conditions and series arcing. Some AF devices will be able to detect this failure mode, but not all. This should be considered when selecting a device for your application.
2. Arcing Cross-Talk
It is obvious that a circuit protection device should trip when a fault occurs, but it is also important that the device trips only when the fault occurs on the circuit it is protecting. Hence the reason for the cross-talk immunity test. The idea is to setup a system with two circuits: one protected by an AF protection device with a normal load, and one where an arc fault occurs. Wires from these two circuits are then twisted together for a defined length and an arc fault is generated on the second circuit. The objective is to determine if the first circuit (the one protected by the AF device) would trip due to coupling.
Try Before you Apply
There are many factors that should be considered when evaluating AF protection devices and Lectromec plans on covering them in future articles. But remember that arc fault protection is not a one-size-fits-all solution. Before using for a given application, it is recommended that the system performance be verified for your configuration. If you are looking to verify performance of your devices and how they work under operational conditions, please contact Lectromec. Our engineers can help design the test that will help you select the best device for your application.