Circuit breakers must protect circuit branch wiring. Current sensors provide the primary method for this protection. Measuring input branch current offers the easiest approach to accomplish this protection.

Arc fault detection uses voltage, current, light or combinations of these sensor types. The arc signature determines sensor choice for arc detection. The arc signature represents data features characteristic of arcing. Filtering frequencies of interest magnifies these features. Common DSP techniques such as fast Fourier transform (FFT) also enhance signal characteristics.

Final applications create additional requirements. Circuit breakers commonly interrupt 10,000A. Arc fault interrupters stop arcs from 5A to 500A. Arcing signals produce frequencies up to 10MHz and beyond. Cost, size, manufacturability, linearity, and temperature stability affect sensor selection.

Analysis of arcing current data shows arcing as time varying pink noise. This creates a wide frequency spectrum. Figure 2-3 shows this as a vertical line on the FFT that rolls off with frequency. Detection occurs at MHz level frequencies. Magnitude varies randomly with time and across frequency.

AC arcs create a period where voltage builds while current remains zero. The arc gap voltage must reach sufficient magnitude to ionize air in the gap. This current discontinuity creates greater magnitude of arcing characteristics when measuring current versus voltage. Circuit breaker standard UL1699 sets a current threshold for arcing. These factors make current sensors advantageous for arc detection in branch circuit protection.

Voltage offers advantages in other applications. Voltage has a narrower defined dynamic range. This leads to simpler filtering and DSP requirements. Light sensors do not work for this application. Arc detection must occur anywhere along the entire circuit.

Current sensors considered include current transformers, Rogowski coils, and shunts.