In mysteries, the phrase “Follow the money” is often said during the course of the investigation. In the power quality realm, standards primarily address voltage characteristics, such as voltage sags, swells and fluctuations that cause light flickers and rapid voltage changes and interruptions. Follow the current, see where it goes, find the problem.

In actuality, generators that produce the voltage are rarely the cause of power quality problems. They make a nice, clean sine wave without distortion or variation. It is the current drawn by the loads that changes things. Current through the source impedance, which is the combined effect of all the wiring and transformers looking back toward the generating source from the load, creates a voltage, leaving less voltage for the load. If the current is harmonic-rich, as with nonlinear loads, the voltage gets distorted.

The scene of the crime

Following the current was the basis for troubleshooting a problem in a single-family residence where a 15A AFCI/GFCI breaker tripped two evenings in a row. The breaker had been installed during a renovation in 2010 that replaced the knob-and-tube wiring and upgraded the service panel. 

The circuit had a ceiling fan with light, kitchen ceiling light fixture, two switched interior light fixtures, one receptacle and a switched exterior motion sensor spotlight. The spotlight had two 25W bulbs that were rated at 120V:238maA. The one receptacle had a digital alarm clock and air filter plugged in.

Around 8 p.m., all of the aforementioned lights in the house went out. Checking the breaker panel, the homeowner found the 15A breaker tripped. Resetting it, the AFCI LED was briefly lit. Pressing the test button tripped it again. After resetting, everything seemed to work as before.

The hypothesis and clues

The next evening, the same event occurred. The circuit tripped off shortly after it had gotten dark outside. As before, the ceiling fan and kitchen light were on, but neither drew significant current. However, a light on the ceiling fan had been flickering. This time, the breaker had the AFCI LED lit. It cleared upon reset. 

This led to the hypothesis that the breaker trip was not due to an overcurrent condition, as there wasn’t a large enough load that turned on then. If it had been a hard fault, then the breaker would not have cleared when reset. Hence, the problem was likely initiated by the motion sensor light turning on when night fell.

With only a digital voltmeter for troubleshooting, a simple test was derived. Everything else was left operating as it had the past two days, except the switch that powered the outdoor motion sensor spotlight was turned off. The first and second nights came and went without a breaker tripping, which seemed to confirm the hypothesis. The controller portion of the light fixture was also around 15 years old, but the bulbs were LED replacements.

The decision was made to replace the motion sensor fixture with a new one, lower in power but with adequate lumens for the area. The wiring to the fixture was 12 AWG, as that was used throughout during the renovation. There was no apparent damage from water, critters or the previous installation.

When the new fixture was first energized, the breaker tripped again. This time, the AFCI and GFCI LEDs on the breaker were both lit. Since the trip was directly correlated to the energization of a very-low-power load, it supported the prior conclusion that it wasn’t an overcurrent trip. 

Removing the switch in the box and the cover to the junction box in the attic showed no damage that would have caused an arcing or ground fault, nor did the wiring. Subsequent energization of the new motion sensor light by turning on the switch with light in the test mode never caused a breaker trip. Activation of the motion sensor in the dark later that day also didn’t result in a trip.

Research into motion sensor lights and AFCI breakers showed other complaints of similar problems. A typical breaker works on detection of overcurrent condition on the line conductor, where the higher the overload, the shorter the time it takes to trip. 

The GFCI components of the breaker measure the current in the line and neutral conductors, and if the difference between the two currents is above the limit (typically 5 mA), then it will cause the breaker to trip. AFCI components look for the signature of an arcing fault current in the line conductor, discriminating between normal and unwanted arcing conditions. That discrimination portion is a complex scheme that had some issues in earlier models.

While it would have been nice to find the smoking gun (or wire or component), the initial energization of the new sensor light was considered to be the source of the last mistrip. And that flickering light in the ceiling fan turned out to be just a defective bulb. Like in detective stories, not all clues gathered point to the perpetrator.

shutterstock / Hennadii H/ StockSmartStart