Greetings and thanks to all the smart and experienced electricians and engineers here in the forum...i'm an electrician with over 20 years of experience, and I feel like I should know the answer to this question already, so I'm a bit embarrassed to be having to ask....but I think its important to always keep learning. Here's the scenario:
A customer reported isolated power loss in her residence: two dimmers in the same 2-gang box suddenly stopped turning on their respective fixtures (one an interior chandelier and the other an exterior sconce). Wiring method in this particular switch box is NMC. She reports all other power and light to be functioning normally. Service is 120/240v 1P 3W, located in Los Angeles. All breakers are located in the service, there are no subpanels. I did not open the dead-front of that panel on the day I was there, due to heavy rain.
Testing the voltage on the hot that feeds both dimmers, the meter initially read 95 volts, hot-to-neutral and hot-to-ground. Neutral-to-ground voltage is zero. It then suddenly and abruptly dropped to 75 volts for no apparent reason. In the process of reseting all branch circuit breakers in the panel, one at a time, (none were labeled), I discover that turning one particular breaker off drops the voltage on the "problem" hot to zero, AND, turning off the ADJACENT breaker in the panel, while the problem circuit is energized, actually restores voltage on the problem circuit to normal. Although I did not open the dead-front and check the voltage at the breaker wire lugs, the breakers in question are full size and mounted in the relatively new panel in the same way that a full sized 2 pole breaker would be, likely having 240V potential between their respective loads.
No other loads in the house (at least none that are both known AND accessible) seem to be connected to EITHER of these two breakers. As a side note, in addition to the lack of adequate labeling, there are other indicators that a non-professional modified the wiring in this small house: missing NMC connectors, poorly mounted j-boxes, etc., so I'm not too surprised that these two lighting outlets seem to be the ONLY loads on one 20A breaker, and that at least one OTHER breaker has little, if any, load connected to it at all.
I suspect that some type of short circuit condition is the cause of the low voltage reading, and that the short was cleared by shutting off the second breaker....but I feel like I should be able to tell more about the type of short from the information that I have described. Is it line to line? Or line to ground/neutral in a combination circuit?
I left the second breaker off, and advised my client not to turn it back on unless and until further troubleshooting was done to discover and correct the true cause of the problem, even if turning it back on did not SEEM to have an impact on the lighting circuit in the same way that it was while I was there.
Thanks in advance for your time and input, and I'll try to answer any questions you might have about any details that I have not covered in the description above.
A customer reported isolated power loss in her residence: two dimmers in the same 2-gang box suddenly stopped turning on their respective fixtures (one an interior chandelier and the other an exterior sconce). Wiring method in this particular switch box is NMC. She reports all other power and light to be functioning normally. Service is 120/240v 1P 3W, located in Los Angeles. All breakers are located in the service, there are no subpanels. I did not open the dead-front of that panel on the day I was there, due to heavy rain.
Testing the voltage on the hot that feeds both dimmers, the meter initially read 95 volts, hot-to-neutral and hot-to-ground. Neutral-to-ground voltage is zero. It then suddenly and abruptly dropped to 75 volts for no apparent reason. In the process of reseting all branch circuit breakers in the panel, one at a time, (none were labeled), I discover that turning one particular breaker off drops the voltage on the "problem" hot to zero, AND, turning off the ADJACENT breaker in the panel, while the problem circuit is energized, actually restores voltage on the problem circuit to normal. Although I did not open the dead-front and check the voltage at the breaker wire lugs, the breakers in question are full size and mounted in the relatively new panel in the same way that a full sized 2 pole breaker would be, likely having 240V potential between their respective loads.
No other loads in the house (at least none that are both known AND accessible) seem to be connected to EITHER of these two breakers. As a side note, in addition to the lack of adequate labeling, there are other indicators that a non-professional modified the wiring in this small house: missing NMC connectors, poorly mounted j-boxes, etc., so I'm not too surprised that these two lighting outlets seem to be the ONLY loads on one 20A breaker, and that at least one OTHER breaker has little, if any, load connected to it at all.
I suspect that some type of short circuit condition is the cause of the low voltage reading, and that the short was cleared by shutting off the second breaker....but I feel like I should be able to tell more about the type of short from the information that I have described. Is it line to line? Or line to ground/neutral in a combination circuit?
I left the second breaker off, and advised my client not to turn it back on unless and until further troubleshooting was done to discover and correct the true cause of the problem, even if turning it back on did not SEEM to have an impact on the lighting circuit in the same way that it was while I was there.
Thanks in advance for your time and input, and I'll try to answer any questions you might have about any details that I have not covered in the description above.
