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Thread: Voltage reading through a GFCI

  1. #1
    Join Date
    Oct 2007

    Voltage reading through a GFCI

    I recently had a situation with a low voltage landscape lighting transformer that is puzzling me. The transformer was connected to a GFCI, which was reading normally (~117 volts). In the transformer, there is an outlet designed to accept an accessory timer. When testing across this outlet, I received voltage readings of between 90 and 104 volts. However, the transformer would not operate. The GFCI did not trip.

    Upon further investigation, I determined that the homeowner had unplugged the transformer under full load. This resulted in damage to the neutral on the power cord. A continuity check on the neutral confirmed that it had completely failed and that it was unable to conduct any electricity. Replacing the power cord fixed the problem.

    I would appreciate it if someone can explain to me why I might have received a reading of 90-104 volts when testing on the outlet since, ordinarily, when you test between hot and ground, you receive the same reading as testing between hot and neutral.


  2. #2
    Join Date
    Feb 2003
    Seattle, WA
    Quote Originally Posted by cduncanf
    . . . ordinarily, when you test between hot and ground, you receive the same reading as testing between hot and neutral.

    That is only true if the neutral and ground are connected to each other. Ordinarily, they are, at the main panel. But in your case, the broken neutral eliminated the path to the N-G connection at the main panel. Thus, any voltage reading is possible, since you are essentially measuring voltage between two points that no longer have anything to do with each other.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  3. #3
    Join Date
    Dec 2006
    found the same thing on a diy overhead once when i checked voltage with my meter every thing read fine. put a wiggy on it and nothing.turned out he used a splitbolt on his neutral and didnt get it tight enough, over time it corroded enough that it wouldnt carry any load.
    it still had enough of a connection to carry my digital meter but with the wiggy that actually pulled something it wouldnt
    sounds pretty close to the same scenario.

  4. #4
    1. I've heard that the timer receptacle inside a landscape lighting transformer is not a good place to check line voltage. Does this have any validity? The connection to the GFCI is direct.

    2. I've heard of other cases where the timer receptacle yields strange values. What other causes (aside from a broken or compromised connection to neutral or hot) are possible?

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