I recently posted a question about cutting the ground wire in an isolation transformer to make it truly isolated. After reading the responses, I decided the best advice was to reconnect the ground wire and just use a three prong to two prong adapter between the isolation transformer and the DUT. After I reconnected the ground I checked and confirmed that I had ground wire continuity from the wall outlet to the transformer outlet. Then, just out of curiosity, I plugged in one of those outlet testers from Home Depot (the ones with the three lighted indicators that tell you if the outlet is wire correctly). I expected that it would say that I had an "Open Neutral" since there is no connection between primary and secondary coils, but what it said was that I had "Open Ground". I checked the tester in a house outlet and it is functioning fine.

Is there some physics about transformers that would confuse this tester or is it telling me that even though I think I have good continuity through the ground wire, it isn't good enough?


2 Answers 2


That's the correct indication for the transformer in its original configuration.

With a normal outlet, the neutral and the ground are bonded together at the service entrance, so the tester will light two lights: one between line and neutral and the other between line and ground, since the same voltage appears across both pairings.

With the tester plugged into the transformer, there's voltage between line and neutral created by the transformer secondary, but now there's no path between your isolated neutral and ground, so now that light on the tester no longer lights, indicating "open ground".

  • \$\begingroup\$ Thanks. That makes sense. Can you also help me understand why my multimeter reads 170v between the neutral and ground (on the transformer outlet) and only 61v between the hot and ground? (of course it still reads 120v between hot and neutral) \$\endgroup\$
    – user13758
    Oct 3, 2012 at 16:56
  • 3
    \$\begingroup\$ The reason is that the secondary of the isolation transformer is floating, and due to capacitive coupling it will charge to around 60V (half of the supply voltage), so what you are seeing is ~60V + 120V at one side, and ~120V - 60V at the other side (the signs are due to the 180 degree phase shift) \$\endgroup\$
    – Oli Glaser
    Oct 3, 2012 at 17:05

Daves answer makes sense if the light off indicates a problem. If there is a light on that indicates open ground though, then it's the same reason, just different connection of the neon light - because the ground is not present (it's now isolated from neutral) to stop a potential appearing between Live and Neutral:

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