GFCI and an Open Ground
Given: A 120 VAC GFCI supplying power to a 30 AMP RV. The RV has one problem, the 3rd Prong Ground wire at the 30 AMP plug is not making connection. (The RV chassis is floating because there are no ot...
Forum Discussion
road-runner wrote:
I did say that, and upon looking deeper than I originally intended to I see that you are correct. For more FYI, here's what I suspect is an early implementation of grounded neutral detection that used 120 Hz signal injection via a separate transformer: http://www.masterworksservices.com/files/GFCI_by_Sam_Goldwasser.pdf
And here is a chip approach to 120 Hz. signal injection that I believe "could" use a separate transformer, but in practice likely just adds a coil to the current sense transformer: http://media.digikey.com/pdf/Data%20Sheets/Texas%20Instruments%20PDFs/LM1851.pdf
I have no clue what devices use the LM1851 or FAN4147, if one is obsolete, and so forth. Intuitively the FAN4147 sounds like a more advanced approach to me, and a single 2-coil transformer is probably required for the oscillator to work. I suspect somebody did a good job thinking out of the box to come up with this approach.
The LM1851 is what I originally found. I think what makes it the more primitive approach is that absent any feedback, it may not be able to inject the 5mA required to trip the sensor when the neutral/ground short is high resistance. The amplified approach can sense much smaller currents using positive feedback. I agree that this is some clever engineering indeed. These are simple, yet elegant devices that are often misunderstood.
