โApr-17-2018 08:04 AM
โApr-25-2018 04:43 PM
road-runner wrote:
I think what's behind all the downstream grounded neutral debate is that when somebody diligently goes to the web to see how a GFCI works, they have a 90+% chance of pulling up an explanation that totally omits that grounded neutral detection even exists. And for the <10% that learn that it exists, they'll be led to believe that there's exactly one way that it's done, which is the trap I fell in to.
โApr-25-2018 01:37 PM
myredracer wrote:It's best for any of these GFCI info sources to not make broad specific statements on how the grounded neutral detection works. Better to present things as an example of how it can be done. As wnjj pointed out after digging into spec sheets, in at least a couple of the implementations the 2nd coil does not do any detecting, and the separate transformer has given way to an added coil on the current detecting transformer. Maybe there are one or more cases where the 2nd coil really does detect, so I'm trying to be cautious. Continuing the trend, I just found a chip that needs only a single coil winding for everything. I didn't see any explanation of how this one works. http://www.icbase.com/File/PDF/ONS/ONS54501409.pdf
Here's a couple of diagrams that show how the 2nd coil detects a grounded neutral current flow.
โApr-25-2018 10:16 AM
โApr-25-2018 08:30 AM
BurbMan wrote:BB_TX wrote:
OK, my head is starting to hurt. After all the reading I still cannot grasp how the circuit can detect a fault between the neutral and an open (not connected) ground. So I will just accept that it happens. I was a systems engineer with little experience at the circuit level, so that may be my weakness here.
You are thinking about this all wrong BB....
The GFCI is designed to detect an imbalance in the current from the hot to the neutral. Even without a ground connection, an imbalance is created when YOU become the ground, ie you are getting shocked!
In the example given by the OP, the lack of a ground connection to the trailer will not trip the GFCI since it is monitoring the hot/neutral balance. Because the trailer is not grounded, if any part of the skin becomes electrified, you will get a shock when you touch it, because YOU become the ground path from the skin to the ground. In that case the GFCI will see that current is leaking from hot to ground THROUGH YOU and trip the circuit.
โApr-25-2018 07:04 AM
BB_TX wrote:
OK, my head is starting to hurt. After all the reading I still cannot grasp how the circuit can detect a fault between the neutral and an open (not connected) ground. So I will just accept that it happens. I was a systems engineer with little experience at the circuit level, so that may be my weakness here.
โApr-22-2018 07:03 PM
โApr-22-2018 06:50 PM
โApr-22-2018 06:35 PM
ktmrfs wrote:
also, remember that a GFCI will NOT provide 100% protection against electricution. If you are across hot and neutral AND insulated well enought from any ground path, a GFCI won't trip. Example. in the trailer no stab jacks down, dry ground, rubber soled shoes and you get across hot and neutral. Good chance there is not a good enough ground path to cause a GFCI to trip.
Same thing in a house. rubber shoes on dry hardwood or vinyl floor and you get across hot and neutral. good luck.
โApr-22-2018 06:33 PM
Harvard wrote:
Assuming there are now two faults, one being the floating chassis and the second being the person standing on EARTH being exposed to the 120VAC supply to the RV. Yes, the GFCI would trip.
But if there is only one fault, the open ground (floating chassis), a person receiving the "tingle" while providing the path from the 40VAC CHASSIS to EARTH.
No, I do not think the GFCI would trip.
โApr-22-2018 08:15 AM
Harvard wrote:nebster wrote:
So here's a follow-up question to confirm something I have been presuming for a while:
If the RV is powered via a line+neutral shore cord, with no EGC, and an inline GFI is provided in the shore cord... is a hot chassis event with a human inadvertently providing a low impedance path to earth ground something that the GFI will (should) protect against?
Assuming there are now two faults, one being the floating chassis and the second being the person standing on EARTH being exposed to the 120VAC supply to the RV. Yes, the GFCI would trip.
But if there is only one fault, the open ground (floating chassis), a person receiving the "tingle" while providing the path from the 40VAC CHASSIS to EARTH.
No, I do not think the GFCI would trip.
โApr-22-2018 07:53 AM
nebster wrote:
So here's a follow-up question to confirm something I have been presuming for a while:
If the RV is powered via a line+neutral shore cord, with no EGC, and an inline GFI is provided in the shore cord... is a hot chassis event with a human inadvertently providing a low impedance path to earth ground something that the GFI will (should) protect against?
โApr-21-2018 11:40 PM
wnjj wrote:
Yes.
โApr-21-2018 11:31 PM
nebster wrote:
So here's a follow-up question to confirm something I have been presuming for a while:
If the RV is powered via a line+neutral shore cord, with no EGC, and an inline GFI is provided in the shore cord... is a hot chassis event with a human inadvertently providing a low impedance path to earth ground something that the GFI will (should) protect against?
โApr-21-2018 10:46 PM