# Thread: GFCI on ungrounded system (system grounding, not equipment grounding)

1. Senior Member
Join Date
Oct 2007
Location
New Jersey
Posts
6,127
Originally Posted by romex jockey
But this is an interesting bit of electrical theory we've going here.....

It would appear the contention is that gfci's can't work w/o a path for current to travel

So if we've an isolation Xformer (floating in space?) , H&N or H&H going through a toroidal to a load , said load side would need a source to loose current to?

Is that the jist?

~RJ~
Yes, that's why I withdrew my comment. Imagine a current source, panel, GFCI circuit breakers, Romex, etc floating in space. The Romex doesn't have an EGC in it. At the end of the Romex is a receptacle with a toaster plugged in. If we strip the Romex somewhere and grab the hot, nothing happens. We're not touching ground so no flow, except for the capacitance current as we get charged up to the conductors potential, which others have pointed out may or may not be enough to trip the GFCI. Only if I make contact to a ground potential that is connected to the neutral back at the panel will current bypass the normal neutral path and cause an imbalance.

In fact, if I grab the hot and neutral at some location without being grounded, the GFCI won't trip, because now I'm a parallel load to the toaster and the outgoing and return currents are still the same.

2. Originally Posted by romex jockey
But this is an interesting bit of electrical theory we've going here.....

It would appear the contention is that gfci's can't work w/o a path for current to travel

So if we've an isolation Xformer (floating in space?) , H&N or H&H going through a toroidal to a load , said load side would need a source to loose current to?

Is that the jist?

~RJ~
That's the just of it. It's the same reason one could argue that adding an equipment grounding conductor to an old two wire romex, living room circuit in a stick frame house makes it more dangerous. Until you brought that EGC into the room there was no chance of a ground fault, also a GFCI wouldn't work there either.

3. Originally Posted by kwired
I wouldn't say it comes down to only that, but yes capacitive charging current can trip a GFCI.
Originally Posted by winnie
No system is ever completely isolated from 'ground'. 'Capacitive charging current' has already been mentioned. Even an 'ungrounded' system will have capacitive coupling to ground, and this current can be large enough to cause a significant shock. If you touch a 'hot' conductor then one phase will be grounded through you, and your body will try to carry the capacitive charging current of the other two phases.

-Jon
And for completeness I guess we should throw in "leakage current" which is technically a different phenomenon than capacitance. A system could have a very high resistance fault which would also make the system not completely floating.

4. Originally Posted by MechEdetour
Ok so this actually answers one of my questions. Don't really need to have a neutral per say... two hots of a grounded system will still allow a GFCI to provide protection. That "path to the source" I am pestering about will allow the leakage imbalance to occur.

I'm still not convinced leakage current will "exist" on an ungrounded system because it will not have a return path (through earth) to the source.

To hopefully add some clarity: If an ungrounded system has no faults - meaning there are no unintentional connections between ungrounded conductors and dead metal parts/equipment - and everything is hunky dory... Will a GFCI function if I have wet hands?

To push it a bit further, in the case where a single ground-fault does exist, and now the system is unintentionally grounded, will a GFCI function as intended?

It almost seems like a GFCI will only function on a grounded system, or an ungrounded system with a single ground-fault present. Amiright?
When I mention "leakage current" in context with GFCI's I am talking about any current no matter how small that passes through the CT in the GFCI in one direction but does not come back in the other direction - it leaked outside the intended current pathway. If this happens and the current involved exceeds 4-6 mA threshold, a properly functioning GFCI will trip and won't care what is grounded and what is not grounded. All those devices are looking for is unbalance current through the CT, so if you connect one line to ground but have an ungrounded supply, there is no current flow in this fault path - until there is a second fault, your first fault is the first ground reference point of the system and it then becomes a lot like a grounded system once you establish a ground point. Now a ground fault elsewhere on the system can have current flow that is outside the CT current monitor in the GFCI - which will cause it to trip after it goes above the 4-6 mA level.
Last edited by jumper; 01-13-18 at 01:50 AM.

5. Senior Member
Join Date
Jul 2007
Location
Clark County, NV
Posts
1,222
A GFCI will let you be killed if the current electrocuting you is from hot to neutral, or staying within its monitored conductors. Else, it will trip within its limits if working correctly. While a "grounded system" has a specific potential to [electrical] ground, I don't think whether or not the system is grounded would affect the protection of a GFCI. It will still trip if the current leaving is not coming back along the intended conductor. Right?

6. Senior Member
Join Date
Oct 2007
Location
New Jersey
Posts
6,127
Originally Posted by MAC702
A GFCI will let you be killed if the current electrocuting you is from hot to neutral. Else, it will trip within its limits if working correctly. While a "grounded system" has a specific potential to [electrical] ground, I don't think whether or not the system is grounded would affect the protection of a GFCI. It will still trip if the current leaving is not coming back along the intended conductor. Right?
Sure, but where is it going if not to ground? If you grab the hot and neutral but aren't touching anything else, the GFCI won't trip. If the effective resistance across your body is less than about 24,000 , you'll have more than 5 milliamps flowing.

7. Originally Posted by MAC702
A GFCI will let you be killed if the current electrocuting you is from hot to neutral, or staying within its monitored conductors. Else, it will trip within its limits if working correctly. While a "grounded system" has a specific potential to [electrical] ground, I don't think whether or not the system is grounded would affect the protection of a GFCI. It will still trip if the current leaving is not coming back along the intended conductor. Right?
That is what I have been trying to get across this whole thread. It don't care what is grounded or not, it only wants to see that whatever goes out comes back, if not it went somewhere besides the intended path, that makes it trip.

8. Senior Member
Join Date
Jul 2007
Location
Clark County, NV
Posts
1,222
Sure, but where is it going if not to ground? If you grab the hot and neutral but aren't touching anything else, the GFCI won't trip. ...
Correct. The rest is what I'm asking, too, because I've no experience with "ungrounded systems." Does the current have the ability to fault to something other than its intended conductors, even if they don't have a specific potential to an electrical "ground?" I'm probably asking a bonehead question and might be wasting the thread's time...

9. Originally Posted by MAC702
Correct. The rest is what I'm asking, too, because I've no experience with "ungrounded systems." Does the current have the ability to fault to something other than its intended conductors, even if they don't have a specific potential to an electrical "ground?" I'm probably asking a bonehead question and might be wasting the thread's time...
A fault is a connection to anything not intended, isn't it? Even if no current flows as a result, but once you have that first fault, you set things up for fault current to be possible if there is a second fault.

10. Senior Member
Join Date
Oct 2007
Location
New Jersey
Posts
6,127
Originally Posted by kwired
A fault is a connection to anything not intended, isn't it? Even if no current flows as a result, but once you have that first fault, you set things up for fault current to be possible if there is a second fault.
Not necessarily. Say we have our ungrounded system. You open the Romex at point A and the hot touches a piece of completely UNgrounded steel. You open the Romex at point B and touch the hot to the same steel at a different point; a second fault. Nothing happens, because both points are at the same potential.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•