winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
In another thread, the comment was made that a GFCI would not work on an ungrounded system.
Rather than respond in an already confused thread, I decided to move that discussion into a new thread.
IMHO a GFCI would function to reduce the severity of shocks if installed on an ungrounded system. The basic method that a GFCI uses, detecting any net current flow on the sensed conductors, will work just fine from any supply suitable to power the GFCI electronics.
Where the function will be different is that the characteristics of any ground fault will be different on an ungrounded system, and this will change the response of the GFCI.
In an idealized 'ungrounded' system, a single ground fault will cause _no_ current to flow. On such a system, a GFCI would not detect such a ground fault, because no current was flowing through it. Of course, on such a system, such a ground fault would not cause a shock! You can't get shocked by having zero current flow though your body.
In the real world, there is no such thing as an ungrounded system. The closest that we get is a system that is not intentionally grounded, but which is still 'capacitively' grounded. Because of this capacitive grounding, a ground fault will complete a circuit, and some current will flow through it. The magnitude of this current flow depends upon the total size of the system.
For industrial 'impedance grounded' 480V systems, the 'capacitive charging current' can be several amps. I imagine that a 'residential' ungrounded system (not that the NEC permits this) consisting of a small 120/240V transformer, non-metallic wiring methods, etc. would have a very small capacitive charging current.
If the capacitive charging current of the system is low enough, then ground fault current could never be high enough to trip a GFCI. The worst case could be if the ground fault current was as high as it could be without tripping the GFCI. A 4mA _continuous_ shock would not be enough to trip a GFCI, but might be quite uncomfortable. Of course, if the capacitive charging current is low enough, than not only would the GFCI not trip, but the shock would not even be perceived.
-Jon
Rather than respond in an already confused thread, I decided to move that discussion into a new thread.
IMHO a GFCI would function to reduce the severity of shocks if installed on an ungrounded system. The basic method that a GFCI uses, detecting any net current flow on the sensed conductors, will work just fine from any supply suitable to power the GFCI electronics.
Where the function will be different is that the characteristics of any ground fault will be different on an ungrounded system, and this will change the response of the GFCI.
In an idealized 'ungrounded' system, a single ground fault will cause _no_ current to flow. On such a system, a GFCI would not detect such a ground fault, because no current was flowing through it. Of course, on such a system, such a ground fault would not cause a shock! You can't get shocked by having zero current flow though your body.
In the real world, there is no such thing as an ungrounded system. The closest that we get is a system that is not intentionally grounded, but which is still 'capacitively' grounded. Because of this capacitive grounding, a ground fault will complete a circuit, and some current will flow through it. The magnitude of this current flow depends upon the total size of the system.
For industrial 'impedance grounded' 480V systems, the 'capacitive charging current' can be several amps. I imagine that a 'residential' ungrounded system (not that the NEC permits this) consisting of a small 120/240V transformer, non-metallic wiring methods, etc. would have a very small capacitive charging current.
If the capacitive charging current of the system is low enough, then ground fault current could never be high enough to trip a GFCI. The worst case could be if the ground fault current was as high as it could be without tripping the GFCI. A 4mA _continuous_ shock would not be enough to trip a GFCI, but might be quite uncomfortable. Of course, if the capacitive charging current is low enough, than not only would the GFCI not trip, but the shock would not even be perceived.
-Jon
