Current path

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pruter49

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My question is derived from single phase power at 120 volts, current travels by the ungrounded(hot) conductor to the load and returns on the grounded conductor(neutral) to the source. If the source voltage becomes 240 volts, then each leg to ground carries 120 volts but how can it have a return path if they are 180 degrees out of phase, this would be a phase to phase short if not for the load? A gfci operates by sensing the unbalanced load between grounded and ungrounded in a 120 volt circuit but how does it operate for a 2 pole breaker at 240 volt? Also how does current return on a 3 phase system to the source with no neutral?
Thank you for your time any help would be appreciated.
 
I think you need to get yourself a good basic electrical theory book. I would suggest either Mike Holt's or Tom Henry's.

The load doesn't care what the voltage of the circuit is or the phase angle is. It just needs to be a complete path from the source, through the load, and back to the source. Whether this is done by ungrounded and grounded conductors or 2 ungrounded conductors, or even three ungrounded conductors makes no difference. All current that leaves the source comes back to the source via these conductors.

Sometimes it helps to draw a picture. In any case, get yourself a good theory book or read some basic theory areticles online.
 
pruter49 said:
My question is derived from single phase power at 120 volts, current travels by the ungrounded(hot) conductor to the load and returns on the grounded conductor(neutral) to the source. If the source voltage becomes 240 volts, then each leg to ground carries 120 volts but how can it have a return path if they are 180 degrees out of phase, this would be a phase to phase short if not for the load? A gfci operates by sensing the unbalanced load between grounded and ungrounded in a 120 volt circuit but how does it operate for a 2 pole breaker at 240 volt? Also how does current return on a 3 phase system to the source with no neutral?
Thank you for your time any help would be appreciated.
Click to expand...
The single phase xformer is not really 180 dg out of phase. We will freeze time to demonstrate what is happening by going to 2 batteries in series. One baterry is 1.5 volts from - to+ +being you 120 and negative being your grounded leg When you have 2 bateries in series and you are using the grounded midpoint as your reference in 1 direction you will read + 1.5 volts but in the other direction you will read -1.5 volts giving you a total of 3 volts across both batteries. When you are using a midpoint ground like on your 120/240 xformer it is just like the battery scenario where in order to keep the same reference ground you are actually reversing the measuring test leads giving you a seemingly negative voltage. it is not really 180 dg out of phase you are just measurig both directions from the middle of the winding seemingly out of phase.
 
pruter49 said:
My question is derived from single phase power at 120 volts, current travels by the ungrounded(hot) conductor to the load and returns on the grounded conductor(neutral) to the source .
Click to expand...

Current leaves and returns on both conductors 60 times per second. Don't get caught up on the thought that current is traveling out to the load on the ungrounded conductor and returning on the grounded conductor.
 
pruter49 said:
. . . this would be a phase to phase short if not for the load?
Click to expand...
Actually, this is correct for any electrical load. Were it not for the impedance of the load, every supply circuit would be a direct short.

The fact that we ground one circuit conductor has no effect on the voltage between any two points, be it 120v or 240v.

Read this: http://forums.mikeholt.com/showpost.php?p=708650&postcount=4
 
I hope it changes back the other way

I hope it changes back the other way

OK now for the gfi question. At the point of which the ground fault protection originates there is a device that sences current on each wire leaving and returning to the device. The wires leaving the device will have a magnetic field equal and opposite the wires returning to the device which will effectively cancel each other under normal opperation. When badness happens and current does not return it also does not add up to zero back at the gfci letting it know the badness is happening so that it can safely trip and stop the ground fault from occuring.
 
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