From the point of view of the physics of the case I think it is better to consider that the current in the circuit is just one current and for an electrically short line is everywhere in time phase.
If you lump all of the loads after the GFI into one equivalent load then you have a simple series circuit and there is just one current through the wires of the GFI.
The cancellation is in the magnetic fields that link the coil of the GFI. This cancellation arises because you have two equal currents, as long as there is no stray paths from the black wire to the green wire, going opposite directions and this produces equal and opposite magnetic fields in the GFI coil.
Mathematically, you can regard opposite phase currents if that seems easier and as long as you maintain the same conventions as to + and - the answer will be correct.
But physically, there is still just one current. When the + end of the source goes more + it attracts more electrons from the whole circuit simultaneously making all of the current in the circuit have the same time phase.
Beautiful, David Simmons, that’s exactly what happens. To the coil the load might as well be a length of wire, a short even.
Each electrical cycle produces two magnetic fields, both “positive” in that they go from zero to maximun and back to zero. The only difference is that one is N/S and the next is S/N. So if you bring same current of the circuit back through the coil, in the opposite direction, it sees nothing because the N’s and the S’s cancel each other.
On the other hand, if the return current is smaller, the magnetic field will be smaller and not fully cancel the outgoing field. The difference in these two fields will induce a voltage in the coil, cause a current flow in the relay, and if strong enough, trip the gfci’s contacts.
Don’t try this at home dept;
Once, at work, I plugged a pigtail (suicide cord ;)) into a gfci, grabbed the hot, and touched ground (earth) with a finger on the same hand. The gfci tripped and I didn’t feel a thing. Easiest lunch I ever won. IT guys are so easy.
I love Janis Joplin.
I don’t know why the first, uncorrected version of my post got posted but it did. I think my excerpt below could be confusing because I wrote that there is just one current, which is true, and this paragraph speaks of two equal currents.
“The cancellation is in the magnetic fields that link the coil of the GFI. This cancellation arises because you have two equal currents, as long as there is no stray paths from the black wire to the green wire, going opposite directions and this produces equal and opposite magnetic fields in the GFI coil.”
I rewrote it to say: … This cancellation arises because a current goes out through the coil in the black wire and the same current comes back through the coil in the white wire. This current going out and back through the coil produces equal and opposite magnetic fields in the GFI coil and therefore no induced coil voltage.
Means the same thing but doesn’t screw of the terminology about how many currents there are. Somehow the corrected version didn’t make it. There was some problem in posting and I had to try a couple of times so I guess I didn’t notice that my correction had been lost.
…IN THIS THREAD??? Don’t think I missed the implication in that phrasing
Crafter_man, you said you are an EE. Can I ask what kind of application? I’m not doubting the EE title but wonder if maybe you work on board level stuff and not distribution level type stuff.
I do think that your comment about “US engineering school” was a bit out of line and shines a poor light on the rest of US engineering (science in my case actually) school graduates. I do remember well the arrogance of the engineers in the engineering school. I don’t know about where you went to school, but at the state school I went to, the arrogance was completely unfounded.
I certainly have no reason to suspect the US schools are generally any better than those on the bottom half of the world.
Just curious… why would you doubt I work on distribution level systems? Have I ever said something incorrect w/ regard to AC distribution?
As for the other stuff, the comment was directed at Desmostylus. Inappropriate in GQ, I agree.
I don’t want to get too heavily into RL stuff here, but yes, I did graduate from an engineering school. BSEE from the University of Cincinnati. I’m now pursuing my MSEE from another school located not far from UC.
Me again. This conversation has been getting pretty esoteric and way over my head.
Things are still fine. I have the transformer connected to the mains and taking wires out the 110v side and running them to your typical 110v wall outlets. Using a few apparatuses, let’s see, a 5-speaker computer system, computer printer (every so often) (not the computer itself), computer network hub (24/7) stereo system (every so often), dremel charger (every so often) all appears good to go with no problems.
Voltage readouts are the same as before ~90v and ~200v on blade to ground. 110v across the two blades.
Bardos: Glad to hear you’re still with us. The setup you’ve described could be dangerous. I certainly wouldn’t recommend it.
Having said that, if you are going to continue using it, you really should consider investing a few bucks in a Ground Fault Circuit Interrupter. Failing that, don’t use more than one piece of equipment that relies on earthing. That is, use as many things with two-pin plugs as you like, but only one thing at a time that has a three-pin plug.
Crafter_Man: Let’s just forget it.
Q.E.D.: Deja vu? Didn’t user_hostile also question your “180° out of phase” explanation of GFCI’s in your (I think) very first post on this board?
What the OP uses is a 2:1 step down transformer. There is nothing wrong with this if the device itself is UL (or its Spanish equivalent) approved. It isn’t being used as an isolation transformer so there is no need to keep the grounds separate.
Given the large number of appliances, my forst idea of using a portable, plug-in transformer into which the appliance is plugged isn’t feasible.
So, I would wire the transformer just like normal house wiring. I would connect one side, either side, of it to the 220 v. system neutral (white wire in the US) and carry the safety ground (green) through into the 110 v. system. I would also have a GFI in the 220 v. side and a dedicated circuit breaker or fuze in the 110 v. side. It was stated that the transformer was a 1 kw unit so the breaker should be no more than 10 Amp. And the transformer rating should be checked to make sure it is intended for continuous use.
The object is to make sure that should the transformer insulation, or that of any other 110 v. device, fail, all of the safety features of a grounded power system would work. In any case the person using the devices is grounded to the 220 v. system ground through some impedance. Shoe soles, rugs, concrete or whatever, and you can’t count on that always being a high impedance. So I don’t think allowing the 100 v. system to float with respect to the ground of the 220 v. system is the safe way to go.
That’s because it depends. Sometimes it does, sometimes it doesn’t.
I understand this isn’t the case of the OP, but let’s say I use a 1:1 isolation transformer w/ 120 VAC on the primary. From a “risk of electrocution” standpoint, it is safer to keep the secondary floating. This is the point I was trying to make, and I challenge anyone to prove me wrong. From a ground fault perspective (i.e. from the NEC’s viewpoint), it is better to ground the secondary.
Note, however, that there may be an increased “risk of electrocution” when using a step-down transformer. This will depend on the primary voltage and leakage current.
[Bardos: Glad to hear you’re still with us. The setup you’ve described could be dangerous. I certainly wouldn’t recommend it.
Having said that, if you are going to continue using it, you really should consider investing a few bucks in a Ground Fault Circuit Interrupter. Failing that, don’t use more than one piece of equipment that relies on earthing. That is, use as many things with two-pin plugs as you like, but only one thing at a time that has a three-pin plug.]
me again. Why dangerous? and what could happen by using two three-pronged devices at a time?
bardos
