I mounted a metal electrical box on the top of the transformer to house a spst switch and a GFI outlet.
I used a standard polarized line cord to the primary via a switch.
I put the switch on the primary side of the transformer.
There was also an available shield wire on the primary side of the transformer which I decided to connect to the neutral side of the input in case it helps noise or anything like that.
Now...I was wondering if there was any real sense of using a GFI outlet on the secondary and I still question it.
After I was done I pluged in a test radio and all wored well. So I decided to see if the GFI was doing anything.
I first put a 1k resistor in series with my amp meter on the 10 amp scale and using aligator clips I cliped one end to one side on the secondary AC coming out of the GFI into the radio and put the oter end to earth ground. There was no sign of any current at all so I cliped it to the other side of the AC coming out of the GFI to the radio and again the other end to earth with the same resultant zero current.
I couldnt get any current at all so I removed the 1k resistor and tried a lamp and that was the same so I finally removed the meter and all and went directly to earth from either side of the AC output from the GFI and there was no current at all so the GFI remained connected to the load.
I guess the transformer's inherent design is such that one can never establish any current flow to earth from either side of an isolation transformer's secondary... so how could a GFI ever have any benefit there?
I haven't given this a great deal of thought but I was wondering if it would ever make sense to tie one side of the primary to neutral and also tie one side of the secondary to neutral. Would you still acvhieve "isolation" ? ... Would the GFI work and diferently?
I understand that the way the GFI works is that it is always looking to see if the current leaving one side of the outlet matches the current returning on the other side... and if it detects any difference it knows that there must be a short and shuts off. But if there could never be a viable "electrical-path" on a secondary of an isolation transformer to earth... then there wouldn't be any shorts so there could never be an imbalance in current right?
Any thoughts?
Regarding why you have a GFCI on your isolation transformer, this is to protect you should a short ever occur between the primary and the secondary of your transformer. Currently, with your transformer in proper order, the GFCI serves no purpose, as you have already found out. If you take a 1K resistor and connect it from the small prong hole of your bench outlet to a water pipe, you may very well trip the GFCI, because the small prong hole of your household outlets is the "hot" hole, and is out of phase with earth with a potential of 110 volts. The large prong hole is neutral since its wiring eventually connects to the neutral line of your home, which is connected to earth so that it has the same potential as earth.
The secondary of your isolation transformer is in no way electrically connected to the primary, and so it has no electrical relationship with earth, even though the wiring which feeds the primary does due to its connection to earth. It is magnetically connected, which is how it receives its energy (shifting magnetic field in primary generates current in secondary in the same way a moving magnet generates current in a coil in a generator). There may be some capacitive coupling between the two coils, which leaves you with a very mild shock risk--perhaps a small tingle. This may or may not trip your GFCI. Since you've already tried connecting your secondary to earth, and it didn't trip the GFCI, the capacitive coupling is obviously not strong enough to trip the GFCI.
Regarding earth, a lot of people have this idea that it has some magical purpose in electricity. It does not. The only reason why there is any current flow between your AC receptacle and earth is due to how your home and the utility company's wiring is wired. The secondary of the power transformer up on that pole in your back yard has a center tap. The entire coil puts out 220 (or 240 in modern day wiring) volts alternating current. The center tap is connected to earth for various reasons. Each side is out of phase with the center tap and with eachother. From the center tap to each side you get 110 volts (or in modern wiring, 120 volts in most localities). It is only because of this connection that you get any relationship between the wiring in your home and earth (or a water pipe, which is connected to earth simply by its being buried under soil). Now you may wonder why our electrical wiring is connected in this way, since it creates such a shock hazard. There are very good reasons for this, which I will not get into. I am at work right now, and they are quite complicated. Some countries don't have the center tap (if present) connected to earth. Our country has the entire electrical system connected so that it somehow relates to the soil in the ground. One reason is for static charges. Different parts of the earth and sky are at different potentials. Connecting the entire system which spans over many cities tends to unify the system. Other reasons have to do with devices (such as motors and radios) leaking current due to poor insulation quality, etc. This is part of where it gets complicated.
A set of books which I purchased about a year ago explains this thoroughly. It is the Library of Practical Electricity of 1917, by Croft (I think that this is his name). Though this 8 book set is very old, it explains in great detail the wiring methods in homes and factories and power utility plants. Since it was written in a time closer to when electrical wiring first started making it into homes, its information is more closely connected to the reasons why such practices as the ones which are still used to-day were ever developed. I strongly recommend that you read this volume of books if you ever come across it (eBay is a good place to look). When you first acquire the volume, it may seem dull when compared to the technical information found in a radio book. After you finish reading the entire set, though, you will be amazed and pleased with the information found in these 8 books. A little of the information, such as the theory about what light consists of, is no longer valid, but most of the information still applies to to-day's world. I found out recently that the very strange and dangerous constant current system which was once widely used for street lighting (especially carbon arc, but also incandescent) is still used in my city (Milwaukee). I asked a city worker, who happened to be working on some street Christmas decorations, why the lights on my street in Bay View surge bright and dim sometimes. He told me that in certain parts of the city this very old system is used. It is bizarre. The lights are all wired in series, and are all of the same amperage. If one burns out, a special device connects across the lamp (either a mechanical device or an insulative device which the current breaks through and shorts across). The remaining lights continue to burn, and a regulating device (now greatly improved) reduces the voltage supplied to the lights accordingly. Thus current is maintained at a constant, and voltage is varied accordingly. With the old systems, if the circuit opened up (broken wire), the voltage would climb uncontrollably, causing dangerous arcs and fires. At one time such a system was used in factories, but was considered highly dangerous by 1917. I guess it is still safe for street light wiring.
Thomas
Thomas
I, quite coincidentaly, just received such a text as a Christmas gift from my daughter ( she says it cost her all of one dollar at www.abebooks.com)
You may find it interesting reading as well.
The title is "Hawkins Electrcal Guide No.9" © 1915 !
It is a small-ish leather-bound (like a little old bible) guilt edged book.
Interestingly enough it adresses the exact street-lighting subject you described. (relating to varying brightness of older street lamps...and arc-lamps described as 'series-lamps' and the constant current apparatus etc.)
As it relates to arc lights it states:
There are several types of common arc lamps:
"seires-fed" , "shunt-fed" and "differential-type"
1.) Series-Fed: The carbons are initially in contact, but as soon as current flows in the series coil, the carbons are pulled apart and an arc is formed. if the arc be too long, the resistance is increased and the current lowered so that the pull of the solenoid is weakened and the gap reduced.
2.)Shunt-fed: The crbons are held apart before the current is turned on, and the circuit is closed through a solenoid connected across the gap so formed. All the current must pass through this coil at first, and the plunger of the solenoid is arranged to draw the carbons together. The pull of the solenoid is opposed by the force of springs or gravity, etc. In operation, on turning on te current, a high voltage exists across the gap between the electrodes, and the solenoid; this overcomes the opposing force, and thus brings the carbons together. When the carbons touch, the voltage drops, and the opposing force, overcoming the weakend solenoid, pulls the carbons apart, thus lengthening the arc. When the voltage rises too high, the shunt coil again reduces the gap till equilibrium is restored.
(negative effect is:)--- The high resistance thus interposed makes it difficult to start te dynamo.--
3.) The Differential lamp: This is a combination of series and shunt types of lamp.
The effect on the dynamo is:
They increase the apparent resistance of the arc as the current rises, and correspondingly assist the dynamo in maintaining the current constant.
Here are two of the drawings about the series-arranged, street-lamps and controls from the text that you may enjoy looking at.
www.pbpix.com/CarbonArc-1.jpg
www.pbpix.com/CarbonArc-2.jpg
Also, what is interesting to note if it is covered in your book, are the various types of lightning arrestors used back then and possibly still to this day. Some involve blow-out arc gaps, some mechanical and some simply depending upon the rising of the arc due to the hot draft of air created. Some use chokes and some use other types of arc gaps. Some use a combination of both as well as some resistances, since the arc gaps would tend to pass high frequency lightning and the resistances and inductances would tend to pass lower frequencies. Some interesting methods were covered, too, regarding dealing with alternating current vs. direct current. Different methods were used for arresting lightning with the two types of currents due to the tendency for direct current to arc more. I forgot all of the details on this, but it's amazing. Also is the interesting discussion about electrolytic lightning arrestors.
Have fun with the books.
Thomas
P.S. In my book there is mention about old houses from the 1800s of better design where the floors are filled with cinders for thermal insulation, sound insulation, and pest control. Odd, but interesting just the same.