Isolation transformer and electrons - Page 1
Forum section: electronics / beginners

author: nForce
author_rank: Frequent Contributor
timestamp: December 02, 2018, 05:45:22 pm
content: I have a basic question:When we use isolation transformer we have a physical electric isolation between two systems. If we connect a wire between a system on the other side of an isolation transformer and ground then the current would not flow.But how do electrons know on the other side of an isolation transformer that we don't have a current loop? They just feel a potential difference, so they should flow because they don't know nothing about isolation.Thank you.

author: bson
author_rank: Unknown
timestamp: December 02, 2018, 06:08:49 pm
content: An electric potential causes charge to shift around, it's the force acting on electrons. A transformer is coupled through a magnetic field, so there is no potential between the two sides. (Capacitors are coupled through an electric field, hence they do have a potential.) The potential is between the individual taps on each side, not between the sides. Since there is no potential between the sides you can connect one of the taps on the secondary to ground or any other primary-side reference. The electrons simply have no force acting on them, hence no current.

author: Zero999
author_rank: Super Contributor
timestamp: December 02, 2018, 07:16:18 pm
content: As far as the secondary coil is concerned, the electrons are being moved around by the changing magnetic flux. The primary coil could be replaced with a spinning magnet, and it would behave in the same manner.

author: IanB
author_rank: Super Contributor
timestamp: December 02, 2018, 07:28:09 pm
content: Actually the electrons do flow, however the size of the current is too small to be of significance. The electrical potential on the secondary side of the transformer is fluctuating at the AC frequency and the ground potential is not. So a small AC current can flow due to the capacitance between the ground and the secondary.For normal low voltages the current is too small to matter, but for large electrical transmission transformers where the secondary voltage is (say) 500 kV this capacitance effect is very significant and considerable precautions have to be taken to prevent arc over.

author: nForce
author_rank: Frequent Contributor
timestamp: December 02, 2018, 08:34:01 pm
content: Quote from: bson on December 02, 2018, 06:08:49 pmOk, but the magnetic field also moves charges (qv x B), the Lorentz force? And there is also a magnetic potential not only electrical one.

author: nForce
author_rank: Frequent Contributor
timestamp: December 05, 2018, 07:53:33 pm
content: Sorry, but I am still waiting for an answer on my last question.

author: SG-1
author_rank: Regular Contributor
timestamp: December 06, 2018, 12:07:54 am
content: The impedance of the secondary side is reflected back across to the primary side with the same ratio as the transformer turns.With a perfect transformer:If you place a dead short on the secondary that impedance is reflected back across the magnetic circuit to the primary side as a dead short & you get a lot of electric current.If the secondary is open that impedance is reflected back across to the primary side as an open circuit & you get no electric current in the primary side.In the real world:Because of internal losses, etc... some small primary current is always present when the transformer is energized.

author: nForce
author_rank: Frequent Contributor
timestamp: December 07, 2018, 02:30:06 pm
content: Quote from: nForce on December 02, 2018, 08:34:01 pmI meant by what bson wrote: "A transformer is coupled through a magnetic field". But I don't understand why is there no current, what about Lorentz force? Electrons also feel a force, because of a magnetic field.

author: SG-1
author_rank: Regular Contributor
timestamp: December 07, 2018, 04:15:52 pm
content: The magnetic force on the electron is perpendicular to the conductor, not axial.

author: IanB
author_rank: Super Contributor
timestamp: December 07, 2018, 04:21:24 pm
content: Quote from: nForce on December 07, 2018, 02:30:06 pmI don't think any of us understand your question. Where should there be current, due to the magnetic field?

author: lordvader88
author_rank: Frequent Contributor
timestamp: December 07, 2018, 05:22:38 pm
content: I'll add some questionsIf u have a cloud of free electrons, and in that cloud u put a transformer with AC source on one side, and a closed loop on the other side, what does the electron cloud do ? Or would they just all repel due to Coulumbs law.Ok what if it was a plasma of hydrogen, so just all free electrons and protons, what would they do ?In an ideal transformer with a magnetic core, is the magnetic flux literally confined only inside the core ?

author: boB
author_rank: Frequent Contributor
timestamp: December 07, 2018, 05:34:55 pm
content: The charges flow in the secondary due to the will of Faraday's law

author: nForce
author_rank: Frequent Contributor
timestamp: December 07, 2018, 07:00:13 pm
content: Can someone explain in detail this:QuoteWhat about magnetic potential? So magnetic potential does not move charge around?

author: IanB
author_rank: Super Contributor
timestamp: December 07, 2018, 07:17:02 pm
content: Quote from: nForce on December 07, 2018, 07:00:13 pmA changing magnetic field will cause charges to move along a conductive path. In a transformer the conductive paths are the wires that make up the windings. There is no conductive path between primary and secondary so the magnetic influence cannot cause charges to move in that direction.

author: nForce
author_rank: Frequent Contributor
timestamp: December 07, 2018, 07:32:23 pm
content: Ok, IanB but capacitors are also isolated between plates:QuoteSo no current with capacitors?

author: GeoffreyF
author_rank: Regular Contributor
timestamp: December 07, 2018, 08:03:57 pm
content: Electrons have no feelings. In fact when there are a lot of them and they have a good running start they can be downright heartless, even taking joy in one's death or burning.

author: IanB
author_rank: Super Contributor
timestamp: December 07, 2018, 10:22:57 pm
content: Quote from: nForce on December 07, 2018, 07:32:23 pmCorrect. There is no net current through a capacitor other than leakage. The average current between the two poles of a capacitor is zero in steady state or repetitive cycles assuming there is no dielectric breakdown.Maybe what you are trying to think of is eddy currents? Any conductive objects in a changing magnetic field will experience induced eddy currents due to the influence of the magnetic field. So if you as a human were exposed to a changing magnetic field of sufficient intensity you could be electrocuted by the eddy currents induced in your body.

author: SG-1
author_rank: Regular Contributor
timestamp: December 08, 2018, 05:34:55 pm
content: The same principals apply to each side of the transformer. If the circuit is closed then you have current. If the circuit is open no current.To know why requires a look from physics, not engineering.With an open circuit their is no internal e-field axial to the conductor, or along it's length. There is no emf to move the charge.Here are some notes I made from some IEEE papers.

author: nForce
author_rank: Frequent Contributor
timestamp: December 09, 2018, 06:08:43 pm
content: OK, now I know what means coupled through a magnetic field. It's just like with optocouplers, which circuits are also isolated.I have made a drawing, where we have a transformer which has 1:1 turns. So the magnetic flux is changing in the core of a transformer, because we are changing the AC voltage on the primary. Because of that the voltage between one tap and another tap of a transformer on the secondary side is also changing. Lets freeze the picture in time, when we have 120 V potential on the top and 0 on the bottom tap. We are standing on the ground which is 0 V. We touch the bottom tap and because it's 0 V, and we are standing on the ground which is also 0 V, there is no current. Now we touch the top tap which has 120 V potential and we are standing on the ground which is 0 V. Now what?You are going to say, yes there will be no current, because we have isolated system and we don't have a current loop to flow. Ok, but electrons do not know nothing about isolated system, they just see a voltage difference. So we are touching the top tap 120 V potential, and we are standing on the ground 0 V potential.

author: IanB
author_rank: Super Contributor
timestamp: December 09, 2018, 07:30:34 pm
content: Quote from: nForce on December 09, 2018, 06:08:43 pmBut this picture is wrong. The transformer output winding is isolated so we don't know the potential of any part of it. It is undefined. All we know is that one end of the winding is at a 120 V higher potential than the other.QuoteTo be more precise, the ground is at 0 V because we have decided to use this as a reference.QuoteAs noted above, before we touch the bottom tap its potential is undefined. We do not know what it is. After we touch the bottom tap its potential becomes 0 V, because we have connected it to ground and all connected parts of the system equalize to the same potential.QuoteNaturally there is no current because potentials have equalized and there is no driving force.QuoteBefore we touched the top tap its potential was undefined. After we touched the top tap its potential became 0 V, because we have connected it to ground and it will equalize its potential through the connection.QuoteYou are correct, there is no current because there is no circuit. But also there is no current because the top tap is at 0 V potential after we touch it, the same as the ground, so there is no voltage difference to produce any current.Note that to say there is "no current" is not 100% accurate. In order for potentials to equalize there will be a small movement of charge. This will manifest as a current, but it will be a transient effect. The current will flow for a very short time and usually will be very tiny (depending on the capacitance of the system and the magnitude of the voltages concerned).

author: ArthurDent
author_rank: Super Contributor
timestamp: December 09, 2018, 09:53:00 pm
content: GeoffreyF – “Electrons have no feelings.” GeoffreyF is correct and I have never met an electron that was hell bent on doing me harm and in a one-on-one interaction they cause no problems at all. However, in a large crowd they can do a great deal of harm if you cross them.All these metaphysical descriptions of electrons with ‘free will’ is just confusing the issue. There also seems to be some mystique about isolation transformers where there is no similar concern about step-down and other transformer ratios. You might guess that an isolation transformer does just what it says-isolates. There are also some things that just have to be taken as real and ohms law is one of those things.Simply put, if there isn’t a complete circuit there will be no current flow. The following modified drawing displays it pictorially.

author: Zero999
author_rank: Super Contributor
timestamp: December 09, 2018, 10:39:32 pm
content: Oh I see, it seems this is about the common misconception of what ground is, rather than how transformers work.In order for current to flow, there needs to be a complete circuit. Any electrons coming out of the negative terminal of a battery, must go back into the positive. The same is true for the transformer, except the direction of the current flow is continuously changing.Another interesting fact is that within a closed circuit, the net charge remains constant. When a capacitor discharges through a resistor, the number of electrons and thus charge in the entire system doesn't actually change. All that happens is electrons are transferred from the negative plate of the capacitor, to the positive plate, via the resistor. The number of electrons inside the capacitor stays the same.Ground is just a reference point, from where all voltages are measured. It makes no sense to measure the voltage between a stake in the ground and a floating transformer secondary. All you'll read is a ghost voltage, leaking through the transformer's inter-winding capacitance. The primary coil is acting as one plate of the capacitor with the neutral of the mains connected to the soil and the secondary is the other plate of the capacitor. If the voltage source powering the transformer was to be completely isolated from earth, which is actually impossible, as there will always be some capacitive coupling, then you'd read zero between earth and any part of the circuit, because no current would flow.

author: Shock
author_rank: Super Contributor
timestamp: December 09, 2018, 10:46:49 pm
content: Quote from: nForce on December 09, 2018, 06:08:43 pmThe problem with the question you are asking is you are defining a 0V that does not exist on the secondary side in the circuit shown.Between L1 and L2 (the secondaries) is the 120VAC difference in potential. Those two connections are no longer Live/Neutral/Earth (or what you want to call them) as they are galvanically isolated.Ground/Earth to the secondary is just like any other conductor, it might as well be a piece of wire. It's not 0V to the secondary, it's nothing while not connected. While not connected the only situation it would be called it 0V is in reference to the live side if the Neutral is at the same potential as Ground/Earth.The electrons won't see a voltage difference between the secondary and Ground/Earth because there is none. There has to be some kind of completed circuit in order to induce the flow of electrons.

author: Shock
author_rank: Super Contributor
timestamp: December 09, 2018, 11:02:35 pm
content: You can find how electrons flow in a circuit in any textbook but going back to your original question of how do they know. A simplified analogy would be how evaporation and precipitation works. This is a completed circuit and has flow (of moisture/water in this case) aided by the warming and cooling of the earth.If we introduce an insulator and isolate the earth from the sky no complete circuit exists and no flow occurs. There might be plenty of potential to flow in the oceans but without the loop nothing will occur.

author: nForce
author_rank: Frequent Contributor
timestamp: December 10, 2018, 05:34:09 pm
content: Quote from: Shock on December 09, 2018, 11:02:35 pmIf I reference to this picture. Because earth has everywhere potential of 0 V, how do electrons find the correct path through earth to the transformer? If we have a town with 20 transformers, and we create a short circuit somewhere on the grid, how do electrons know where to flow to that one transformer of the total 20?

author: IanB
author_rank: Super Contributor
timestamp: December 10, 2018, 05:43:26 pm
content: Quote from: nForce on December 10, 2018, 05:34:09 pmMaybe a better answer:If the electrical distribution transformers are connected to ground at the point of supply (as they normally are), then if you make another connection to ground somewhere else on the network (your short circuit), then there is a potential gradient through the ground between the two connection points. Current will flow in the direction of decreasing potential through the ground, which will take it back to the source.This is like the direction of current flow in a lake. If there is an outfall at the edge of the lake then water will flow towards the outfall to maintain a level surface. You cannot see the potential gradient that causes the water to flow across the lake, but it is there.

author: Shock
author_rank: Super Contributor
timestamp: December 10, 2018, 07:54:34 pm
content: Quote from: nForce on December 10, 2018, 05:34:09 pmFirstly the analogy was just to give you an idea how current flows in a circuit. Don't read into it too much. I'd also get away from this earth is 0V idea, it's just earth a circuit path (not a great one either).Not sure I understand your question, but if you are referring to mains distribution, it works like any other circuit, the least path of resistance will be taken as the return path. Which is going to be the neutral line over some ground/earthing rod in soil.The earthing of the neutral throughout distribution systems from what I read a while back was to prevent dangerous DC voltages on the AC lines from lighting strikes and then added to homes then as a separate path to appliances.QuoteNot sure I understand your question. If the one transformer had a completed secondary circuit allowing more current to flow it would cause the magnetic field (magnetic flux density) to increase which increases current flow on the primary.There are ideal components and real components, if the transformers primary is connected it's actually using a small amount of current even if the secondary is open. So the first part of the answer is that current is already flowing.When you have two conductors that have a voltage potential across them it does not need to find a pathway from the electrical station/substation/transformer/battery etc, it already has one and is sitting there waiting for the circuit to be completed. The magic happens straight away.

author: ArthurDent
author_rank: Super Contributor
timestamp: December 10, 2018, 09:23:01 pm
content: nforce – “If I reference to this picture. Because earth has everywhere potential of 0 V, how do electrons find the correct path through earth to the transformer? If we have a town with 20 transformers, and we create a short circuit somewhere on the grid, how do electrons know where to flow to that one transformer of the total 20?”Once again, the language of science is math, not feelings, and electrons don’t willy-nilly choose to do or not do something, they follow the laws of science. First you have to understand how grounding works in a power distribution system. There are countless utility poles and many of them have grounding rods driven into the physical ground for system safety but in a single phase residential distribution system besides the hot line there is always a neutral wire that is connected to these ground rods at many poles and connected to neutral throughout the entire system without relying on ground except for safety. Here is a drawing showing how in a single MV (medium voltage-maybe 2500V) feeder circuit is grounded. Every so often there is a step-down transformer that has a 240V center-tapped winding and the center tap is also connected to ground, both at the pole and in any house it’s connected to.The distribution system is the 2 feeder wires (hot and low) with as many loads (transformers) connected as needed up to the power rating of the distribution line. If there is a short at any transformer secondary, it has a primary fuse that will blow so it won’t take the entire distribution line down but all the houses fed from the transformer output will lose power.If there is a short on the 2500V distribution line causing the distribution line fuse to blow at the sub-station then all the houses connected to all the transformers on that entire distribution line will lose power.If there is a short on the sub-station feeding the distribution lines going to neighborhoods then all the houses connected to all the distribution lines connected to the power sub-station will lose power and earth ground could care less.

author: nForce
author_rank: Frequent Contributor
timestamp: December 11, 2018, 04:24:58 pm
content: Hmm, but I want to know from electrons perspective. Maybe IanB gave a better answer.So if I understand correctly: When we make a short circuit on the grid, those electrons flow from a transformer through wire into ground. Now the transformer has electron "holes" and the earth (our planet) has negative charge. Because of that the transformer tap, which is grounded suck electrons from the ground, to fill the "holes". Now everything is again balanced the earth and the transformer. Altough our planet is very large, so excess of electrons would be negligible.Sorry but my question is pure physics, and not engineering.

author: Shock
author_rank: Super Contributor
timestamp: December 11, 2018, 06:01:36 pm
content: This is where the electrons are going in a completed AC circuit.

author: ArthurDent
author_rank: Super Contributor
timestamp: December 11, 2018, 06:06:29 pm
content: nForce - "So if I understand correctly: When we make a short circuit on the grid, those electrons flow from a transformer through wire into ground. "No, no, no! Read what I posted. The ground (earth) is only a safety connection that the zero volt reference wire (neutral) is connected to at many points along the distribution route. If all the wires from the poles to the ground were removed the system would still function because, as I said:there is always a neutral wire that is connected to these ground rods at many poles and connected to neutral throughout the entire system without relying on ground except for safety.The electrons have no perspective, they could care less, but they are absolutely law abiding and follow ohms law to the letter.

author: nForce
author_rank: Frequent Contributor
timestamp: December 11, 2018, 07:12:13 pm
content: Quote from: ArthurDent on December 11, 2018, 06:06:29 pmI meant ground, as an earth. We are standing on the earth and touch the hot wire.

author: nForce
author_rank: Frequent Contributor
timestamp: December 11, 2018, 07:24:41 pm
content: Quote from: Shock on December 11, 2018, 06:01:36 pmYes, so the electrons are switching between earth and the wire, and between earth and the transformer. But my point was about the earth (planet).

author: 001
author_rank: Super Contributor
timestamp: December 11, 2018, 07:27:34 pm
content: google Kirhgoff law

author: IanB
author_rank: Super Contributor
timestamp: December 11, 2018, 09:08:39 pm
content: Quote from: nForce on December 11, 2018, 07:12:13 pmIt's still not clear that you understand.Get a loose piece of wire and wave it around in the air. What is the voltage of the wire? Answer: it has no voltage. It is not 0 V, it is not zero, it is simply undefined. We do not know what the voltage is. If you touch it, no current will flow because there is no voltage.Now imagine the Earth as a big ball of wire floating in space. What is the voltage of the Earth? Again, we do not know. It is undefined. If you stand on the Earth no current will flow. The Earth is isolated because space is an insulator.So now, consider a transformer. It will have two wires coming out of it. Neither wire has any voltage relative to ground because the winding is isolated. Neither wire is hot, and neither wire is neutral. If you touch either wire while standing on the ground nothing will happen.Next, suppose we connect one wire to a rod in the ground. We have nowmadethis wire neutral, and the other wire has beenmadehot. It wasn't hot before, but now it is.So now, what happens if we touch the hot wire? Now there is a circuit made and current can flow. The current flows through you, through the Earth (a big ball of wire), through the rod in the ground, and back to the transformer.When you say "short circuit" you need to understand that "circuit" means a closed loop, out and back again. So a "short" circuit is a shorter loop than the one intended. That's it.

author: Gregg
author_rank: Super Contributor
timestamp: December 12, 2018, 02:52:49 am
content: The following is an attempt to provide the OP with another viewpoint toward understanding the question posted; it may not be 100% correct due to simplification.nForce,It may help to let go of thinking of electrons actually flowing like so many basic explanations comparing electricity to water suggest. It may be closer to reality to consider the electrons more like the balls of a Newton’s cradle where a force on one end of the set causes an almost equal reaction on the far end. Electrons are not free to flow like water in a hose; a good conductor just has the electrons not as tightly attracted to the nucleus as an insulator. From the electron’s perspective, it gets kicked from one nucleus to the next and thereby kicking another electron on down the line. If the potential continues or reverses, it may be a different electron that gets kicked away, but none of them go very far of and by themselves.If a force is applied to an isolated wire to cause electrical potentials like shown in the graphic posted by Shock with neither end connected to make a circuit, the electrons don’t jump off the ends of the wire; the wire in this instance acts like a capacitor, the charge potential varies from end to end but there is no circuit. The graphic probably would be closer to reality if it showed the blue dots being packed tighter at one end and farther apart at the other rather than disappearing from the ends.To get back to the transformer: the secondary is an isolated wire with potential(s) being induced by the magnetic fields created by the primary. Even if the primary has a connection somewhere in its circuit connected to ground, from the perspective of the electrons in the secondary they are like stars in the night sky; they may have potential energy but no measurable interaction. To put the transformer question into a very basic scenario: take two batteries (your choice) and connect a terminal of one to the earth ground; try measuring any potential to ground from the second battery and because there is no circuit there is no potential from either terminal of the second battery to ground even though each battery has its own potential.In other words potential is just potential, nothing happens unless there is a circuit. Most electrical parameters need to be considered as circuits first and what is happening in the circuit all the way from the source back to the source. The secondary of a transformer is considered to be an isolated source. Potential is how far you need to keep your fingers away. Amperage is the damage it can do if the circuit resistance or impedance is lower than needed to keep it under control.Old school AC welding machines are an example of transformers similar to the posted question. These devices were merely a transformer with a few taps on the primary that stepped the voltage (potential) down to a level less than lethal while stepping the amperage (ability to melt metal welding rods and add heat to the substrate) up with a core that easily saturated before drawing more current than the input could provide. One arbitrary output terminal was connected to the metal being welded, usually considered “earth”. Several of these welders could be connected to the same potential steel via the building structure, ship hull etc. but the different welders wouldn’t swap electrons and interact because they were different circuits and the potential only existed for each individual circuit. More modern welding machines are usually SMPS DC but the output still has to be isolated.Then there are autotransformers. Transformers with isolated secondaries can be used as autotransformers such that the secondary potential can be either added or subtracted from the primary; however caution needs to be exercised as to the connections. Then there are autotransformers that have only one winding a configuration most Variacs utilize. Look it up for further details.

author: Shock
author_rank: Super Contributor
timestamp: December 12, 2018, 04:36:00 am
content: Quote from: nForce on December 11, 2018, 07:24:41 pmThe earth as a whole is considered neutral. It has the ability to conduct (and alternate) electricity in a completed circuit. Though not the most reliable conductor of electricity which is why ground/earth rods are sunk into favorable conditions depending on the soil type, moisture, depth.There is actually a high voltage transmission line system called "Single Wire Earth Return" which just uses one conductor and the earth to complete the circuit over long distances.As mentioned, the Neutral side of the mains is connected to Earth throughout the distribution network at various points (at least here it is). Because the mains is now Earth referenced (not floating) making a connection between Live and Earth to complete a circuit will cause electrons to move back and forth like I showed in the previous image.How far do the individual electrons travel in a Live to Earth circuit? Not very far at all, they would alternate similar as with wires given the same propensity to conduct. Do they still alternate in the ground, earth, soil? Yes where it's conducting and not 100% insulating.What path do they take to complete the circuit? The majority would be "shared paths" of least resistance to the nearest neutral line I would imagine. Perhaps there is also some capacitive effect as well, I'm not sure. But we know electrons alternate through conductors, even poor ones. They tell us this at school.

author: SG-1
author_rank: Regular Contributor
timestamp: December 12, 2018, 07:00:02 am
content: nForce, I think you would find the electricity articles here very interesting:http://amasci.com/elect/elefaq.html

author: wbeaty
author_rank: Contributor
timestamp: December 12, 2018, 07:48:46 am
content: Quote from: Gregg on December 12, 2018, 02:52:49 amOK, but electrons also flow too. An electric circuit is like a flywheel made of electrons, where if you push on one part, the whole thing turns as a unit. (So, it's like a circular Newton's cradle! Or like a loop of hose that's completely full of metal spheres, all lined up. Jerk one sphere, and fast waves fly all around the loop, but also, all the spheres move ahead, like a turning wheel.)Quote from: Gregg on December 12, 2018, 02:52:49 amNot true. Electrons in metals are free to flow, very much like water in a hose. They flow slowly as a group, while the waves (the energy) zoom fast. The "flywheel" turns slowly, but the whole thing turns as one, so energy can flow almost instantly to every part of the "wheel."In a good conductor, the electrons have left their original atoms, and are orbiting around all through the metal. In other words, they're jumping between atoms all the time, inside all metals everywhere. This is more like a vibration than like a flow. An "electric current" is when they jump slightly more in one direction. They all move along in one direction, while also jumping/vibrating. (Air and wind is similar, where individual air molecules are always wiggling, even when the "air" is still, with zero wind. The energy in wires is like sound waves. The electric current is like slow wind.)But inside an insulator, the electrons don't do this, but instead stay in their individual atoms. If metals are full of "electron liquid," then insulators are choked with solid "electron ice."

author: wbeaty
author_rank: Contributor
timestamp: December 12, 2018, 08:06:28 am
content: Quote from: nForce on December 02, 2018, 05:45:22 pmSimple: in circiutry, electrons flow in loops only.But don't just accept this answer; instead ask why.Why do electrons only flow in loops? It's because if we wanted them to flow in a one-way direction, we'd need kilovolts, megavolts, to apply enough force. Electrons are actively held inside of wires by their enormous attraction to the positive charges of atom nuclei. The electrons of the metal can flow in a loop, but can't leave the metal circuit.If electrons in a circuit are like a flywheel, then it's very easy to spin the flywheel, but very hard to pull chunks out of it. That's why electrons stay inside the isolated circuit: an immense voltage-force would be needed if you wanted to push any of them out of the loop and into the Earth.Why? It's because metals are half electrons, half protons, all in balance and strongly attracting. If we make electrons flow in a circle, then they always stay near some protons. But if we try to make them flow in one direction, then we're fighting against an immense-but-unnoticed attraction-force. We're pulling them away from the enormous positive charge inside the copper.---Also, always remain aware that one wire doesn't "have a voltage."Instead, each wire has many voltages at the same time (voltage WRT the ground is usually kilovolts, voltage WRT the sky is megavolts, WRT the sun or moon is even higher.) In other words, voltage is like altitude, and your body doesn't "have altitude." The altitude of a single point is meaningless. Altitude is always measured between two points. So, your body has many altitudes at the same time: altitude above the floor, altitude above the dirt, altitude above the average local landscape, altitude above sea-level, altitude above the center of the earth... and voltage behaves like that.The Earth isn't actually at zero volts. Instead, we'll find several megavolts between Earth and sky (ionosphere.) We could say that the sky is at +1.5MV and the Earth is at -1.5MV. Or say that the sky is at zero volts and the Earth is at -3MV. Both are true at the same time, because Earth's surface has many voltages all at the same time ...because voltage is always measured between two points.The word "electric potential" means something like length-in-an-E-field. And length only has meaning when it's measured between two points.

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2018, 10:59:06 am
content: Quote from: nForce on December 11, 2018, 04:24:58 pmIn the distribution transformer scenario, the earth simply acts as a return conductor for the current path. If one side of the transformer isn't connected to earth, then there will be no return current path, so no current will flow, as is the case with the isolation transformer, when the other side isn't earthed.

author: Brumby
author_rank: Unknown
timestamp: December 12, 2018, 11:48:35 am
content: YES!I've been wanting to express something like this - but haven't had the time to put the words together. Thanks towbeaty!!I would make one change, which I have highlighted...Quote from: wbeaty on December 12, 2018, 08:06:28 ameg Lightning.Quote

author: Brumby
author_rank: Unknown
timestamp: December 12, 2018, 12:37:49 pm
content: The first thing you need to do is set aside electrostatic examples such as lightning. That deals with single discharge events on not a continuous current flow.The other thing you need to understand - and there has been a LOT of effort in this thread to try and make this point - is that there is no such thing as an absolute voltage. It is ALWAYS a voltage at one pointwith respect toanother point. Whether you realise this or not, you prove this point every time you make a voltage measurement. If you connect one probe from your multimeter to a voltage source (say a 9V battery) and don't connect the other probe, then you aren't going to get a voltage reading.Further, if these two points are notelectricallyconnected in some way, then you cannot measure the voltage between them - because thereis no differencein their potential. To prove this, connect one probe from your multimeter to the positive terminal of one 9V battery and connect the other probe to the negative of a second 9V battery - with no connection between the batteries. Again, you aren't going to get a meaningful voltage reading.However, once you make a connection between these two batteries, you have defined voltage relationships and youwillget a voltage reading that is meaningful. Note that if you make exactlyoneconnection between the two batteries, you will not create any short circuits, but depending on which of the four possible connections you could make (with the probes connected as above), then you will get a voltage reading of 0V, +9V or +18V. Swap the multimeter probes and the sign of the voltages will change.In all these cases, the multimeter completes a circuit when meaningful measurements are given.The primary and the secondary of a standard transformer are two separate electrical circuits. The fact that they are coupled magnetically has NO impact on their behaviour as separateelectricalcircuits (other than the transformer action).

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: December 12, 2018, 01:04:55 pm
content: Interesting point is that an electric current is rather like a hydraulic circuit, in that the 'fluid' (electrons) doesn't have to go through the whole length of the pipe (cable) to do work. Electrons going in one end, jostle the rest along, so some come out the other end. Not the same ones that went in, though. In fact, the to-and-fro movement of the electrons in a 50Hz AC circuit is quite short. So it's more like a vibration than a flow.https://en.wikipedia.org/wiki/Drift_velocity#Numerical_example

author: ArthurDent
author_rank: Super Contributor
timestamp: December 12, 2018, 02:43:20 pm
content: Hero999 – “In the distribution transformer scenario, the earth simply acts as a return conductor for the current path. If one side of the transformer isn't connected to earth, then there will be no return current path, so no current will flow, as is the case with the isolation transformer, when the other side isn't earthed.”That is absolute incorrect. If you had read my post #27 and post #30 you would know that the earth grounding of the neutral wire (that is the actual return conductor for the current path) is a safety feature and the earth isn’t relied on for carrying current.The reason for this is simple, the earth’s resitivity is a variable that varies wildly from fairly low in some areas to near infinity in rocky areas. Power distribution has to be known and controlled and utilities can’t have one customer’s lights dim or flickering while the neighbor’s lights are very bright. Two wires are used and ground is only a safety connection.This confusion is possibly caused by the misunderstanding of what the proper definition of ‘ground’ is. Here is a good description to help clear the confusion many have and is titled “An Introduction to Ground: Earth Ground, Common Ground, Analog Ground, and Digital Ground”.https://www.allaboutcircuits.com/technical-articles/an-introduction-to-ground/More specifically, in the case of power distribution being discussed, here is a document that describes a power distribution system and the ground connections used for safety.http://www.samlexamerica.com/support/documents/13007-0612_GroundedElectricalPowerDistribution.pdf1.0 CONDUCTORS FOR ELECTRICAL POWER DISTRIBUTIONFor single-phase transmission of AC power or DC power, two conductors are required that will be carrying the current. These are called the “current-carrying” conductors. A third conductor is used for grounding to prevent the build up of voltages that may result in undue hazards to the connected equipment or persons. This is called the “non current-carrying” conductor (will carry current only under ground fault conditions).2.0 GROUNDING TERMINOLOGYIn electronics, Ground is considered to be a common point, a point of zero potential and an infinite sink of electrons. In reality, most points we consider “grounded” are far from this perfect state. For purposes of electrical power transmission and distribution, the term “Grounded” indicates that one or more parts of the electrical system are connected to Earth, which is considered to have zero voltage or potential. In some areas, the term “earthing” is used instead of grounding. Connection to Earth may be made using Earth Electrodes like ground rods, buried wires, metallic pipes and other conductors in contact with the Earth.Earth Ground is described in the context of lightning protection, safety and operational performance and is used for the following functions:• Dissipate lightning strike energy in a manner that protects the surrounding area. Please see details under the Section 6 titled “Grounding System and Lightning / Ground Fault Protection”• Provide a low impedance connection back to the AC power mains Ground or Neutral to reliably clear fault conditions by blowing a fuse or by tripping a circuit breaker. A “fault condition” occurs when a live, ungrounded current carrying conductor comes in contact with the exposed metal parts of electrical equipment• Reduce the step potential gradient to safe levels• Form a natural sink for atmospheric and radiated noise• Provide an electrical antenna counterpoise.

author: nForce
author_rank: Frequent Contributor
timestamp: December 12, 2018, 03:31:24 pm
content: Quote from: Shock on December 12, 2018, 04:36:00 amSo electrons also alternate in the earth, when conducting in the loop?

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2018, 04:17:03 pm
content: Quote from: ArthurDent on December 12, 2018, 02:43:20 pmI'm aware of that. I did read your posts, but they refer to distribution transformer, rather than the original poster's set-up. The original poster is also in Europe and you refer to a US distribution system which is very different.Yes, the earth can act as a current return path. In most installations, the neutral conductor should only be intentionally used as a return current path, but the earth is a common return path, under fault conditions.Note that in some installations, the earth is indeed the sole path for the return current.https://en.wikipedia.org/wiki/Single-wire_earth_returnWhat I was attempting to explain was that in the case of an isolation transformer, the secondary is not connected to earth, therefore there is no return path for the current, via earth. If a meter were connected from anywhere on the secondary to earth, it will ideally read 0V. In the case of a distribution transformer, the secondary is connected to earth, so connecting a meter between the live and earth would result in the full supply voltage being read.

author: ArthurDent
author_rank: Super Contributor
timestamp: December 12, 2018, 08:54:24 pm
content: Post #40 Hero999 – “In thedistribution transformer scenario,the earth simply acts as a return conductor for the current path.”Post #46 Hero999 – “I did read your posts, but they refer todistribution transformer,rather than the original poster's set-up.As in your post #40 I am indeed talking about the distribution transformer scenario, as you were.Hero999 – “The original poster is also in Europe and you refer to a US distribution system which is very different.”Whether it’s the U.S. or Antarctica they still use wires to make a circuit. They do not rely on ground for a return path, only for safety. Back in the 1800s for telegraph (low current, low voltage) they had used it but switched. Rarely, in extremely high voltage long distance transmission, ground has been used as a return where the resistance of the ground and the current used is relatively small compared to the voltage. It has been tried and kind of works in these special applications. What was being discussed here was a medium voltage neighborhood distribution system where there is a physical wire used as the return and the ground is just part of the protective or safety circuit, not a return path as you indicated in your drawing. If the ground had any typical resistance at all you would never get full voltage to the load.Also in your drawing you show a 3-phase system which is mainly used for larger industrial applications and not residential. Even with that, the assumption of ground being the return path is incorrect. Here is how it is described:“Wye vs. Delta ConnectionsStarting with Wye, the connection consists of a total of five wires: (3) hot, (1) ground and (1) neutral. The configuration closely resembles a letter Y, with the neutral component connected at the middle, which is also where all the lines converge. It is at this point wherein the voltages are all equal.It is important to point out that the phase and line current are also considered to be equal. Multiplying the phase voltage by 1.732 (or square root of 3) will result in arriving at the voltage present between any of the two lines. In a Wye system, 120V can be measured from any hot wire to neutral. Additionally, 208V is measured from hot wire to hot wire. This is also the same for Delta configurations.By comparison, a Delta circuit inside a transformer appears as a triangle with equal sides, resulting in a closed path. In most cases, such wiring configurations do not have a neutral and is present on the secondary side of the transformer. The three phases are connected at every meeting point on the triangle. Moreover, a Delta system is equipped with four wires: (3) hot and (1) ground.”Here is an excellent video from ElectroBOOM that cover a lot of the points discussed, both 3-phase and single phase, and makes it clear. I recommend his videos because they also have entertainment value while showing the pitfalls of doing it wrong.As far as an isolation transformer working I see that you agree with my previous diagram from post #20 on that point.

author: IanB
author_rank: Super Contributor
timestamp: December 12, 2018, 09:04:23 pm
content: ArthurDent, you seem to have completely missed the flow of the thread and the context in which answers are being provided. It's also a bit silly and condescending for you to be trying to correct someone (Hero999) who obviously would not make the mistake you are implying.Let's be clear here:the earth acts as a return conductorfor the current flowing through a person standing on the ground who touches a hot wire. If the transformer neutral wasn't grounded at one or more points along the way, the earth would not act as a return path to that transformer.The misconception that seems to arise from the OP, as much as we can try to understand it, as that somehow the earth/ground is a magical 0 V sink for all voltages anywhere. This is not true. The ground only acts as a magical 0 V sink in as much as it acts as a return path for current back to a source that has a grounded neutral conductor.

author: Shock
author_rank: Super Contributor
timestamp: December 12, 2018, 09:11:04 pm
content: QuoteSo again defining the parameters, if there is AC voltage potential between two points then you're ready for current to flow. By completing the circuit or loop it will conduct and at the electron level they will alternate directions at the frequency of the mains.Like I said previously (unless you want to dive deep) just consider the earth to be a wire and if it's in circuit (and not isolated) electricity will conduct.

author: ArthurDent
author_rank: Super Contributor
timestamp: December 12, 2018, 10:35:51 pm
content: IanB – I don’t see where anything I have posted has been incorrect in my posts describing the proper use of grounding, getting shocked, or correct wiring principles. Please look at the last diagram I posted. The diagram of the Isolation transformer I show and showed previously in post #20 is exactly what Hero999 later showed to demonstrate whether you get shocked or not on primary or secondary of an isolation transformer-there is no disagreement there at all. I clearly show that touching ground and a hot wire referenced to ground is dangerous but that has nothing to do with the correct circuit return being a wire as universally used.It is the continual claiming that ground is used as the return current path for an electrical circuit, not just for safety, that I see as absolutely wrong. Hero999 even linked to rarely used single wire feed with earth return so using earth for the return circuit instead of a wire is what they meant and are presenting, not a grounded person accidentally touching a live wire and getting shocked. I have presented links to technical references to show that using a return wire and not relying on ground is universally accepted and I don’t see any disagreement with the references. I also don’t see what I have posted as condescending, instead saying that I’m silly and wrong without being able to explain how anything I have said is incorrect is what I see as somewhat condescending.So simply put it is this. I have continually said and shown that a person can get shocked by touching a live wire that is referenced to ground. Probably all of us have gotten shocked at one time or another. I have also said that in an proper electrical circuit there is a neutral wire that is also bonded to ground (why else would you use an isolation transformer?) and it is the wire, not the somewhat resistive ground, that is the circuit return path back to the source. How is that in any way incorrect?

author: IanB
author_rank: Super Contributor
timestamp: December 12, 2018, 10:47:04 pm
content: Nothing you have said is incorrect, but neither have a seen a "continual claiming that ground is used as the return path for an electrical circuit", so there seems no need to correct that. There has been perhaps a misleading drawing, and a reference to the rare use of single wire earth return, but this is not the same as "continual claiming". On the whole everyone just seems to be violently agreeing with each other.Anyway, I'll let it rest.

author: cdev
author_rank: Super Contributor
timestamp: December 12, 2018, 11:06:32 pm
content: Some power systems in Australia, New Zealand and elsewhere use ground for returns. Because of the lower cost. But not many.https://en.wikipedia.org/wiki/Single-wire_earth_returnThere is a history of power systems in Australia on the IEEE site that goes into a lot of detail about the systems' evolution there.

author: Zero999
author_rank: Super Contributor
timestamp: December 13, 2018, 01:33:15 pm
content: Quote from: ArthurDent on December 12, 2018, 08:54:24 pmSigh. . .I've never said that the earth is routinely used as a path for the return current on low voltage domestic installations. You've has completely missed the point and taken the thread off topic by being the first to mention the distribution transformer. I apologise for making matters worse by replying to you and mentioning single wire earth return, which as you've correctly stated, is only applicable to high and medium voltage distribution.QuoteYou're wrong: a three phase system is routinely used both for domestic and industrial distribution in Europe. Single phase distribution transformers are very rare. We don't have many pole mounted transformers either. One three phase transformer will typically serve a whole street or block. In a domestic, single phase setting, every third house in a street might be on the same phase. For example in a street with 9 houses, numbers 1, 4 & 7 might be on phase 1, houses 2, 5 and 8 on phase 2 and houses 3, 6 and 9 on phase 3.QuoteYes, it illustrates why a meter will read 230V between live and earth, when connected directly to the mains but will read 0V between the secondary and earth on an isolation transformer. It demonstrates that when the transformer's secondary is earthed, there's a possible return current path through the earth, yet there is none when the secondary is floating. The ground current path is shown between the meter and transformer's earth connection. I repeat it's not supposed to illustrate the correct way to measure the mains, just that the full mains voltage will be present if one of the meter's probes is earthed and the other connected to live. Note that the isolation transformer's primary is connected between live and neutral, not earth!

author: nForce
author_rank: Frequent Contributor
timestamp: December 13, 2018, 03:43:27 pm
content: Quote from: Shock on December 12, 2018, 09:11:04 pmIf we take an ampermeter and connect it in the earth let say 10 meters below ground, then we will measure AC current if we have single wire earth return power system?

author: IanB
author_rank: Super Contributor
timestamp: December 13, 2018, 06:57:28 pm
content: Quote from: nForce on December 13, 2018, 03:43:27 pmIf you have an ammeter, it has two wires. You are going to stick one wire into the ground? Where are you going to attach the other wire?Where does the single wire earth return transmission system come into the picture? Is it part of the test or is it far away?

author: nForce
author_rank: Frequent Contributor
timestamp: December 13, 2018, 07:14:12 pm
content: We should be in a cave, below ground. One wire goes on one side of the wall the other goes on the other wall. Because of power grid and power stations nearby or above ground, we should measure AC current.

author: IanB
author_rank: Super Contributor
timestamp: December 13, 2018, 07:20:51 pm
content: Yes, those are called stray currents. They always exist. Occasionally they are harmful and can cause galvanic corrosion on buried metal objects. But mostly they will be negligible.Stray ground currents are why the London Underground has two insulated power rails and does not use the running rails to return the traction current.

author: Zero999
author_rank: Super Contributor
timestamp: December 13, 2018, 07:27:55 pm
content: Quote from: nForce on December 13, 2018, 07:14:12 pmLike this?Yes, some of the current will flow round the cave and some through the meter.As mentioned above, they can cause problems, if the system isn't implemented correctly, especially if the ground isn't conductive enough, resulting in a high potential difference gradient, near the transformer.

author: nForce
author_rank: Frequent Contributor
timestamp: December 13, 2018, 08:13:28 pm
content: Why didn't you tell me about this at the beginning?It took 3 pages of this topic just to explain to some beginner that yes an AC current flows in the earth because of the power grid and it's called a stray current.I didn't know about this, because in convential circuits there is always a return wire. The earth does not look like a wire.

author: Zero999
author_rank: Super Contributor
timestamp: December 13, 2018, 08:35:03 pm
content: Quote from: nForce on December 13, 2018, 08:13:28 pmWhy did it take three pages for you to make it clear what you were asking?Go back to your original post.Quote from: nForce on December 02, 2018, 05:45:22 pmHow was anyone supposed to know you were asking about stray currents? All you talk about is an isolation transformer and electrons?Consider being more descriptive and posting some schematics, rather than a few cryptic statements.

author: IanB
author_rank: Super Contributor
timestamp: December 13, 2018, 10:07:08 pm
content: Quote from: nForce on December 13, 2018, 08:13:28 pmExcept...Quote from: IanB on December 11, 2018, 09:08:39 pm

author: Brumby
author_rank: Unknown
timestamp: December 13, 2018, 10:42:55 pm
content: Quote from: nForce on December 13, 2018, 08:13:28 pmAs already stated - but I will rephrase: To getgood answersyou need to askgood questions. We aren't mind readers and if all we know of you is what is in one or two posts, then we need you to help us understand exactly what it is you are asking.QuoteThat is a rather simplified statement and not properly qualified.Stray currents are unintentional, but occur as a function of the physics of a situation. Fault currents are caused by a safety earth at the customer's location having a path to Live. In a single wire earth return system the current through the ground in normal operation is, by design, the full current of the load.QuoteYou are being too literal. The Earth may not look like a wire, but it certainly DOES look like a conductor when you look at it properly - and this is the trick in understanding a few things with electricity ... what is a conductor may not seem like it should be. Breathe on a high impedance circuit and the moisture that condenses from your breath will affect that circuit. The human body does not look like a wire, but it conducts electricity - just hold the probes of your multimeter while on a resistance range. This is also why electrocution is a risk.

author: nForce
author_rank: Frequent Contributor
timestamp: December 14, 2018, 06:17:31 pm
content: Ok then, thanks for your help.I have one question: Our electrical outlets at home are voltage sources. Because the voltage is constant and the current varies. But what are limitations that outlets would be current sources?

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2018, 07:28:10 pm
content: Quote from: nForce on December 14, 2018, 06:17:31 pmWith a constant current source, the loads would need to be connected in series, for the current to be constant. If the loads were connected in parallel, the current would vary, depending on the loads, similar to connecting devices designed for operation off a constant voltage source in series. The open circuit voltage of a constant current source is theoretically infinite, with the power being proportional to the resistance, so open circuits need to be avoided and rather than disconnecting a load, it should be replaced with a short circuit, which will result an relatively little power dissipation.

author: rstofer
author_rank: Super Contributor
timestamp: December 14, 2018, 07:32:06 pm
content: This question of transformer return current is easy: The electrons have to complete a circuit back to the transformer from which they came.Utility transformers have one side grounded and called the Neutral. Every ground fault has to get back to this neutral point to complete the circuit and it does it by traveling through some path (perhaps dirt) back through the point where the neutral was grounded.This is actually easier to think about when you consider a 3 phase transformer in a Y connection with the center point grounded. A phase to ground fault MUST get back to that center point by any means necessary. The resistance of the round trip path partially determines how much current will flow.On some higher voltage installations, that midpoint is not grounded with a piece of wire but rather with a resistor. This is done to limit the available fault current and is quite common in motor control centers for large motors.On other, older, installations, the mid-point isn't grounded at all. This means the first ground fault won't trip the circuit, it just guarantees that the other phases are at some high voltage relative to ground. We used to have neon lights on the phases that, when one went out, indicated the presence of a ground fault somewhere in the system. This type of installation is no longer in use. The reason is that the second ground fault (assuming the first wasn't cleared) is now a phase-to-phase fault and the fault current is much higher.

author: rstofer
author_rank: Super Contributor
timestamp: December 14, 2018, 07:41:16 pm
content: I think you might find some 1-wire medium voltage installations in rural areas of the US. Apparently, New Zealand and Australia are also using 1-wire distribution schemes for rural electrification:http://www.nzdl.org/gsdlmod?e=d-00000-00---off-0hdl--00-0----0-10-0---0---0direct-10---4-------0-1l--11-en-50---20-about---00-0-1-00-0--4----0-0-11-10-0utfZz-8-00&a=d&cl=CL1.18&d=HASH01a419b0834cf9dbfb8643d0.9.fc

author: rstofer
author_rank: Super Contributor
timestamp: December 14, 2018, 07:45:01 pm
content: Four simple equations would suffice to describe all of these situations:https://en.wikipedia.org/wiki/Maxwell%27s_equations

author: Brumby
author_rank: Unknown
timestamp: December 15, 2018, 01:48:20 am
content: I think a lot of that might be a bit too advanced for the OP just yet. We are just getting sorted out on the fundamentals of current paths.Quote from: nForce on December 14, 2018, 06:17:31 pmIf we are talking about household general purpose mains outlets, the big issue here is practicality.For a constant voltage supply, you present a given voltage at the outlet and the devices that get plugged in will only draw the current they need. Aside from efficiency factors, there is no inherent waste of power.For a constant current supply, the source will attempt to push the same amount of current through a device, irrespective of its needs. As a result, the voltage presented will rise or fall as the case may be in order to maintain that constant current. For a known load having a specific current requirement that does not change during operation, this could work - but for a constant current supply that wants to supply more current than needed, there will need to be a dummy load to "bleed off" the extra current that the load does not want. Very wasteful.If there is any fluctuation in the current requirements, then the source is going to "throttle back" when current demand rises and "open up the throttle" when it falls in order to keep the current constant. This becomes a practical problem in many cases - such as starting and running an electric motor. Have the supply provide the current for running and it will not provide the necessary current to get it going from a standing start. Have the supply provide the current for starting and it will want to continue to supply that same current during the running period. The voltage response will skyrocket.Another problem would be with a power board. Have a general purpose outlet provide a set amount of current and you plug in one device. Let's assume that this setup works because the current supplied matches the current required. Now plug in a second device with the same requirements (for simplicity). That constant amount of current will be shared between two devices, so each will only get half. Most devices will not work properly, if at all, in such a situation. Fill a 4 way power board and it gets even worse.But more than that the question has to be asked: Where do we place the constant current regulator? Do we place one at each power point in a house? Do we place it on each power circuit in a house? Do we place it on the mains supply to a house? Do we place it on the pole transformer that services several houses? With the variations in loads, there is no real answer you can give here. It's the power board problem all over again - at a bigger scale.The only way around this is to have the constant current supply programmed to adapt to each load independently. This would require the electronics to provide a constant current at every distribution point - and that electronics would need to be cheap, reliable and able to handle variations across a wide range of devices from a night light to a vacuum cleaner. There would also need to be a communications protocol between the device and the supply so that the supply knows what current to deliver.This is all getting way too messy.What is needed is a system where the devices get to choose what current they require, without having to go through some convoluted process. A system where adding extra devices through a power board, for example, just works. A system that is simple, reliable and inexpensive.Such a system does exist: Constant voltage supplies. Present a given voltage and the device will only take the current it needs. Paralleling loads is not a problem - so long as the capacity of the circuit is not exceeded. There is no need for electronics in basic systems. Reliability is high and costs are low.The only time constant current supplies are generally beneficial is where you have a single functional unit - such as an LED - that operates more effectively and safely with such a supply. This is one of the key functions of an LED driver.

author: IanB
author_rank: Super Contributor
timestamp: December 15, 2018, 02:29:09 am
content: There is one case where constant current circuits have been used, and that is lighting circuits. In particular, street lighting (in the past), and airfield lighting (maybe still today).In the case of street lighting there would be one long loop containing many lamps in series, and a constant current power supply designed for a specified current with a large compliance voltage (maybe 1000's of volts). Each lamp used in such a circuit is specified with two numbers: the rated current, and the output lumens.Because the lamps are in series, there are certain challenges with this system. For instance, if a bulb blows it will break the circuit and all lights will go out. Unfortunately, if a bulb blows the full compliance voltage will also appear across the break in the circuit, leading to a bright, hot arc with lots of smoke, flames and molten metal. Therefore each bulb has a special bypass link that fuses and shorts out the bulb if it blows, preventing the arc and keeping the other bulbs lit.Why might such an awkward arrangement be used? Presumably because it lowers the wiring cost and also because keeps all bulbs at the specified brightness. There is no issue of a bulb at the end of a long wiring run being dim due to voltage drop in the cable. Every lamp will give the specified brightness no matter how far away from the power source. This might be an advantage on an airfield where every lamp must perform exactly to specification.The wiring cost would be lower because 50 bulbs use the same current as 1 bulb, and therefore you don't need fatter cables to carry 50x the current.

author: Brumby
author_rank: Unknown
timestamp: December 15, 2018, 03:28:47 am
content: That's a neat idea.The point of note is that the load requires a particular current and the supply is made to provide exactly that one specific value of current. There is no variation in the load current requirement of the lighting system.The high compliance voltage, however, will require attention to insulation requirements.