A short produced from an isolated power supply would like to travel to ground? - Page 1
Forum section: electronics / projects

author: rajhlinux
author_rank: Contributor
timestamp: May 24, 2021, 08:41:32 am
content: Hello everyone my fellow electrical nerds, hope all is doing swell and well.Alright so I am on a debate with this person on YouTube, he thinks he is super duper smart and knows it all.He is showing off some weird looking isolated power supply.I told him that a short produced from the isolated power supply would rather like to travel to ground, is this a valid statement?Would a short produced from any kind of isolated power supply would like to travel to ground?From what I learned, high potential energy will always like to travel to least resistance path to ground.It also means that the short produced would also like to travel to the user's body and to ground, is this also a correct statement?If there is no least resistance to ground other than the user's body whos feet is touching ground, this also means the short produced from the isolated power supply would like to travel to the user's body to ground? This is correct, right?Edit: I have attached a diagram to explain the scenario.Thanks.

author: richard.cs
author_rank: Super Contributor
timestamp: May 24, 2021, 08:56:57 am
content: No these are not correct statements. There's nothing special about ground, other than that it is somewhat conductive and that many common power supplies are in some way connected to ground. An isolated supply is not connected to ground, so no complete circuit involving ground can be formed via a single additional connection. Therefore no current will flow to ground.

author: Gyro
author_rank: Super Contributor
timestamp: May 24, 2021, 09:01:39 am
content: It's difficult to understand what you mean by an "isolated power supply".Sorry to disappoint you, but if it is an isolated power supply in the traditional sense (a battery, or mains psu with primary - secondary isolation), then the preferred short path would be between its terminals.Any current to ground would be at worst, a little leakage current as there is no defined return path for the current.

author: rajhlinux
author_rank: Contributor
timestamp: May 24, 2021, 09:06:14 am
content: Thanks for your reply, so where does this short spark would rather like to travel to? I know that the current from the isolated power supply would not travel to ground but it's confusing about an arc blast or arc spark produced from a short of an isolated power supply, where will the arc blast or arc spark energy would rather travel to?

author: rajhlinux
author_rank: Contributor
timestamp: May 24, 2021, 09:10:51 am
content: The isolated power supply in question is a transformer.So an arc spark produced from a high voltage with high amperage isolated transformer would really try to travel through it's isolated conductors and would not rather like to travel to a conductor which is earth ground?

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2021, 10:36:01 am
content: The path of least resistance to the current will be across the secondary of the transformer. A relatively small current will flow, if one side of the secondary is connected to earth ground, because there will be some capacitive coupling between the secondary, earth, the prirmary and power supply. The current will depend on the frequency, voltage and the capacitance.

author: rajhlinux
author_rank: Contributor
timestamp: May 24, 2021, 12:10:39 pm
content: So what about static electricity? Why do people of professional work places needs to mandatory discharge their body from any static electricity?The electrons from static electricity is no different than that of the electrons of the arc spark produced from a short of an isolated power.Static electricity simply means when high potential of energy is sitting at one spot of an object waiting to be discharged to a lower potential energy object, such as ground which it loves to go to.So with this in mind, doesn't the same fundamental principle applies to high potential energy of a spark arc? Is a spark arc completely different than a static arc ? Both are the same thing, both are high energies of electricity which now the circuit does not apply anymore and these spark arc energies are same as static energy which would rather choose to flow through a conductor to ground.What is the difference between a spark arc from an isolated power supply and a spark arc from static electricity? We all have seen spark arcs from both static and non static forms, isn't the spark arc behaves the same for both static and non static electricity?So I need to believe that a spark arc from static electricity would like to go to a conductor to ground but a spark arc from an AC isolated power supply would rather not choose to travel to a conductor to ground?

author: Zero999
author_rank: Super Contributor
timestamp: May 24, 2021, 12:47:05 pm
content: This should be in the beginners section. . .Static electricity is a capacitive discharge. All objects, have a property known as self-capacitance, as opposed to mutual capacitance, which occurs between two conductors. The earth, has a self-capacitance of 710 µF, which is huge, compared to that of any object on earth, so can be ignored. An object becomes statically charged, when it has an excess, or deficit, of electrons, with respect to the surroundings, which in this case is earth. The self-capacitance depends on the physical dimensions of the object.https://en.wikipedia.org/wiki/Capacitance#Self_capacitanceTo complicate matters further, all objects have a mutual capacitance, with respect to earth, as well as self-capacitance. The mutual capacitance depends on the objects proximity and area, with respect to earth.In the case of a DC power supply, such as a battery. If we connect the cathode to the earth, a tiny current might briefly flow, if the battery has a small static charge, then it will just sit there and no current will flow. The battery will have a certain capacitance, so if the anode is now connected to earth, a brief current will flow again. If we connect the battery to a high frequency oscillator and connect the output to earth, a current will flow through the oscillator's output, which will depend on the frequency, the capacticance of the battery and resistance to earth.

author: IanB
author_rank: Super Contributor
timestamp: May 24, 2021, 01:24:05 pm
content: Quote from: rajhlinux on May 24, 2021, 12:10:39 pmThink about voltage as a difference in potential between two conductors, and think about a difference in potential as a movement of electrons (charge) from one place to another.In general terms, static electricity occurs when electrons have moved between the earth ("ground") and some other object like a person wearing insulated shoes. Since the electric charge came from the earth, the earth is where it wants to return to balance things out. There is a potential difference between the person and their surroundings. (It can work both ways, with positive charge or negative charge.)In the case of an isolated power supply, the electrons have been moved from one terminal of the power supply to the other terminal. There is a potential difference between the two terminals. Therefore the electrons want to go back to the terminal from which they came. One terminal has an excess of electrons and the other terminal has a deficiency of electrons. The earth or "ground" is not in this picture (it is not in any closed circuit involving the power supply) and therefore no current will try to flow to ground (not enough that you can measure).The whole point of an isolated power supply is that it is isolated ("insulated") from its surroundings. This isolation prevents any tendency of electric current to flow from the power supply to outside objects.

author: rajhlinux
author_rank: Contributor
timestamp: May 25, 2021, 12:04:39 am
content: Alright this is what I have in mind and would like to know what the answer is:https://drive.google.com/file/d/1Fel3GFr19U0_L3fLTRh_YsLbWMuaSEea/view?usp=sharing

author: IanB
author_rank: Super Contributor
timestamp: May 25, 2021, 03:03:27 am
content: QuoteHere, you are starting with an incorrect premise. The above quote isnotanything said by the electrical engineering community.If you start with an incorrect premise, you will reach incorrect conclusions.Remember what a "short" is: a "short" is an abbreviation for a short circuit. What does a short circuit mean? It means that where the current is intended to take a certain path round the circuit, it unintentionally takes a shortcut and completes the circuit through the wrong path. This doesn't change the start and end of the circuit at all, only the path taken.In your diagram, points "A" and "B" are source and destination in the circuit. Point "C" does not represent a shorter path from "A" to "B", so in no way can can point "C" be considered a short circuit. What may happen in some circumstances is that a spark travels from "A" to "C", and from "C" to "B" all at the same time. In this case, point "C" is merely a stopover on the journey. But if the spark originated at "A" then "B" is always the destination.

author: rajhlinux
author_rank: Contributor
timestamp: May 25, 2021, 08:57:34 am
content: Quote from: IanB on May 25, 2021, 03:03:27 amInteresting, so a spark discharge is not the same form as a spark from a static discharge?Because I thought the same laws of physics apply between a spark arc discharged from a dynamic current source and a spark arc discharged from a static current source.I hope we are clear that the discharge of a spark arc is dynamic from both sources of AC current and static current.It just does not make any viable sense down to the electrons, the name of science, why a spark arc from a dynamic source would behave differently than that of a spark arc from a static source.I would like to read valid documented information about this, or perhaps conduct this test myself...Static current is also an isolated current source, there's nothing of a static current source that has any type of link or connection to earth ground, yet we see electronic scientists/engineers and fire rocket builders discharge all the static current from their body down to a grounded conductor by touching the grounded conductor. Now I myself, have witnessed discharge of spark from my body down to a grounded conductor.Here in Wikipedia, it states that: "and even see the spark as the excess charge is neutralized when brought close to a large electrical conductor (for example, a path to ground)"https://en.wikipedia.org/wiki/Static_electricityhttps://en.wikipedia.org/wiki/Static_electricitySo to be clear, when a spark arc is created with anisolatedAC high voltage power supply, the spark arch discharged from a high potential conductor point "A" would rather choose to be "neutralized" of a lower potential at it's original destination (point "B") than a grounded conductor near by (point "C")?If this is true, is it because point "B" has a much lower potential than the grounded conductor point "C"?Which conductor has a lower potential during the spark arc event, point "B" or point "C" ?

author: IanB
author_rank: Super Contributor
timestamp: May 25, 2021, 01:30:01 pm
content: Quote from: rajhlinux on May 25, 2021, 08:57:34 amI didn't say that. A spark is a spark. Any differences might be due to energy, shape, duration.QuoteThey do.QuoteIt doesn't behave differently.QuoteIt's not isolated. Ground is part of the circuit. A static charge is a charge difference relative to ground potential. Allowing a static discharge to flow back to ground is completing the circuit.QuotePoint "B", naturally. Point "C" is neutral, since it is not part of the circuit.

author: Zero999
author_rank: Super Contributor
timestamp: May 25, 2021, 07:10:37 pm
content: There seems to be a fundamental misunderstanding about the difference between a circuit being powered by a battery and static electricity.Imagine you have a fan, in a duct, forming a circuit, with one side of the fan connected to the other. The air can move around in a circuit and even drive a load such as a turbine. One side of the fan will have a slightly higher pressure, than the other. This is analogous to a battery, with the higher pressure region being at a more negative voltage, than the lower pressure region. Now think of a sealed chamber with a slightly higher, or lower pressure, than the ambient barometric pressure. If the seal is broken, the air will move, until the pressure in the chamber is equal to barometric. This is analogous to an electrostatic discharge.There needs to be a complete circuit for a current to flow continuously. A current can briefly flow, due to an imbalance of charges between an object and its surroundings, but it will decay to zero, once an equilibrium has been reached.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 25, 2021, 07:42:08 pm
content: In static discharge, the charged element (say person) has a moderate (100s of pF) capacitance to (usually) ground. That capacitance completes the circuit.Likewise, the chassis of an "isolated" electrical apparatus still has some capacitance to ground. That capacitance can be charged up and cause a spark discharge. However, this is largely different from the potential between two terminals of the device such as a high voltage power supply. That arc can only return to the device itself.This is basically applications of Kirchhoff's current law which requires that current always flows in loops. If you are ever puzzled about something like this try to find the current loop and that will usually answer your question.

author: james_s
author_rank: Super Contributor
timestamp: May 25, 2021, 07:49:40 pm
content: What do you mean by "ground"? It is typical to refer to the node of a circuit that connects to the negative terminal of the power supply as "ground", but that is not the same as an earth ground that is literally connected to the earth. Electricity from the utility only seeks a path to earth because it is deliberately referenced to earth, the neutral bus in your panel is at some point physically connected to a conductive rod driven into the earth. An isolated supply implies one that does not have either side tied to ground, so current from that supply will not try to flow to ground. All you have is a potential difference between two points, and all the current can try to do is take the path of least resistance between the two points of different potential.

author: Zero999
author_rank: Super Contributor
timestamp: May 25, 2021, 09:25:17 pm
content: In the context of electrostatics, earth, or ground, is a very large object, with respect to the charged object in question. I most cases it's simply planet earth.

author: IanB
author_rank: Super Contributor
timestamp: May 25, 2021, 09:43:23 pm
content: Quote from: rajhlinux on May 25, 2021, 08:57:34 amYou might care to look closely (for example) at how a Van de Graaff generator works. It is essentially a constant current source that feeds a small current from the base (ground reference) up a post carried on a belt and deposits it into the electrode at the top. When the electrode eventually discharges, the charge flows back to ground, thus completing the circuit. You could in fact connect a galvanometer from the top electrode back to ground and observe a small continuous current when the generator is running.

author: rajhlinux
author_rank: Contributor
timestamp: May 27, 2021, 01:53:54 am
content: Quote from: james_s on May 25, 2021, 07:49:40 pmWhen I mention about ground for this question I am referring to a conductor connected to earth's soil at least 7 feet deep into the soil. All I wanted to know is the behavior of the spark not the current. I just wanted to know where would the spark would mostly likely to go as it discharges and ionizes in the air, it always travels to a point which has the least potential. So the question is only based about the spark in relation to the scenario. I was assuming all sparks would travel to ground. Because my assumption was that once a spark was generated, it now behaves as a spark like any other type of spark and will always want to go to earth's ground since earth's ground has the least potential energy.But it seems this is not the case and seems like a spark do indeed behaves differently with respect to it's electrical current scenario. It seems to me from everyone's statement that a spark behaves like a current and will always want to complete the circuit. So in essence, there needs to be more clarification in electrical engineering and science texts books that a spark DOES NOT always NATURALLY chooses to go to earth's ground, a spark would rather choose to complete it's circuit.It does make sense that a spark does not choose to take the path to ground and rather take the path to complete the circuit. Simply because the spark is high potential energy and would rather go to least potential energy. So if the voltage was 50K AC Volts, a spark just discharges and shoots out from the flowing live source conductor, so now it seems this spark discharge would have 50K positive potential energy and would like to go to 50K negative potential energy, I hope this is the correct way of viewing it...It's pretty obvious that 50K negative potential energy is way much more lower potential energy than earth's ground which is naturally set at 0 volts, so hence the spark discharge would rather complete it's circuit and travel to the source conductor which is 50K negative AC volts which makes logical sense and not take the path to earth's ground which is always 0 volts. This scenario is off course to my original question with respect to the isolated high voltage AC power supply based on a transformer. This is all about the spark's discharge behavior and nothing more.They see seeing is believing and would like to conduct this test... although it now seems it's pointless.

author: rajhlinux
author_rank: Contributor
timestamp: May 27, 2021, 01:55:24 am
content: Quote from: Zero999 on May 25, 2021, 09:25:17 pmSo are you saying that the discharge of the spark would flow to earth's ground?

author: james_s
author_rank: Super Contributor
timestamp: May 27, 2021, 02:03:11 am
content: Quote from: rajhlinux on May 27, 2021, 01:53:54 amOk so you're talking about earth ground. As I mentioned before, the reason electricity from the wall tries to find a path to ground is that one side of the generator and distribution network is intentionally connected to ground via metal rods driven into the earth. There are good reasons for why this is done but they aren't really important here, the important thing is that it is set up that way, it is NOT an isolated supply.If you have an isolated supply, the term "isolated" implies that it does NOT have one side connected to ground. A battery for example gives you two wires with a potential across them, you could touch either one of these wires while standing barefoot on wet ground and no current will flow because the supply is isolated and there is not a complete circuit. If you connected one of the wires to a metal rod driven into the earth, stood on the ground and touched the other wire then current would flow, whether you get a shock depends on the voltage of the battery.

author: IanB
author_rank: Super Contributor
timestamp: May 27, 2021, 03:49:34 am
content: Quote from: rajhlinux on May 27, 2021, 01:53:54 amYou seem to have this idea that a spark is different from an electrical current. But it is not. A spark is just an electrical current flowing through the air.An electrical current flowing through the air of short duration is called a spark. An electrical current flowing through the air of long duration is called an arc. In both cases the air is a conductor of electric current just like a wire. In fact the conductive path of an electric arc has almost zero resistance, and this can make arc discharges extremely dangerous.You seem to have this idea that a spark will just shoot out from a high voltage electrode and then decide where to go. But this is not normally the case. Normally you have to bring some other conductor close by so the spark will have somewhere to jump to. You must have seen this in any number of electrostatic demonstrations?Also, I do not believe any engineering texts will say a spark naturally chooses to go to earth's ground. There is no reason why they would say this as it is wrong. If you have such a text, please give a reference to it.

author: Zero999
author_rank: Super Contributor
timestamp: May 27, 2021, 08:08:57 am
content: Quote from: rajhlinux on May 27, 2021, 01:55:24 amYes, if you have an object, carrying a static charge and bring it near the earth's ground, a spark will jump towards ground. The object will then be at the same potential as the earth and no more current will flow.The reason why this doesn't happen with an isolated power supply, such as a battery, is the entire circuit is at the same potential as the earth.Going back to thepneumatic analogy. The battery is equivalent to the fan in the duct. It shifts electrons round in a circuit. If you were to connect one side of the battery to earth, no current would flow to earth, the same as drilling a hole in the duct, wouldn't cause a net flow of air, in or out of the duct, after the pressure has equalised. A static discharge is like a sealed chamber with a higher, or lower ambient pressure, than the surrounding atmosphere (ground). Once a connection is made, electrons/gas molecules move and the charge/pressure equalises.