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In an AC supply which is not earth grounded, I see a small amount of current flowing to ground from both wires of the supply (live wire has higher) even though the circuit is incomplete.

It is due to capacitive coupling? Can anyone explain it?

And why do I get more current from the live wire to ground than the neutral to ground even though neither are grounded?

Here is a photo for context.

enter image description here

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I see where the diagram is attempting an isolated system. However, that is wrong. The system is not isolated: it is bonded to earth. You can't miss the neutral-ground bond in the diagram; it looks like a human!

What can be said about isolated systems is "the first ground fault is free". It doesn't do any harm, because the systems aren't connected except for that, and that does not complete a circuit. This first ground fault effectively becomes the neutral-ground bond. That can work in an industrial setting where there is staff maintenance doing frequent testing to detect that "first ground fault" and fix it before there is a second.

In your ideal diagram, the human is not shocked because he's the only ground fault. However if there's a second ground fault, he's dead.

Capacitive coupling doesn't tingle. What's more, capacitive coupling can't occur at all unless wires are running in parallel for some distance. You are trying to use "capacitive coupling" as a catch-all for all unexplained current and that's just wrong.

We often get people on diy.se who say they've replaced their GFCI 3 times and can't understand why it trips when they use their expresso machine. Duh, but they are too vain to accept their precious little appliance could possibly have a ground fault. Don't fall into that trap.

Newsflash: Your machine isn't isolated. If it's supposed to be isolated, then it has a ground fault. That's why it shocks you when you touch it.

The reason more current flows hot-earth than "neutral"-earth is that the ground fault is closer to "neutral" than to hot.

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  • \$\begingroup\$ What do you mean by "ground fault" and "the first ground fault is free"? Also, how the system he refered to, shall be isolated? \$\endgroup\$ – DannyS Jun 3 '18 at 8:44
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    \$\begingroup\$ @dannys A ground fault is when an electrical system connects to ground in an unexpected and unintended way. m.littelfuse.com/products/protection-relays-and-controls/… \$\endgroup\$ – Harper Jun 3 '18 at 11:31
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    \$\begingroup\$ @DannyS: Read "the first ground fault is free" as meaning there is no penalty (electric shock) as it just neutralises one of the isolated supply lines. Now if you touch the other wire there will be a penalty (electric shock). \$\endgroup\$ – Transistor Jun 3 '18 at 18:19
  • \$\begingroup\$ @Transistor, but if you touch the second line while still keeping the first one right? So in turn you happen to be the ground. \$\endgroup\$ – DannyS Jun 3 '18 at 18:57
  • \$\begingroup\$ It could be you, someone else, moisture, faulty wiring or anything else you can think of causing the first fault. \$\endgroup\$ – Transistor Jun 3 '18 at 20:03
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Yes, assuming there is not some other physical connection or fault causing the ground loop, you most likely have some level of capacitive coupling to ground from the mains.

This could be from any number of wires or other elements of your equipment being too close to a grounded object. This could also be coming from your isolation transformer itself.

The fact that you're getting a higher reading on your live vs your neutral leads me to suspect it's due to your isolation transformer. Your mains power is most likely earth referenced with a low resistance path between neutral and ground. If your transformer is wired in phase, and the coupling is at the transformer, the proportionally reduced difference in potential will translate to your "isolated" secondary.

The truth is that not all isolation is made equal. Even between transformers from the same manufacturer both marketed for isolation, there are significant differences in quality and coupling.

One major difference you should check for is if your isolation transformer has a Faraday shield. This helps minimize capacitive coupling and leakage current, and is found in higher grade isolation transformers.

This difference can be seen in two isolation transformer datasheet schematics:

N-76U without shield

enter image description here

N-48X with shield.

enter image description here

Even with shielding, it's still not bullet proof. The marketing pitch from a higher end medical grade isolation transformer claims:

Faraday shield reduces the cumulative leakage current of the Isolator and connected equipment to levels less than 100 microamps.

The goal of most power isolation is not to be perfect, but to reduce the potential to "safe" levels in the event of someone accidentally touching a live line. You're still not supposed to touch it!

The take away is that "isolated" does not necessarily mean perfect or zero current. As long as your source is coming from something that is ground referenced (mains) and/or your AC lines have anything grounded anywhere near their magnetic field, you will have some potential for coupling and current flow. Please stop touching power lines.

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  • \$\begingroup\$ First of all I am using an DC to AC inverter as the isolated supply. There is no solid connection to ground with any of wire. The rest is same: I get slight tingling on live wire but nothing on neutral. Neutral to Live is 250VAC RMS \$\endgroup\$ – ObsessionWithElectricity Jun 4 '18 at 18:46
  • \$\begingroup\$ My inverter is sitting on a wooden desk. \$\endgroup\$ – ObsessionWithElectricity Jun 4 '18 at 18:48
  • \$\begingroup\$ @ObsessionWithElectricity Sorry. Skimmed over the comments. In the future, please update valuable information to to question rather than placing in comments. A pic of your equipment would help too. Winny already pointed you to a question that most likely has the answer you need. \$\endgroup\$ – Phil C Jun 5 '18 at 1:37
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In an AC supply which is not earth grounded, I see a small amount of current flowing to ground from both wires of the supply (live wire has higher) even though the circuit is incomplete.

It is due to capacitive coupling? Can anyone explain it?

The circuit looks like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Air is just a gap, anytime you have two conductors and a gap, you have capacitance and if you have AC, current can flow. You'll still have some current flow and a mild shock. The values of this simple circuit depend on a wide variety of conditions, but may stand in for an approximation.

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  • \$\begingroup\$ But the question is about current to earth from an 'isolated' supply. You've missed that bit. \$\endgroup\$ – Transistor Jun 5 '18 at 21:45
  • \$\begingroup\$ It doesn't matter what it is, if your touching an isolated supply or a regular supply and standing on a highly insulated surface, the return current will go through air and turn you into an antenna. \$\endgroup\$ – Voltage Spike Jun 5 '18 at 22:03
  • \$\begingroup\$ I don't know that antenna is the correct word. In any case, your answer doesn't make your point clear. \$\endgroup\$ – Transistor Jun 5 '18 at 23:04

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