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Electrician apprentice here.

Earth potential is said to be 0 volts.

Let’s say I have a star-configured transformer which supplies me with voltage. Let’s also say that the transformer is perfectly balanced and that the 0 potential center point is grounded to earth.

Imagine then that we have some sort of electrical appliance downstream. That particular appliance is grounded as well, by use of say a copper rod.

So a TT system.

If a ground fault suddenly occurs at my electrical appliance the current will suddenly run in my local grounded copper rod.

However, I am told that this happens only because the circuit is completed by the grounded transformer on the supply side.

But if the earth potential is already zero, why would the current move between the local ground rod and the supply ground rod? Wouldn’t earth merely be what closes the circuit? I mean, after all a closed circuit isn’t necessary in order for a current to exist. Only a potential difference is, right?

David
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In a TT system the impedance between supply ground rod and local load ground rod is never going to be zero ohms so there will always be a voltage between the two when current flows. With a live fault to local load ground, current will circulate back to the supply star-point via earth.

There is nothing magical about ground that allows it to soak up current - a closed loop is needed for current to flow and the ground/earth is part of that loop.

Andy aka
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  • If a closed circuit is needed in order for current to flow, then how does lightning strike earth? – David Jul 03 '19 at 13:53
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    @David Lightning is a static discharge. In that case you can think of the ground and the "sky" as two capacitor plates which is producing an arc when a breakdown voltage is reached. Closed circuit is needed for continuous current. – Eugene Sh. Jul 03 '19 at 13:57
  • Also, I still don’t understand why the current will circulate back to the star-point via earth. Is it because earth potential in this case is not 0. Is it because earth potential instead is star-point potential + Z multiplied with A of the distance between the two grounded rods (impedance times current)? – David Jul 03 '19 at 14:01
  • @David there is nothing magical about earth - it is a conductor and can be regarded as a wire if that helps. Would you have the same question why current can flow in a perfect super-conductor when there is absolutely no volt drop or potential? – Andy aka Jul 03 '19 at 14:06
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But if the earth potential is already zero, why would the current move between the local ground rod and the supply ground rod? Wouldn’t earth merely be what closes the circuit? I mean, after all a closed circuit isn’t necessary in order for a current to exist. Only a potential difference is, right?

No. You need a complete circuit for current to flow, not just a voltage. Otherwise, you'd never need a neutral in AC circuits, and you wouldn't have to wire battery-powered circuits back to the battery's negative terminal.

Also, especially with TT installations, beware that some zeros are more zero than others. We say that the Earth is at zero volts. But don't assume that the voltage at a metal rod hammered into the ground will always be zero. There may be a resistance of tens or hundreds of ohms between the rod and the general mass of the Earth. As soon as a current flows, the voltage of that rod may rise significantly above zero.

Simon B
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A closed circuit is very necessary. What is a case where current flows without a closed circuit?

We see lightening and other static discharge which my seem to do so, but these create temporary shorts between objects closing the circuit until the charge equalises.

So having just your consumer with an iron stake in the ground would be equivalent to running a wire from your consumer back to your source but just dropping it on the ground near by, not making any connection.

As long as you imagine 'earth/ground' to be a wire or plane connecting your stuff it all makes sense. Just ignore what it's made out of.

hekete
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  • So if we remove the grounding at the source (IT earthing system) it wouldn’t be possible to receive an electric shock seeing that the circuit would never be able to close? – David Jul 03 '19 at 15:37
  • @David technically that is correct (if you only touch one point in the circuit). But it turns out that is actually more dangerous. Consider if two people touch the circuit and both of them are touching the ground, suddenly what was safe to do with one person now kills both people. In reality ground faults occur in distribution systems all the time (trees, dirt, water, etc). You could also still get a shock from the initial equalisation of potential and because you have capacitance, high enough voltage AC will still cause dangerous current flows. – hekete Jul 04 '19 at 05:20
  • @David Grounding makes a part of the circuit always safe. There are other considerations, like protecting the distribution system from faults that could damage it or cause wild voltage fluctuations damaging stuff connected to it. You also need a way to keep neutral 'neutral' when using single phases paired with it. – hekete Jul 04 '19 at 05:25
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nobody realized what does a "fault" mean. A fault is a deffective isolation between some live point of the appliance to the chassis. If the chassis is grounded, some current will flow from phase to earth via the chassis, the grounding strap, and finally the rod. As ground has similar potential as the neutral, this current is similar to what would flow through a resistor put between phase and neutral of one of the secondary windings. According to the more or less defective isolation, that current would be higher or lower. If there is no earthing cable and somebody touches the chassis with a hand and earth with the feet, the current will try to get the earth through that person's body. That's why we prefer earthing the chassis.

Felix
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  • Even if the chassis is grounded, if one were to touch the chassis with a hand and the ground with the feet, wouldn’t you still receive an electric shock (assuming no RCD). I mean, your body would just constitute a parallel circuit, wouldn’t it? – David Jul 03 '19 at 15:51
  • @David Yes, but when the chassis is grounded there are now two parallel conductors when the human body enters the circuit. Some can/will flow through the human, but most will flow through the chassis. The human typically will have a much higher impedance than the safety connections, so the lethal currents will not flow through the human (usually). AND hopefully the current flowing through the safety ground will trip a circuit protection element and shut the whole thing down before a human comes in contact at all. – Aaron Jul 03 '19 at 15:56
  • @Aaron Ok, but during circuit theory I learned that in a parallel circuit the voltage will be the same and only the current will be different. So I guess the answer to my question is similar to the scenario where a bird sits on a power line: that is, although the bird constitutes a parallel circuit the voltage drop over the bird is only equal to the small voltage drop that occurs over the equally short part of the power line it sits on. In other words, when the current passes through a human that touches a live grounded chassis that human doesn’t experience the 230V/Z but rather just a few V? – David Jul 03 '19 at 16:06
  • @David Yes. The model is more dynamic than that, and contains body resistance, contact resistance (human to appliance), chassis resistances, etc. but essentially yes. – Aaron Jul 03 '19 at 16:14
  • But what if we remove the grounding at the supply side as in an IT earthing system? Where will the current then flow to? Will the system be safe as far as electric shocks goes of the kind we have talked about? – David Jul 03 '19 at 16:31