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enter image description here

If the live wire in above illustration touches to the metal case, a high short circuit current will flow to the earth through the safety earth wire and the fuse will trip.

Now imagine a 9V battery staying on a wooden table. If now its plus terminal is tied to the earth through a wire almost no current will flow.

Above two points shows that it is not the case that the earth is a reservoir that sucks all electrons, but simply current needs a loop to flow.

In the battery case it is obvious that there is no loop because the negative terminal is floating.

But what is the loop in the first AC case? Is that the AC generator star connection? Can you draw this loop to illustrate better?

floppy380
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  • A ground loop is any time you connect two conductors that are connected to ground *and each other*. Which makes a loop, and when you make a loop, a magnetic field flowing through the loop creates currents in the conductors that make up the loop. – Voltage Spike Jan 13 '18 at 18:56

2 Answers2

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What is different is that for a battery there is not other connection to ground, this is not the case with the mains.

In the mains, the neutral connection is actually grounded as it enters your building, or at the transformer.

As such shorting the live wire to the case is really shorting the live wire to the neutral side of the transformer through the ground line creating a loop, and, as you have expressed, large current flows till the fuse of breaker trips.

schematic

simulate this circuit – Schematic created using CircuitLab

Which is why this is dangerous...

enter image description here

With an isolation transformer you isolate the power from ground and there is no return path.

enter image description here

Trevor_G
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  • Woow I didn't know transformer's secondary was tied to the earth. Is that the case for all step-down transformers in a distribution system? – floppy380 Jan 13 '18 at 15:56
  • @doncarlos all? It is pretty much standard globally, yes, individual installations may vary. – Trevor_G Jan 13 '18 at 15:59
  • I meant the intermediate distribution transformers between a power plant 115kV and 2.4kV, not the transformers for houses. Anyway maybe thats a bit specialised question. Your answer replies exactly what I was asking about. – floppy380 Jan 13 '18 at 16:02
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    But I think they are 3 phase transformers and I guess star connected so not the secondary but the star point is wired to the earth? – floppy380 Jan 13 '18 at 16:06
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    Depends on the country you are in. In the UK and US Ground and Neutral should be connected together at the feed into the building. – RoyC Jan 13 '18 at 16:08
  • @Trevor_G Your edit was great, can you also add another one which illustrates if an isolation transformer is used for the man it wouldn't be dangerous? It could be a very complete answer! – floppy380 Jan 13 '18 at 16:14
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    @doncarlos sheesh. do you think I have all day.. oh ya.. so I do – Trevor_G Jan 13 '18 at 16:29
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    Another great answer! Plus one for "You" figure! – Dumbo Jan 13 '18 at 23:56
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Any mains wiring installation requires a safety earthing system, or grounding electrode system - typically to grounding rods or a steel water pipe.

Then, the transformer's output is grounded: One conductor is picked to be "neutral", or a hypothetical point in the middle is picked using transformer tricks, and that is pegged to the grounding electrode to create an artificial relationship with ground. Of course this is also tied to those green ground wires in the wiring.

Why? To give a return path for fault currernt, and to keep the system voltages from "floating" or "rattling" at arbitrary random potentials. For instance if the 2400V supply transformer had a weak primary-secondary fault, it might "pull up" the secondary outputs up to 2400V, so now your voltages are 2280V, 2400V, and 2520V. Trouble is, your stuff is not insulated for 2520V.

In residential, the neighborhood poletop transformer has a ground rod and a neutral-ground bond. Each house's main service panel also has a grounding electrode and its own neutral-ground bond. They must all be in sync, or you will create current loops through your lawn, and make your meter spin. But if you own your own transformer, you can ground any way you want.

Imagine you have a supply transformer with secondary taps at 0V, 120V, 123V, and 240V. You connect 0V to pole L1, 120V to neutral, 123V to ground, and 240V to pole L2. At this point you would have common North American split-phase power, with L1 and L2 both 120V from neutral. But with a twist: neutral would be biased 3V away from ground. That's weird, but it still serves all the purposes of grounding: it keeps neutral near ground, gives a path for fault current, and assures sane voltages on all the conductors.

Why does your hot-ground fault complete a circuit? Because we added that neutral-ground bond so it would do that.

If you prefer, you could run an isolated system where hot and neutral have no relationship with ground, it is unbonded. In that case, your first ground fault becomes your ground bond. I have had this happen with a panel fed from an onsite transformer that unknowingly did not have a neutral-ground bond. An L1-ground fault biased the entire system 120V from where it should've been, with L2 now 240V from ground and neutral 120V from ground. I found this out when I deenergized a circuit on L2 and flashed its hot to ground just to make sure it was off; rather than get a blinding arc flash, I got a tiny fizzle and the circuit lit back up! Not good, though.

At this point, your second ground fault then starts tripping breakers.

An isolated system makes sense when you have engineering staff on premises regularly checking for ground faults. They catch the first one before a second one occurs.

No one has ever claimed that earth draws all electrons to it. A lot of people think electricity wants to "return to earth", that's only true for lightning and ESD. Artificial electricity wants to return to source.

Harper - Reinstate Monica
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  • "*At this point you would have common North American split-phase power, with L1 and L2 both 120V from neutral. But with a twist: neutral would be biased 3V away from ground."* This example breaks down because North American service entrances don't have a separate ground and neutral -- the neutral is grounded, on both ends. This puts a near-short circuit across 1.25% (3V) of the secondary windings and L2 is closer to 117V than 120V. A constant fault current would run through the neutral and the customer's grounding electrode back to the pole ground if the neutral isn't grounded at the pole. – Michael - sqlbot Jan 13 '18 at 22:49
  • @Michael-sqlbot I said immediately before that, **If you own your own transformer, you can ground any way you want**. The double grounding both at the transformer and at the panel is not a characteristic of *North American* wiring, but rather the *standard practice of Edison companies* providing power to homes. I quite agree inducing a neutral bias in discord with a power company supply is a great way to spin a meter faster than you can write checks to pay for it. – Harper - Reinstate Monica Jan 14 '18 at 00:08
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    +1 for Artificial electricity wants to return to source. – Trevor_G Jan 14 '18 at 00:34
  • @Harper, I see your point, now. I didn't make the connection from one paragraph to the next. – Michael - sqlbot Jan 14 '18 at 01:32