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If neutral is connected to ground and current desires ground then I would think it would go to ground. But my understanding is that it is kept off of ground until it's "upstream" of the breaker box, or unless a malfunctioning device shorts hot to ground. So what "prevents" the electricity from going to the ground that the neutral is connected to?

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I answered this exact question here, though the explanation is very technical. The short answer is, there will be current through you if you touch a neutral wire while grounded, but it will be so small you will barely feel it. –  BlueRaja - Danny Pflughoeft Dec 8 '13 at 18:18

5 Answers 5

The short answer is resistance. Electricity will find the easiest path to ground. At the neutral-ground bonding in the breaker panel is a connection to the ground wire that goes to the actual ground. It's a heavy gauge, and connected to grounding bars deeply embedded in the ground to offer the least possible resistance.

For current to travel from the neutral back to the ground wires to where it can electrocute you, it needs:

  1. an imbalance in the house wiring (remember, there are two hots, and with a broken neutral it becomes a circuit with two loads in serial),
  2. a failed neutral since the neutral will be at the difference in current used by the two hots (by design of a center tapped transformer), and
  3. a better path to ground than the ground wire that is connected to the grounding rods.

As long as any one of those three conditions doesn't apply, you should not see any current on the ground wire.

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I doubt that you will ever see an installation where the two hot conductors are perfectly balanced. So you will see some current on the wires that go to the grounding rods. Which is the main reason they are 6 gauge wire, to help pull the voltage down. –  Brad Gilbert Dec 9 '13 at 0:12
    
The imbalance will first go out the neutral. After that, it will appear as an increase in voltage on the lesser used hot. See this question where using the microwave caused someone's lights to get brighter. –  BMitch Dec 9 '13 at 0:20
    
If you have a multimeter capable of measuring milli-volts, and measured the AC voltage from the grounding rod to the dirt a few feet away; you would see that there is some voltage. See also infinite resistor grid. –  Brad Gilbert Dec 9 '13 at 0:48
    
I was looking at the majority of the electricity flow, which is what I think most people are concerned about when wiring a home. The paranoia is from the hot having 120, the neutral on the other end of that 120, the ground connected to the neutral, so metal appliances should have 120 on their exterior ready to electrocute you. Sure, there's going to be some milli-volts from the slight resistance, and there could also be phantom induced voltage, but it's going to be much more like a 9v battery than the 120v hot. –  BMitch Dec 9 '13 at 12:25

A great question and it seems odd to have a ground and a neutral. We have a neutral to keep the ground from being loaded. We dont want the ground to be load, it doesnt have the capacity and its for back-up.

In AC systems, there is current on the "hot" leg and on the "reference" leg. We use a neutral leg for reference. In countries with 240v systems, there is no neutral, but two opposite legs of 120v that provide a reference for each other. You see in AC systems, the voltages are not constant - it alternates back forth +X volts and -X volts. + and - from what? voltage is just potential differences from the reference. In 120v systems, on leg will be -60v from the neutral and then switch to +60v from neutral 60 times a second. In a 240v systems with out neutral wire there is a perceived reference between the two 120v legs that are off opposite phase.

So, yes in theory, you could get the potential difference from a 120v "hot" leg and ground, BUT then your would be loading the ground and it would no longer serve a safety function. In practice when you load the ground, appliance panels and other things that are grounded out become will be loaded and can shock.

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The grounding conductor should always have the capacity to carry the full load supplied by the ungrounded (hot) conductor, in a properly wired circuit. –  Tester101 Dec 9 '13 at 12:01

I think the answers here are looking too deep into this. The simple answer is, that it's not a complete circuit.

Electricity only flows in a complete circuit, and always want's to return to the "source". If there is no complete circuit, electricity simply will not flow.

Looking at a simple diagram of the secondary side of the distribution transformer, it's easy to see the complete circuits.

Simple diagram

You can easily see the 240V circuit.

240V Circuit Loop

And the two 120V circuits.

120V Circuit Loop 1120V Circuit Loop 2

But if you look at the grounding conductors, there is no complete circuit.

Grounding Conductors

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If you have a broken neutral, lack of ground, but still the neutral/ground bonding in the panel, and no second side of the circuit (second hot), then current goes from hot, appliance, neutral, neutral/ground bond, ground, exterior of appliance, person that touches the appliance, water faucet that person is touching, true ground, back up your neighbors ground, to your neighbors neutral, and back to the transformer. The extra resistance to take that convoluted path is why it only goes that way when other paths have near infinite resistance. –  BMitch Dec 9 '13 at 16:14
    
@BMitch Maybe. Or it goes hot -> appliance -> neutral -> service grounding electrode -> dirt -> transformer grounding electrode -> transformer neutral. Whichever is easier. I'm guessing this is why the grounding electrode system is required to be a low resistance path. –  Tester101 Dec 9 '13 at 16:49
    
For an accurate Spice simulation of the electrical grid you would need to place a resistor on all three legs of the transformer to model the resistance of the wire. You would attach a ground connection on both sides of the neutral wire (modeled with a resistor). At which point you will realize that some power goes through the ground if the two halves of the phase have different loads on them. –  Brad Gilbert Jan 4 at 4:33

Because ground is not a return unless you bridge across from neutral, hot, or have a component short to frame ground.

It is a free standing wire/conduit network connected to earth that isn't a part of a circuit till a fault occurs.

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The ground wire is for safety, it should be very close to the same potential as Earth. To that end, there is a legal limit of how much resistance there is from the ground at the service entrance to dirt outside. In the US it's 25Ω, or you can get away with just using two ground rods 6ft (or more) away from each other. (It's cheaper to install two ground rods than spend time and money on properly testing the resistance)

The reason the neutral and ground wires are connected at the service entrance, is to help keep the voltage potential of the neutral to Earth, as low as possible by keeping the resistance to Earth as low as possible.

To have absolutely no current on the grounding wire going to the ground rods, the nearest transformer would have to have equal current on both hot conductors. Since the transformer typically has it's neutral (center tap) grounded at the the same location as the higher voltage side, there would also have to have to be equal current on all of conductors on the high side as well.


So basically there is always some current going through the grounding system, at the service entrance. The main thing, is that the voltage is kept as low as possible; by keeping the resistance as low as possible.

If you want proof that there is some voltage, and therefore current, on the grounding system, measure the voltage from the grounding rod to a point in the dirt a few feet away. If you measure any voltage with a high-end multimeter of even a tenth of a volt (AC); then there is current on your grounding system.
You can even measure some voltage from the grounding rod to the point at which the neutral and grounding system are bonded. Although this will be far less than the other test, as copper and aluminum are better conductors than dirt.

Even if you turn off your power, you will still see some voltage with both of these tests. Assuming there is someone else connected to the same transformer as you, and your neutral and ground wires are still wired properly. The amount of wire the current would have to travel through before it got to your premises would be greater, so the amount of current/voltage would be greatly lessened.

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@BrianWhite If there's no neutral coming from the transformer to the panel, then where does the center tap on a center tapped transformer go? –  BMitch Dec 9 '13 at 16:07
    
@BMitch Was there previously a comment mentioning the lack of a neutral coming from the transformer? –  Brad Gilbert Dec 9 '13 at 17:13
    
yup, but then got deleted as I'm guessing they realized their mistake. –  BMitch Dec 9 '13 at 17:16

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