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I understand that voltage is relative and that the hot wire in house mains is a nominal \$\pm\$ 120v in reference to the neutral wire. I also believe the ground wire is connected to the neutral at the breaker box and then connected to a metal stake driven into the ground. So here's my question:

If the hot wire was to connect to the frame of an appliance like a toaster let's say, and the toaster frame was not grounded, what happens when I touch the metal frame of the toaster, I don't understand why that is a dangerous situation. I mean, I don't see the circuit path. If the hot is say at 120v above neutral (at that moment in time) and neutral is connected to ground at the power box so the hot is therefore 120v above earth ground, how am I involved even if I am well connected to the earth (standing in water for example)? My mind says this, the current flows from hot to the short at the frame of the toaster to me to earth but I am nowhere near the metal stake in my yard and so how is the circuit complete? So I really have two questions:

Q1: Is there a completed circuit here that I just don't understand?

Q2: Does the frame of the toaster have some build-up of charge that is in excess of a neutral body and therefore imparts a discharge like a source to a sink (even if that neutral body was floating off of the surface of the earth)?

Actually, I have one more question. How does the ground wire protect someone from this situation inasmuch as I don't see the circuit path in it anymore than I see it when I am the "ground wire".

Thanks

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    \$\begingroup\$ Your scenario is not clear. I think what is missing is the fact that the case of the toaster is also grounded so you short from line to case is a dead short into ground. Is that what you mean? Then maybe make it clearer. \$\endgroup\$ – placeholder Oct 11 '12 at 14:48
  • \$\begingroup\$ No, I stated that the toaster is ungrounded in this scenario. \$\endgroup\$ – Salbando Jones Oct 11 '12 at 15:01
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    \$\begingroup\$ I edited your question to reflect that fact \$\endgroup\$ – placeholder Oct 11 '12 at 15:03
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    \$\begingroup\$ fyi: I'm assuming you mean 120 V RMS (which is common in the US and other places). That translates to about +/-170V peak to peak for a sinusoidal wave. \$\endgroup\$ – helloworld922 Oct 11 '12 at 17:11
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Your question about protection is a very important one. Every electrician, technician working with heavy machinery, and electrical engineer should know how these things work.

All About Circuits published a very comprehensive online compendium about electronics and electrical engineering. For your question, I recommend reading this article about safety in circuit design.

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In short, current chooses the shortest path to a lower potential. Without a grounded metal case, your body may become a path to a lower potential (earth). Your body may not be the best conductor, having e.g. 1000 Ohms, but it's still many orders of magnitude less than air.

Your kitchen floor is approximately earth potential. Buildings, soil, etc are not very good conductors, but they still have a low resistance compared to, say, air. Earth's soil may have a resistance of a few Ohms, maybe tens of Ohms, it depends on many factors. What's certain is that it does conduct.

Current choses the shortest paths. What's more, a large contact surface will create more paths for the current to flow, thus lowering the resistance of something which at first glance is not a good conductor.

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  • \$\begingroup\$ I read the article and it makes sense. So I guess this means that a voltmeter would read some potential between the hot wire and anywhere I touch on the surface of the earth. Is this because the "hot" line is referenced to earth at the power station? \$\endgroup\$ – Salbando Jones Oct 11 '12 at 15:44
  • \$\begingroup\$ I would say so. If the hot wire is 200V above earth at the power station, and earth is a giant three-dimensional resistor, then a random point on earth will be a few dozen Ohms "away" from the neutral potential at the power station. \$\endgroup\$ – Jonny B Good Oct 11 '12 at 15:48
  • \$\begingroup\$ There are interesting effects because of this non-ideal conductivity of random materials. Sometimes when there's a renovation in some building, a worker accidentally cuts a wire in the walls with his hand drill. You notice some of your sockets seem to have stopped working. You measure their voltage. With no load, it's a full 230V. But when you attach a load to it, the voltage drops to, say, 13V. What happened? It turns out that the broken wire in the wall still conducts. If your load is an 500 Ohm light bulb and the broken wire is ~10kOhm, then you have a 1:20 voltage divider. \$\endgroup\$ – Jonny B Good Oct 11 '12 at 15:53
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    \$\begingroup\$ The link that Johnny B Good provided was very informative. Another article I found at that site explained exactly what I wanted to know. \$\endgroup\$ – Salbando Jones Oct 11 '12 at 16:07
  • \$\begingroup\$ Damnit, I was so close to finding it :) Would be more useful than the one I found. \$\endgroup\$ – Jonny B Good Oct 11 '12 at 16:10
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You are touching the frame of the toaster which is now connected to line, so you are now "hot". if there is any return path it will flow to complete the circuit. In a lot of cases that return path may not exist but can be difficult to determine that, especially for a lay person.

There are lot of electronics that doen't use the safety but in that case the designer must ensure that someone cannot place a conductive rod into the case and touch line/Hot. Typically sealed units meet this requirement.

If you need vent holes, in which it is possible to stick a metal rod then the case must be arranged such that it is likely that the metal rod shorts against the case , completes the circuit to neutral and pops the breaker.

At this is done to prevent people from contacting line/hot because in some scenarios they become part of the circuit.

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  • \$\begingroup\$ Certainly if I then touch a water faucet or provide some path to ground, I complete the circuit. However, are you saying that if I had the toaster outside, but a few feet away from the ground rod and the earth was dry and not conductive, that I would be safe to touch the faulty toaster? (Mind you, I am not looking to do this experiment, it's just a hypothetical.) \$\endgroup\$ – Salbando Jones Oct 11 '12 at 15:25
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    \$\begingroup\$ Even dry earth will conduct. The surface of the rod underground is large, so there is a lot of paralell paths. \$\endgroup\$ – Jonny B Good Oct 11 '12 at 15:29
  • \$\begingroup\$ Lets say you are standing on a rubber mat. Then no you don't complete the circuit and "should" be safe. Keep in mind that there will still be current flowing "displacement current" that charges and discharges your capacitance. \$\endgroup\$ – placeholder Oct 11 '12 at 15:30
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The circuit completes through the ground. It's not very conductive unless it's wet, but it's enough.

A ground wire protects by taking the current back through itself as a preferred path, as soon as the wire touches the case, blowing the fuse at your distribution box.

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  • \$\begingroup\$ I still do not see the circuit path. If I am standing in the kitchen and the ground stake is in the back yard, how can that be even a "not very conductive" circuit? \$\endgroup\$ – Salbando Jones Oct 11 '12 at 15:03
  • \$\begingroup\$ The stake may be in the back yard, but the walls of your kitchen contain grounded conductors which bring that ground a lot closer. So the circuit doesn't have to be your entire house between you and the ground, but only a few feet of wall or floor. \$\endgroup\$ – Kaz Oct 11 '12 at 17:40
  • \$\begingroup\$ @SalbandoJones, think about the risk of metal pipework you may touch at the same time, or anther item that does have it's case grounded. \$\endgroup\$ – Ian Ringrose Apr 23 '14 at 15:00

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