Fundamental question about circuits and grounding

Question

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

Answer 1

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.

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.

Answer 2

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.

Answer 3

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.