Take a look at this picture:
It shows that if there is a fault with a device, current will flow from Live to Earth, hence you don't feel an electric shock.
However, I don't completely understand how much of a safety increase earthing a home appliance causes. From what I've read, "Earth" is a long metal rod driven quite deep into the soil to get a good connection. Most houses are concrete or wood.
From what I understand, if I touch a faulty appliance which doesn't have an Earth connection, the expected path of current is from Live -> through my body -> through a layer of wood or concrete -> into the soil. Surely if you need a long metal rod into the soil to get a good connection, the foundation of wood in your house cannot provide a current path.
I don't deny there is some added protection (perhaps if I decided to touch a faulty appliance while inside a bath, or outside on wet soil), but is not having an earth connection really unsafe?
Best Answer
When a fault occurs in an un-earthed metal cased device the case becomes live. When you touch it you form a circuit. Now you'd think that current couldn't flow through solid wood, etc. Well, that would be the case for DC, but not for AC. You see, you form part of a capacitor. You're basically one plate of a capacitor, the ground being the other. The floor, your boots, etc, form the dielectric insulator between the two.
The capacitor would block any DC current, but it allows the AC to flow quite happily. The equivalent circuit would be:
simulate this circuit – Schematic created using CircuitLab
The resistance of your body, and the resistance of the ground both limit the amount of current, but it only takes a tiny amount to kill you.
Now with the case grounded, and typically this involves a wire connecting the ground direct back to the ground point, which is also connected to neutral, and a fault occurs, the current will flow straight down the earth wire back to the neutral. This is a much much lower resistance connection than any human could provide, so considerably more current flows. This results in the fuse blowing isolating the circuit.
Also, if the fuse were not to blow for whatever reason (too high a rating?) the low resistance path would effectively short out the human reducing the current available to shock you to an absolutely minuscule amount.