Are unearthed metal cases actually unsafe?

Question

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?

Answer 1

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.

Answer 2

if there is a fault with a device, current will flow from Live to Earth, hence you don't feel an electric shock.

There are two aspects to this : if there is a catastrophic fault (low impedance, allowing high current to flow) the traditional fuse will melt, and protect you by disconnecting the live circuit.

However if the fault is a partial breakdown (perhaps moisture or degraded insulation, which may get worse, but only allows a small current at the moment) the case is either at a dangerously high voltage, or safe because the current is conducted to earth.

Now the earthed case is safer because it is at earth potential, and furthermore, if the current reaches 10-20mA, the live circuit will be disconnected when the breaker trips (RCD in Europe, GFCI in the USA).

In this case the un-earthed case is unpleasant but probably still not lethal, because when you complete the circuit well enough to conduct 20mA, the breaker will trip. And unless you have an unusually weak heart, you will probably survive. (You do test the breakers every year to make sure they aren't sticking, right?)

In practice, if you are deliberately designing an un-earthed case, you will adopt "double-insulated" design practice, in which any single failure (such as insulation breakdown in the transformer) cannot leave the case live. One easy way is to make the case itself plastic and therefore non-conductive! The safety approvals procedures and regulations are designed to ensure that someone independently verifies that these safety practices are correctly followed.

Answer 3

IF there is a metal case then under some fault conditions it can rise to full mains potential if not earthed and cannot do so if it is earthed, as mainsis ground (earth) referenced. That alone provides adequate reason for earthing metal cabinets. Your argument is based on a point that has metit in the general sense but it effectively wrong in ths relevant one here.

ie IF you can remove all paths to ground for users of any resistance value then an ungrounded cabinet will not cause an electric shock path to ground.AND a system that was not ground referenced also would not cause shock paths to ground. Some shipboard power system float relative to the hull. When one or other conductor develops a fault to "ground" the system becomes a floating death trap and such faults are promptly located and eliminated.

However in land based domestic power systems the mains are almost invariably grounded referenced, and paths to ground of various impedance and resistance are common.
Further, the grounding system's main job is to reliably activate fault protection equipment when a fault occurs.

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Asleep at keyyyyyyyyyyyyyyboard. Sleep calls. More if unclear.

Answer 4

The whole scenario of your question misses one point - modern appliances (and I speak of Europe and probably several other parts of the world), do not rely on fuses to protect against electrocution.

RCDs (residual current devices) trip when there is a difference in current between the two AC wires of about 10mA to 30mA and these provide "safety" in the absence of an earth wire or earthed metal parts: -

http://www.performing-musician.com/pm/nov07/images/TechNotes_4.jpg

Fuses prevent fires and wires melting.

Answer 5

If you brush the back of your hand against a "live" faulty appliance, but you are wearing rubber boots and standing on a dry vinyl floor, you will not get a dangerous shock. You will feel a slight tingling, from the capacitance effect mentioned earlier. At 60Hz, there isn't enough current to be dangerous. At radio frequencies, it is a different matter!

The big danger is in touching a "live" appliance while also touching plumbing, heating ducts, a grounded appliance or wet concrete floor. This is much more dangerous than the shock you get putting your finger in a light socket or touching both prongs when struggling with a plug, because the current flows through your body and not just between your fingers.

A loose hot wire touching the frame of a grounded appliance will not just rely on the current being diverted to the ground rod. (Depending on the ground rod, the breaker might not trip and the appliance frame could still be at a dangerous voltage.) In Canada the 6-foot ground rod and Neutral are connected together in the main breaker panel. The power company doesn't provide a Ground wire to the house because their uninsulated Neutral wire is connected to the ground in millions of places.

Answer 6

I needed to pass an exam of the electric safety code ages ago at the university, and I remember this one point: Earthed cases are illegal in a laboratory or work environment where there are exposed terminals with live voltages. The rationale is that you survive touching the live voltage when you don't touch the earth at the same time. I don't know if they've changed that code since then.