Earthing and electrocution

Question

I have seen many videos about earthing but I couldn't understanding why the current would flow from live to ground (earth) if one touched a live wire.

Suppose I am in a house and I touch a live wire while standing barefoot on a concrete floor.

Would I be electrocuted?
Isn't the concrete supposed to be non conductive?
Even if the current flowed to ground, isn't there no connection between earth and live wire? Why would the current go to ground?

Also I have another question out of curiosity. The mains tester we use in our home in which we put our finger on the cap and the bulb glows. Is it based on the parasitic capacitance of our body?

Concrete conduct very good if you want to die. Why the current flows? Because the live is referenced to ground. — Marko Buršič, Mar 08 '21 at 11:39
Concrete resistance is mostly from the absorbed water. A concrete sidewalk in a desert climate is much safer than a damp basement. — Mattman944, Mar 08 '21 at 13:16
Yesterday I spilled some salty water on a new concrete floor. Before I could wipe it up, nearly all had been dissolved into the concrete. Most likely that spot would now be fairly conductive, at least as long as that salt water is floating around inside of it. Dry concrete would be much less so. — user1850479, Mar 08 '21 at 19:11

score 2 · Answer 1 · answered Mar 08 '21 at 14:07

Isn't the concrete supposed to be non conductive?

It contains water and ions, which makes it slightly conductive. Looking it up on google, I found a resistivity value of \$ \rho = 50-200 \Omega.m \$ depending on humidity content. So, let's count a surface of 20x20cm, or S=0.04 m2 below your feet, and L=0.10m depth to the nearest rebar. That would make \$ R = \rho L/S \$ = 250 ohms. That is of course a very crude estimation, could be 10x less or 10x more, but it should get the order of magnitude right, and this resistance is way too low to be safe. In this case, it's the large contact surface area with the floor that gets you.

Would I be electrocuted?

That depends on your definition of "electrocuted". Get a good jolt and swear? Definitely. You would only die if:

Enough current goes through your heart to trigger fibrillation, in this case it's quick.
Or enough current goes through your arm muscles to prevent you from opening your hand and releasing the object that's electrocuting you. In this case you get barbecued by the thermal effects of electricity. If it lasts more than a few seconds, the bits that got cooked will have to be amputated.
Or, most commonly, you get a good jolt and swear, the RCD trips, and you fall off the ladder and break your neck.

Note this only applies to low voltage (240V). If you poke a high voltage line with a stick, it's a lot more spectacular.

Even if the current flowed to ground, isn't there no connection between earth and live wire? Why would the current go to ground?

Neutral is earthed at the transformer in some countries, and at the electrical panel in the house in others.

The mains tester we use in our home in which we put our finger on the cap and the bulb glows. Is it based on the parasitic capacitance of our body?

Both capacitance and current through your body. It lights up a lot less if you're wearing insulating shoes.

score 1 · Answer 2 · answered Mar 08 '21 at 11:39

At some point in the electrical system (your house, or substation, depends where you live etc), the mains neutral wire (which is the standard return path for mains phase) is connected to earth potential. So therefore toucing both live and for example something that is earthed (water pipe, standing on not very well insulated floor) will make you a conductive path between live and neutral/earth.

score 1 · Answer 3 · answered Mar 08 '21 at 12:58

For safety reasons, you need to have a local reference on a power system. Otherwise, that system can "float" to any arbitrary voltage with respect to the local environment. This is the reason that your mains are referenced to ground, typically at the first means of disconnect on the premises.

High conductivity is not required for electrocution to occur with AC circuits. Consider the following diagram:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

score 0 · Answer 4 · answered Mar 08 '21 at 11:42

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Current will flow from Phase/Live to Neutral or Ground/Earth. This is the standard arrangement.

Concrete is somewhat conductive. You need only a small current to feel electric shock and moderate current to be electrocuted. Concrete will often be conductive enough for you to feel a mains to concrete shock and may be conductive enough to allow electrocution.

answered Mar 08 '21 at 11:42

Russell McMahon

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Concrete isn't a great conductor itself but it's worth noting that it usually has a rebar skeleton that greatly reduces effective ground resistance. The electricity has to conduct through 4" to 8" of concrete to connect to the rebar which has decent conductivity and immense surface area connected to more rebar, concrete and ground. – K H Mar 08 '21 at 11:57

score 0 · Answer 5 · answered Mar 08 '21 at 12:01

Suppose I am in a house and I touch live wire standing barefoot on a concrete floor. Would I be electrocuted?

Maybe, maybe not. It depends on many factors but you will almost certainly receive an electric shock. Despite what you may believe, concrete is quite conductive. It's certainly conductive enough that you risk being electrocuted if you are in contact with it while at the same time being in contact with a HOT electrical circuit.

Isnt the concrete supposed to be non conductive?

Everything is conductive to some degree. There is no "perfect" insulator. Under the right circumstances a human being can be electrocuted (i.e. killed by electrical shock) by surprisingly little current. Concrete and other "insulating" materials MAY be conductive enough to allow lethal currents to flow through the body. That's why you should ALWAYS shut off power at the source prior to doing any electrical work.

And even if the current flowed to ground , isnt there no connection between earth and live wire? So why would the current go to ground?

Your assumptions here are incorrect. In fact electrical mains are almost always referenced to ground. You will find a grounding rod near your electrical service entrance. So if you your body is grounded and touch a hot wire, current WILL flow through your body. Whether that's enough to electrocute you is impossible to say. It's dangerous, however, and should be avoided!

score 0 · Answer 6 · answered Mar 08 '21 at 19:05

Would I be electrocuted?

You could be.

Isn't the concrete supposed to be non conductive?

To a certain extent. The magnitude of current through the concrete would be sufficient to cause a nasty shock or even electrocution.

Even if the current flowed to ground, isn't there no connection between earth and live wire? Why would the current go to ground?

In split-phase and 3-phase systems, the grounded neutral would permit current flow through ground in the case of an earth fault.

Earthing and electrocution

6 Answers6