Understanding Earth in a Home

I have been reading articles about earthing, and most of them use a person standing on earth, which is soil I presume, as an example to show how current flows back to the ground. What about in a real world example, like in our homes, where the flooring are made of other materials.

For example in this scenario:

In a home, I am completely barefoot, standing on surfaces say such as tile, concrete or parquet. The surfaces are squeaky clean, and bone dry. If I accidentally touched a 240v live wire, will I get electrocuted? The 3 mentioned materials are very poor conductors, hence, am I correct to say that the electrical path can't be completed, because surface material is insulating my foot so there is no direct connection to earth.

Whereas in this scenario:

I am barefoot in my home toilet, the ground is wet and the water touching my feet is flowing into the drain. I have read that copper pipes are earthed. So in this case, if I happen to touch a live wire, there is a path for the current to flow through my body, water, and the copper pipe to earth. Am I correct that say that as well?

In the first case you might feel a tingle, because you are always coupled to the Earth by capacitance in addition to any high resistance paths present.

In the second case there are low resistance paths and deadly current can flow. It doesn't take much if the path is through the heart.

The problem is that touching a live wire is not going to hurt if that is all you are touching. Birds perch on wires carrying many thousands of volts with no effect.

The problem is that once you are touching the live wire, touching anything that is grounded is now lethal. A lot of your house is grounded and all the shocks I have ever got are from touching the ground after touching a live wire first.

If you are lucky the two parts of the body touching are close together. If not you touch with a separate hand and the current flows across your chest. That is why TV engineers often work with one hand in their pocket.

". That is why TV engineers often work with one hand in their pocket."

I thought they were filling in time playing pocket billiards! :)

Weedpharma

Byork: For example in this scenario:

In a home, I am completely barefoot, standing on surfaces say such as tile, concrete or parquet. The surfaces are squeaky clean, and bone dry. If I accidentally touched a 240v live wire, will I get electrocuted? The 3 mentioned materials are very poor conductors, hence, am I correct to say that the electrical path can't be completed, because surface material is insulating my foot so there is no direct connection to earth.

Concrete is very porous to moisture so won't be dry at all at room temperature. Heat it up to 120C for a few weeks and it will be bone dry, but even then it may conduct somewhat due to all the salts. Not an insulator.

Tiles often have a glazed surface which is a very good insulator (a glass, basically), but glazed surfaces do have cracks and the grouting between the tiles is basically cement, same issue as concrete, porous. Unglazed earthenware tile is also porous, note.

Parquet is wood, guess what, wood is porous, so it won't be bone dry at all.

So your best chance is tiled floor if its a glazed kind of tile, but even then would you take the risk? Standing on a glass or polythene plate that really is a good insulator (even in the present of normal atmosphere) is the safest of all, and you'll still get a shock because you are the plate of a capacitor and mains is AC - but it won't be life threatening.

Of course never do this, its a thought experiment only.

In very humid environments every surface is coated with water molecules (to varying degrees) and thus everything conducts enough to prevent static charge building up (but not capable of large currents), since water molecules can exchange H+ ions readily with their neighbours and pass charge around on the surface.

Pure water is in fact a good insulator - the klystrons I used to work on years ago used it as a coolant and insulator with 25kV 40A dc supplies.

But water as found in domestic supplies isn't pure, and conductive enough to be a serious danger where mains electricity is involved

regards

Allan

Brilliant, thank you for all the replies, they really helped. Mark's words on moisture and porousness of materials showed me a new perspective. My questions are purely hypothetical, not going to try it :) I am just trying to grasp the concept of how current find its way indirectly to earth in a building. This also led me to understand the importance of RCDs, and their role in saving lives. Thanks again.