Conducting concrete

[James Wilkinson Sword]

Somebody in one of these two groups recently said that a concrete based house means you're earthed. Concrete is a bloody good insulator!

Sorry, can't find the post it was mentioned in.

[trader_4]

Wrong as usual. Concrete that's been cured and then dried out, kept away from moisture, may not be a good conductor. But building concrete, a foundation is such a good conductor, it's the preferred ground for the building electrical service. An electrode is placed inside it before the pour.

[Stormin' Norman]

Ah yes, the old Ufer ground. That protected a lot of ordinance throughout the years.

[mike]

Can't help with a link, but I recently replaced my water main with PEX. That broke the ground to the house and wouldn't pass inspection. Long story short, research indicated that using rebar in concrete was a trend in establishing a safety ground. Apparently, there's enough conductive salt and water in concrete to make it work, as long as the concrete sits on the ground and you're not in the desert. Contact resistance is high, but there's a lot of area.

I followed the code and installed two ground rods. I did some impedance measurements between the rods and the electrical system ground (before connecting) and determined that the "grounding" was insufficient to do anything more than dissipate static electricity, but the inspector liked it. I'd guess that hooking to the rebar is at least as good.

[James Wilkinson Sword]

It was more about getting an electric shock by standing on the floor of your house while touching something live. This was suggested by someone recently as being a danger. I just measured some concrete to make sure I wasn't being ignorant, and it was off the scale (>20MOhms)

[James Wilkinson Sword]

Why did you want inspection? When you do work on your house, you don't tell anyone.

[James Wilkinson Sword]

Only if the concrete stays wet. Which won't happen unless it rains a lot.

[gfretwell]

Not really true, particularly if it is damp

[gfretwell]

Across what surface area? Why don't you sit on the concrete floor barr assed and grab a hot wire. Have your widow get back to us.

[James Wilkinson Sword]

If your floor is damp all the way through, you have bigger problems.

Concrete floors are for garages, houses should have wood floors suspended above the ground. What century is the USA living in?

[James Wilkinson Sword]

1cm from electrode to electrode = >20MOhms. That's an INSULATOR. No electricity worth considering is passing at all.

[gfretwell]

This was originally designed for ammo dumps in the desert so try again. Where you live, it is always wet isn't it? The effectiveness is in the contact area, not any single point.

[gfretwell]

One where we have not solved the termite, hurricane and tornado problems?

[gfretwell]

Then it should be safe, pull up your kilt, plop down on a concrete floor and grab that wire.

Don't tell me you are one of those scotsmen who wear panties.

[James Wilkinson Sword]

First time I've seen ammo dumps mentioned. We're talking about houses, concrete floors, and the dangers of being earthed.

Yes, but not inside the house.

The top surface of the concrete in the house isn't wet. If it is, your carpet would go mouldy.

[James Wilkinson Sword]

Chemicals are your friend.

Those tend to affect walls and roofs.

[James Wilkinson Sword]

What part of 20MOhms didn't you understand?

I go commando.

[FromTheRafters]

[James Wilkinson Sword]

So as I said before, it's the water conducting. But your floor really really shouldn't be damp.

[gfretwell]

The danger is not being earthed when you are talking about ammo and this was the most effective way to earth them.

It is wet enough to ionize the stuff in concrete. I didn't make this shit up on the spot, google Ufer.