Using rebar as earthing.

[Rampage]

I've just done the 2382 course and in BS 7671 it states that rebar encased in concrete can be used as an earthing rod.
Surely this won't have any contact with the ground an there won't provide a earth path?

 

[malcolmsanford]

Have a look at regulation 542.2.1 and provided that the concrete is not pre-stressed you can indeed use rebar as an earth electrode and it is a fantastic electrode as I have whole installations over here that use this and get an external earth loop impedance of barely 1 ohms in many case, but there are loads of parallel paths involved in that as you can imagine.

 

[johnboy6083]

Its called "Ufer earthing". Concrete is a better conductor of electricity than soil, and it makes sense to insert your earth rod, or mesh (ie rebar) into it. As rebar is used anyway, its silly not to make use of it. The concrete will also have a much greater surface area than a rod or tape. I beleive that lightning can cause the concrete surrounding the rebar to crack, however in this country, this would not put me off.

At work, we install big generators onto concrete plinths, that our company is responsible for providing. The rebar is not bonded down, and we have to use earth rods to tie the genny to ground potential. Im trying to convince the engineer
to use Ufer earthing, as i think it would be more cost effective, and easier.

 

[Knobhead]

Didn’t know it had a fancy name. Just been reading up on it. It’s been one of those things I’ve always done, but without giving a lot of thought to the physics and chemistry of it.

 

[Rampage]

I didnt realise that concrete would conduct so well compared to soil.

Thanks.

 

[johnboy6083]

I didnt realise that concrete would conduct so well compared to soil.

Thanks.

to be honest mate, not many sparks do. Unless you encounter lots of different types of electrical installations, then there will be a lot of things that you dont know. Of course, you could also start to educate yourself on here, and cross reference it with your books, and wiki for example.

 

[Engineer54]

Its called "Ufer earthing". Concrete is a better conductor of electricity than soil, and it makes sense to insert your earth rod, or mesh (ie rebar) into it. As rebar is used anyway, its silly not to make use of it. The concrete will also have a much greater surface area than a rod or tape. I beleive that lightning can cause the concrete surrounding the rebar to crack, however in this country, this would not put me off.

At work, we install big generators onto concrete plinths, that our company is responsible for providing. The rebar is not bonded down, and we have to use earth rods to tie the genny to ground potential. Im trying to convince the engineer
to use Ufer earthing, as i think it would be more cost effective, and easier.

Ufer earthing works best with a large area of slab/footings. A generator or transformer plinth would in all probability, depending on what depth the plinth is in the ground, need a supplementary earth ring around the plinth. Also depends on what your using as your max Ra value. .....Please tell me it's not 200 ohms??

 

[johnboy6083]

Ufer earthing works best with a large area of slab/footings. A generator or transformer plinth would in all probability, depending on what depth the plinth is in the ground, need a supplementary earth ring around the plinth. Also depends on what your using as your max Ra value. .....Please tell me it's not 200 ohms??

The engineer wants below 20 ohms, and ive managed 4

 

[Engineer54]

The engineer wants below 20 ohms, and ive managed 4

Thank God for that!!! ...lol!!!

 

[johnboy6083]

theres not a chance i would leave a rod with anywhere near 200 ohms. On a recent job, the lowest reading was 36 ohms, and that was obtained by using 2rods, (circa 50 ohms) and the fence earthing ring (to reduce touch potential). Im dubious about this, as the fence posts are not very deep, and come winter time, i can see the ground freezing up, and the reading rocketing, and also lets face it, winter time is probably prime time for power failure, and when the genny is most likely to run.

 

[scania164]

Seems like a good way of saving on supply earth sizes?
Opt for a TT with a ufer earth instead of a TN-S?

 

[Knobhead]

To be changed.

Care to elaborate?

 

[Rockingit]

OK, I'll bite..... genuinely curious now. On a recent install there was a largeish (3m DIA) hollow steel silo buried about 4m deep into the concrete/foundations as the former for a cellar. So I thinks 'bingo' - what a great way to lower my Zs values.....so I duly put a 10mm bond from it to the nearest sub board about 5m away. And the net result was barely any difference to the readings. So, did I go wrong somewhere in my assumptions, or is there some part of the plot I'm missing??

 

[malcolmsanford]

The clue here Rock is in the regs and regulation 542.2.3 and the word "embedded"

I'm not a civil engineers but as far as I can see to get a foundation you dig a huge hole in the ground, inside that hole you lay a wick load of steel ree-bar and then pour tons of concrete on it, so you get your "embedded" part.

Again I'm not also a chemical engineer but I know that the chemicals that are used in the curing process for the concrete acts like a "Benotite" and with a vast volume of this and the steel that is embedded, you do get great values at least as good as a TN-S value.

I can only think that your steel "Silo" was not embedded sufficiently into the fabric of the concrete/ree-bar to be making a good connection

As I said previously out here using this I have got values of under a ohm, the lowest I can remember was about 0.72.

 

[Marvo]

Maybe the silo was coated or insulated to prevent corrosion?