[TowerTalk] Re: Floating Ground Steel Building?

[Jim Lux]

----- Original Message -----
From: "Tom Rauch" <w8ji at contesting.com>
To: <towertalk at contesting.com>
Sent: Friday, July 02, 2004 4:31 PM
Subject: Re: [TowerTalk] Re: Floating Ground Steel Building?


>
> If you think concrete is a good conductor, measure it with
> an ohmmeter. Concrete is a crummy conductor, not a good one.
> It actually is someplace between being a very poor insulator
> and a very poor conductor.

But it's large with large surface area, so the overall resistance to ground
might be quite low.


>
> Now if you fill it with rebar (especially if you chemically
> treat it) you can tie it in as a UFER ground, but that
> should NEVER be a primary ground. It should not be the
> ground for service entrances, especially not with people
> living on the concrete.

Except, of course, that the NEC specifically allows UFER grounds, so it
can't be all that bad, pressure from the construction industry
notwithstanding.  It might be that in real life, UFER grounds require no
maintenance, while ground rods do?

>
> As a matter of fact, I think it would be dumb to use a
> concrete floor as a lightning ground. You run the risk of
> any voltage drop, and there could be thousands of volts per
> foot even if the floor was a few ohms per foot (which it
> definitely isn't), being applied to anything contacting or
> near the floor.

>
> I'd make sure the shell of the building is well grounded
> perhaps every dozen feet to an outside ground, and that a
> heavy gauge buried ground buss wire circles the building, if
> you plan on having any towers near the building. The last
> thing you want is a hit flowing across the skin of that
> concrete floor!

The Ufer ground was invented by one Mr. Ufer to provide lightning protection
for ammunition bunkers in areas where the ground conductivity was poor, and
where driving conventional ground rods was difficult.

Certainly, Tom's comment about not wanting lightning currents flowing across
the floor is valid, so the strategy of providing an isopotential ring is a
good one. They're called step potentials in the standards covering grounding
of things like electrical substations, where they have signficant fault
currents flowing into the ground, on occasion.  The idea is that you don't
want a significant voltage differential between the feet of someone walking
around, or who reaches down to pick up a tool.

Some more reading:
http://www.mikeholt.com/documents/grounding/pdf/ufer.pdf  UFER and tower
grounding - Polyphaser recommends radials, etc. in addition to a UFER ground
for a tower.
http://lists.contesting.com/archives/html/Towertalk/1999-09/msg00177.html  a
post on this very list back in 1999 from an engineer at Polyphaser giving
some background  Attributes it to George Ufer.
http://www.scott-inc.com/html/ufer.htm  some more on Ufer grounds.
Attributes it to Herbert G. Ufer (maybe his middle name is George..?) "He
had determined that concrete was more conductive than all but the best soil,
and that this improved semiconducting characteristic would enhance surface
area contact with the surrounding soil."