[TenTec] ufer ground?

[GARY HUBER]

Jim and all others,

I was just the radio and telecommunications guy (who was sent to a NEC 
Lightning and Grounding school AFTER an incident) and had no say in the 
double run of 400 MCM conductors down the elevator shaft. But after going to 
the NEC (National Electrical Code) school I understood the folly of isolated 
grounds and the importance of supplemental grounds bonded to the Main 
Distribution Point's grounding point as well as the requirement to do so. 
At the time we were having lots of problems with new digital Private 
Business Exchanges (500 to 600 at that time) which were going off line. A 
review of every location revealed one or more NEC grounding violations.

The best one can design, build, and hope for is low impedance to ground and 
low potential differences between points, so that every thing rises at the 
same potential. Any potential difference will make a significant financial 
difference or a potentially fatal difference.

73 ES DX,
Gary -- AB9M

-----Original Message----- 
From: Jim Brown
Sent: Monday, August 12, 2013 11:36 AM
To: tentec at contesting.com
Subject: Re: [TenTec] ufer ground?

On 8/12/2013 7:03 AM, GARY HUBER wrote:
> My previous work place used a Ufer ground mat (basement floor rebar of all 
> buildings of the complex) bonded to building steel (and pilings) with 
> copper grounding bars bonded to them. An external ground ring with ground 
> rods was bonded to each vertical building steel member, with air terminals 
> (on each building roof) bonded to building steel. Copper grounding bars in 
> 14th story equipment room were connected to the ground mat with a 250 foot 
> run of a pair of 400 MCM cables. The electrical service neutral and 
> mechanical ground were also referenced to the ground mat.

Everything sounds exactly right about that except the separate 250 ft
run, which is certainly legal, but would just as well be done to
building steel on the 14th floor. The virtue of going to building steel
is that there are many paths in parallel, and they are all bonded
together at multiple points, which does two very important things. The
paths are in parallel, thus minimizing the inductance to earth (a much
greater component of the impedance to earth than the resistance), and
the bonds between them at adjacent points minimizes the potential
difference between adjacent points.  Indeed, bonding all the grounds is
REQUIRED, not optional.

73, Jim K9YC
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