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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