DIYbanter - View Single Post

Bud--

wrote:
On Sat, 25 Sep 2010 20:13:55 -0500, bud--
wrote:

Sorry w - It looks like nobody agrees with you.

Although it is required that the water pipe must ALSO be used if
present, there can be a problem in doing so. All grounds are not
created equal, and when you have multiple ground electrodes, you
introduce the potential for differing potentials. (That is the main
reason for the newest methods used in new construction.)

Ground loops.

In an industrial plant you may have even more electrodes. In general it
doesn't matter because the electrodes are combined into an "earthing
system" and the power system ground reference is likely at the
neutral-ground bond at the service disconnect, where the earthing system
is connected (in general).

On the other hand, suppose you have a system earthed only with a ground
rod, and it has a NEC allowed (and very good) resistance to earth of 10
ohms. If you have a strong surge coming in on the power wires that
produces a current to earth of 1000A, the power system "ground" is
10,000V above absolute earth potential. Most of the voltage drop away
from the ground rod is next to the rod. The power ground is 7,000V or
more above the earth potential 3 ft or more from the rod. I don't see
how additional electrodes are a problem. Apparently the people who write
the code don't either.

One problem THAT may cause is problems with sensitive electronics,
like the control boards in modern electronics.

The NIST guide on surges, which has been linked in many threads,
suggests that the major cause of equipment damage is high voltage
between power and phone/cable wires - probably about the same as what
you said.

That's why the ground reference for an entering cable system must be
near the same as the power ground reference - which requires connecting
the cable entry ground block with a *short* wire to the earthing system
near the service disconnect. And similarly the phone entry protector.

Much of the protection is not that everything is at "earth potential",
but that everything is at approximately the same potential.

So, now this thread has come full circle. G

I'm getting too old to look.

--
bud--