On 10/7/2012 7:50 PM, gregz wrote:
wrote:
On 10/6/2012 11:28 AM, gregz wrote:

The
common connection is the same place ground is, unless it goes to a
different ground stake or stakes.


Not obvious what you are saying.

You do not want multiple earthing systems. You create an earthing system,
which may have multiple earthing electrodes, and connect it at the
service to the power system ground, which is bonded to the service
neutral. Entry protectors for phone and cable must connect with short
ground wires to a common connection point on the power earthing system.
Dish entry protectors also connect there.

Multiple earthing electrodes can be at far different potential during a
surge 'event' or a nearby lightning strike. Much of the protection is
that during an 'event' the building wiring may rise far above 'absolute'
earth potential, but they all rises together.

I was just thinking about the two or three ground rods near my service
entrance. Not sure if there is a code stating things other than so many
feet deep. I never really thought about it, by my grounds must be poor.
They are close to the foundation, on the dry side of the house, and they
might not even go below foundation, because of the landscape.

If using a rod the code wants it to be 25 ohms to earth, or you can
install more than one with no resistance requirement. Easiest is to
install 2 or more. I would guess the 3 rods are the power earthing
system. I have occasionally driven a rod through the floor under the
service. It avoids being close to the foundation and may be in moister
soil. Rods are a crappy earthing electrode.

I got another
ground rod on other side of house off my aluminum porch roof, and iron
railing. I know the light box ground is also connected to the same porch
metals. A strike somewhere might cause significant current right through
house. Another ground rod off elevated deck attached to metal rods in the
air, plus cb antenna.

I would guess the rods off your metal roofs are crude lighting
protection. It might prevent a fire at the point of the strike, but does
not prevent major electrical damage.

For a lightning rod system, the earthing system must be bonded to the
power earthing system. With high lightning currents to earth and
resistance to earth plus impedance of down conductors there can be a
very high voltage between down conductors and other metal. Metal within
6 feet of the down conductors may have to be bonded to them. Protection
from a direct strike isn't simple.

The NEC wants metal support parts of an antenna connected to the power
earthing system. And a protector(for coax just a ground block) that is
connected to the power earthing system where the antenna lead enters the
building. This will not protect from a direct lightning hit to the antenna.


Separate garage, wired from house. There is no ground rods at garage, and
no boxes. I think there might be a code for a ground, not sure. I have no
box in garage, except for junction boxes. There is a ground rod connected
to sheds metal roof.

Greg

The code wants a grounding electrode at a detached garage if fed by a
feeder (with panel in garage), but it is not required if there is only a
branch circuit to the garage.