On Tue, 6 Mar 2007 08:46:02 +0000, Ian White GM3SEK wrote:
>Let's be careful here. "All grounds within a building," as stated above,
>seems rather too sweeping.
North American electrical codes, nearly all of which are patterned on the NEC,
say that you must bond the power system neutral at the service entrance to an
earth electrode of at least minimum quality. You can add as many earth
electrodes as you choose, but they must all be bonded together. The code also
requires that all grounds (earths in British English) be bonded to the power
system ground as well. For example, conductive water pipes, structural steel,
lightning protection, etc. The code states that the lightning protection system
earth electrodes cannot be depended upon for an earth bond -- in other words,
there must be other earth electrodes as well, but they must all be bonded.
>I had understood the requirement in the USA to be as stated by W8JI last
>night: "NEC prohibits multiple independent grounds on systems ENTERING
>the house". That is very different from Jim's wording of "grounds WITHIN
>a house."
>Using the drawing at the top of Tom's "Grounding" page
>(http://www.w8ji.com/station_ground.htm), the NEC requirement seems to
>be that if you install a new ground for your radio, that comes into the
>house at B, there must also be separate ground bonding OUTSIDE of the
>house, running from B to the service entrance at C.
The "outside the house" bond between earth electrodes is one of good
engineering practice. The reason for the bond is to minimize potential
differences between the earth electrodes in the case of lightning. It makes
sense to make that connection outside the house (so that current is not tempted
to jump the conductor and go through house wiring instead) and as short as
possible (to minimize inductance).
>In the USA, must this B-C conductor always be a buried bare wire - in
>other words, helping to reduce the ground impedance?
Making it bare is not required, but is not prohibited. It is a free choice to
the system engineer.
>(The reason I'm asking is that similar bonding is also required in the
>UK, but it is allowed to be an insulated conductor running through the
>house - which is OK for AC safety, but a Very Bad Move for RFI and
>lightning protection.)
As noted above, we agree. :)
On Tue, 6 Mar 2007 02:30:33 EST, Gudguyham@aol.com wrote:
>Separate buildings and structures (like a tower 100ft from the house) are
>allowed to be grounded separately and do NOT need to be connected to the house
>
>ground. If you send a separate ground out the window to the ground for your
>ham shack in the same structure as the electrical system you should bond that
>grounding electrode to the main grounding electrode for the electrical
>system.
I don't agree with this, at least from an engineering (lightning safety) point
of view, if that tower is supporting an antenna connected to the house. In that
case, I would ground the tower as well as I could, and I would bury a conductor
bonding it to the house ground in a manner that didn't conduct currents through
the house (in other words, to a star point outside the house). The reason for
this bond is that we don't want lightning current trying equalize the voltage
between the house and the tower to travel through the house (ham shack), we
want them in the earth. And we want that to be the shortest possible route.
I think it also makes sense for this to be a bare conductor IF soil conditions
are such that the conductor will remain a good conductor. :)
To understand the logic of these earth bonds, remember that most soil is quite
lossy (resistive). If you drive ground rods 20 feet apart, you'll probably
measure lots of ohms between them. On the other hand, the soil is a ground
plane, so the inductance may be fairly low. Also remember that a major reason
for bonding all grounds together is that adding inductances in parallel
reduces the total inductance, following the same equation as resistances in
parallel.
73,
Jim Brown K9YC
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|