[TowerTalk] Back of desk grounding buss

[Jim Lux]

On 3/22/14 2:40 AM, Ian White wrote:
>>> Get the Harger ground bar.
>>
>> Big waste of money.
>>
>> 73, Jim K9YC
>
> Agreed. The topic here is "back of the desk grounding", and in that
> location a massive copper busbar is a big waste of money. Worse still,
> it may indicate a dangerous lack of understanding.
>
> The place for massive copper conductors is <outside> of the building and
> at the common entrance point. These conductors need to be large enough
> to handle huge lightning-induced surges and divert them away from the
> shack. Once again: the aim is to keep large surge currents <out> of the
> shack entirely, or at least to reduce them to some low residual level.

I'm not so sure about *massive*..

AWG 10 wire that is 0.1 inch (2.5 mm) in diameter has a fusing current 
that is well above the typical lightning stroke.

Using a big conductor is more about mechanical ruggedness and wiring 
convenience.  A big conductor is better able to withstand the slings and 
arrows of being outside and getting run into by lawn mowers, hedge 
trimmers, etc.   A big conductor is better able to withstand the 
magnetic forces from the big current pulse.



>
> By the time those residual surges reach the back of the desk, there
> should be no further need for massive copper conductors. The bonding
> strip along the back of the desk is there for a different purpose: to
> provide a low-impedance "local ground" reference for all the equipment
> lined up along the desk. The aim is to provide a low impedance at all
> frequencies from DC through 50/60Hz up to RF, and when you run the
> numbers the main requirement is for low inductance, which is best
> achieved by a wide flat strip conductor or a large-diameter pipe. Aim
> for the low inductance, and you will almost always find that the
> conductor cross-section is large enough for 50/60Hz safety and hum
> suppression as well.

Wide flat strips have low AC *resistance* because of skin effect, but 
the inductance isn't much different from a round wire.

AC resistance is *very important* in applications like RF grounds for 
commercial broadcast antennas, because resistance = heat = lost power = 
lost money, so they use flat ribbon.

Flat ribbon/strip has a high surface area to volume ratio, and AC 
resistance is all about surface area for RF because of skin effect, so 
if you're paying by the pound for the copper, it's the best deal.

Inductance just isn't strongly affected by the shape of the conductor. 
The NBS monograph by Rosa (from 1907, it's one of the first ones 
published) has all the formulas.

Flat strips don't have markedly lower inductance for a fairly simple 
reason..

Consider your ribbon as a bunch of parallel wires.  Each of those wires 
has some inductance L, and you'd think that putting N inductors with 
inductance L in parallel would give you an inductance of L/N.  But the 
problem is that those wires are right next to each other, so they have a 
significant mutual inductance (the magnetic field of wire #4 is tightly 
coupled to wires #3 and #5 next to it, etc.).  That tight coupling means 
that the inductance of the parallel combination just isn't that much 
lower than of one wire.

The inductance of two parallel inductors is:
(L+M)/2
where L is the inductance of a single inductor and M is the mutual 
inductance.



>
> One more time: the main purpose of the strip along the back of the desk
> is to ensure that all the equipment metalwork is closely connected
> <together> at all frequencies from DC to RF. The best term for this is
> "BONDING".



>
> (Thoughtful authors try to avoid  calling it "grounding" or "earthing"
> because a connection to the Mother Planet is not the main aim. You
> should connect the bonding strip to a mains safety ground as well, but
> that was not your main reason for having the bonding strip.)

There is an ongoing effort for the National Electrical Code (in the US.. 
don't know whether the same is true elsewhere) to change the wording 
from grounding to bonding.  There's huge confusion among users of the 
code about terms like "grounding conductor" and the "grounded conductor" 
  (the former is the green wire, electrical safety ground, the latter 
the "neutral" or white wire).

They really want to get people away from thinking about connecting to 
"earth", because in a lot of cases that's not relevant.

Of course, we talk about ground loops, etc. on *airplanes*.  And I was 
in a meeting not too long ago talking about the grounding tree for a 
spacecraft.

That terminology thing will be with us for a long, long time (just like 
Franklin's choice of which direction to have the current arrow go)