Jack Painter wrote:
Right Ian, having equipment spread out over a wide area is an
undesireable condition, but a reality for many operators. Running 3"
wide or greater copper strapping in individual paths to one single bus
"collector" is a challenge, but one worth trying. Equipment stacked or
racked together could certainly be "grouped" and a common bonding
strap run for each group to the SPG. But 20 linear feet of bus bar
could have huge transient voltages developed across it from a nearby
strike. As in a few hundred thousand volts between supposedly
"grounded' equipment. Hence the importance of all equipment connecting
to the same ground point, not via a long run of common bus bar to that
ground point. Make your compromises according to your individual
requirements and station layout.
Thanks very much for those thoughts, Jack.
As I'd guessed, you had some good suggestions, but you can see their
practical problems too. The same applies to an alternative method, which
is to group related items of equipment as closely as possible, and
connect them by the shortest possible straps to a large sheet of metal.
The key feature is that this sheet should be *very wide*, to create a
very low inductance and help minimize any voltage drops along its
length. Conductor thickness doesn't matter in this location, so a sheet
of copper-clad PC board is fine. It's also very easy to solder short
grounding straps onto the PC board, and move them around without having
to disconnect the whole sheet to drill it.
Some people put the sheet on the table, and the gear on top of the
sheet. If there is a shelf carrying another level of equipment above,
another practical way is to fasten the sheet to the underside of the
shelf.
The reason why conductor thickness doesn't matter in this location is
that we don't expect full-bore lightning surges through the station
itself. The ground bonding at the common service entrance should have
diverted the main surge safely to ground, so the equipment bonding
inside the station should only need to deal with very much smaller
currents - smaller, but still large enough to damage sensitive
electronics. Also, the higher-frequency components of the surge will
only run along the surface anyway, so what matters most is width and
surface area rather than thickness of copper.
As Jack says, busbars are a particular invitation to develop voltage
drops along their length. This "ground-sheet" system accepts there will
be some voltage drop along its length, but does everything practicable
to minimize it. A wide sheet is vastly better than busbars, and solid
copper busbars are a particular waste of money - a large sheet of scrap
PC board will do it far better, for almost nothing.
If I had the luxury of a large station, I'd use these sheets as part of
a wider-scale "grouping" system, as Jack suggests.
Just realize that in a nearby-by strike, lightning will find the weak
points in any system and exploit that weakness to the fullest
possibility.
In this climate, lightning is actually quite a minor hazard. What has
driven me to organize and bond equipment together is the need to control
RFI due to ground currents (of which I generate rather a lot). What's
good for the one is generally good for the other... and often good for
received noise reduction too.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek