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[thedug]

Hi,

I had gotten some help earlier on the recpit forum concerning adding tech power for my studio. I just have one quick question. First I'd like to explain what I did.

I installed a new subpanel off my homes' main panel. I have a dual 50 amp breaker that feeds into the new panel. Ther there is red/black/green wires feeding to the new panel. I remeoved the bar connecting the ground bus to the neutral buss. The green wire goes to the the nuetral buss. The red & black wires go to the two power inputs for the breakers. All the outlets are the orange isolated ground outlets. The ground comes to the ground bar, the white to the nuetral bar, the black to the breaker. Now the ground bar is connected to an 8' ground rod that I drove into the ground. This is connected via #6 copper wire.

So to recap, my orange outlet grounds all connect together at the ground bar in the new panel then go to the new ground rod. The neutrals connect at the new panel then go to the ground/nuetral bar in the main panel.

The ground is totally isolated, do I need to connect the two rods together? If so where is the best place to do it? My electrician is telling me that it isn't necessary.

d./

[Padboy]

I'll start off by saying I have zero experience in trying to do what you are attempting, but through the magic of google, I went a looking, and found this -

http://www.ruelsa.com/notas/tierras/pe30ig.html

Pay particular attention to figures 8 and 9. That's what it looks like you've done, and they (Liebert and the NEC)consider that a dangerous way to do business.

Depending on how far apart the 2 grounding rods are, and local soil conditions, you could be setting yourself up for a problem if something goes wrong. As a test, meter the AC voltage between the 2 separate ground rods.

I have done a gazillion remote tv/audio productions, and the only buzz/noise free way to accomplish true noise and common mode rejection is to run the whole 'object' (truck/room/whatever)into an isolation transformer and *then* bind the neutral/ground to that separate ground rod located as close to the binding point as possible.

The 2 TV trucks I designed were built this way. And in practice, I found that the ground rod would sometimes bring the hum back.

So. While I *did* have a green Camloc for the power ground, I also had a secret switch that disconnected it from the truck. As long as all monitors and anything else that connected to the truck were *powered* by the truck, all was cool. The fire marshall and the electrician always bought it.

later,

pb
(hoping the statute of limitations has expired)

[Geoff_T]

Hi

If you connect your new isolated technical ground to the original ground/neutral bus, you'll defeat the object of the isolated ground.

[thedug]

I wouldn't think it would be totally defeated.

It still reduces the pathways to ground.

d./

[Geoff_T]

Hi

In studios the regular ground supplied by the power company can have all manner of devices hanging on it, fridge/freezers with clunky thermostats, air-conditioning units, light dimmers and heaven knows else what. There's a high probability that crud from these sources could get imposed on the ground... especially if it's tied to the neutral bus.

In the studio environment you want a clean, isolated ground that's unique to that environment. You want everything in the studio tied tightly to that ground so that there's minimal potential difference between the different devices.

If you are just thinking in terms of basic ohms law... then joining two grounds together would reduce the overall impedance to ground but this overlooks...

a. How good the other ground is

and

b. All the nasty equipment attached to that ground and what other buildings/premises are hanging on it.


I'd recommend keeping them separate but am open to contrary opinions

[Geoff Doane]

thedug wrote on Sat, 05 June 2004 17:04

The ground is totally isolated, do I need to connect the two rods together? If so where is the best place to do it? My electrician is telling me that it isn't necessary.

If the electrician says they don't have to be connected together, he probably knows.  In our studio complex, the technical ground does connect to the regular ground at the water pipe entrance to the building, and it doesn't seem to cause any problems.

You mention red, black, and green wires from the main panel to the pony panel.  Shouldn't that be red, black and white?  The cases of the two panels should connect to the regular ground, while your isolated ground bar goes to the second ground rod.  You will need to use 3 wire plus ground cable for the individual branch circuits.  3 wire BX is often used for this, with green tape on the red wire (at both ends!) and that becomes the isolated ground.  The regular ground is still connected to the boxes at each end.

Geoff Doane (too many Geoffs here   )

[Geoff Doane]

padboy wrote on Sat, 05 June 2004 21:48

So. While I *did* have a green Camloc for the power ground, I also had a secret switch that disconnected it from the truck. As long as all monitors and anything else that connected to the truck were *powered* by the truck, all was cool. The fire marshall and the electrician always bought it.

On a recent remote I was involved with (Friday), the electrical inspector didn't buy having Camlocs in the ground line!  He insisted a new green wire be run from the service entrance box to the FOUR ground rods (I guess two rods wasn't enough).

Geoff Doane

[Geoff_T]

cbc6403 wrote on Tue, 08 June 2004 12:10

padboy wrote on Sat, 05 June 2004 21:48   So. While I *did* have a green Camloc for the power ground, I also had a secret switch that disconnected it from the truck. As long as all monitors and anything else that connected to the truck were *powered* by the truck, all was cool. The fire marshall and the electrician always bought it. On a recent remote I was involved with (Friday), the electrical inspector didn't buy having Camlocs in the ground line!  He insisted a new green wire be run from the service entrance box to the FOUR ground rods (I guess two rods wasn't enough). Geoff Doane

Hi Geoff

For the benefit of a soul from t'other side of the pond... what is a "camlock" as applied to USA wiring?

On a Neve it's the quarter turn fastener that holds the wood on!

[Geoff Doane]

Camlocs are large, single conductor, electrical connectors used for power distribution for temporary setups.  I think "Camloc" is now owned by Crouse-Hinds.  "Cams" are colour coded for the different phases and can accommodate up to 4/0 wire (about as big around as your thumb).  They are considered to have a current rating of 400 amps.  They are not foolproof, since you can mix them up, or disconnect the ground, which is why I suspect the electrical inspector didn't like to see one in the ground.

Geoff Doane