If you have a hot air baloon in a high wind you tie it to Star Jones. That's star grounding. There's no danger of lightning strikes unless, of course, you are Ms. Jones ;^)
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Well, now that the humor has been nicely covered............
First, your AC sub panel, if it is to meet code, will be grounded to the main panel, but that's not what star grounding refers to.
Star grounding simply means that the ground side of all equipment in a system (or of all devices on one chassis) meet at a single point. This is done to reduce the incidence of "ground loops" inside a piece of equipment, or between pieces of equipment. The word "star" is used because if you were to draw a circuit diagram of such a grounding arrangement, it would look like a star, with all the ground legs radiating out from one point.
In audio, especially in systems where the components are connected with single ended (RCA) interconnects and all the power plugs are three-prong (with ground pins,) ground loops are easily formed. Here's why (in big general terms!): with single ended interconnects, one of the two signal conductors in each interconnect cable is connected to the chassis of the component on each end of the cable. In addition, each chassis is also connected to the ground pin on its power plug.
Let's just take an interconnected amp and preamp OK? Now if you plug both units into an electrical outlet, that means that the ground pins of each of their power plugs are "connected" because the "ground" terminal of all the electrical outlets in the house is the same "ground." So now you have a loop -- in other words the two chassis are connected -- once by the ground pins of their power plugs, and again by the signal interconnects, thus forming a loop!
You need to break this loop to eliminate 60 cycle hum (why is another topic) and you can't break it at the interconnects, or you'll lose your signal. So the only other place to break the ground loop is at the grounded power plugs. If one of the pieces of equipment didn't have that pesky ground pin on its power plug, the loop would be broken; and everything would still be grounded (via the interconnects) through the ground pin on the remaining piece of equipment!
In practice, since all interconnects lead either into or out of the preamp, we choose the preamp to be the only component that's actually "grounded" at the wall outlet, through the ground pin on its power plug. All other equipment uses a little adapter on the power plug (sometimes called a "cheater" plug) that eliminates the ground pin connection to the wall. The equipment is still "grounded to the wall" via the interconnects connecting it to the preamp. So now the preamp becomes the center of a ground "star", the single point through which the entire system connects to ground.
So how does star grounding apply to an A/C sub panel? What my electrician did is run a 40 amp 65' 8 gauge non-armored A/C cable from the main A/C panel to the sub panel. I then have 4 20 amp 7' runs of 10/2 romex to my A/C sockets. The only ground rod that I have is a 10' 5/8" commercial grade ground rod that goes to my main A/C panel. Should I have a separate ground rod for the sub panel or a separate ground wire from the sub panel to the main panel or some other configuration? The system sounds great, but maybe there's something that I can do to get a lower noise floor. I plan on using cryoed breakers and already have good A/C sockets. I also plan on using Virtual Dynamics A/C wire to replace the 8 and 10 gauge wire that I have presently. Thanks! Stan
65' #8 awg wire, that's kinda small wire for the distance. I am just guessing but the electrician installed 8-3 with/ground nmc, romex, for your sub panel feed. 2 hots, 1 neutral, and bare ground. The bare ground is terminated on the neutral/ground bar in your main electrical panel, just where it should be. Your sub panel is 120/240 volt, 40 amp rating.
Star grounding, "Nsgarch" said it well.
Inside your sub panel there is a separate bar for grounding. This ground bar is where the electrician connected the bare grd wire from the #8-3 w/grd feeder. This grd bar is also were the branch circuit equipment grds connect to. A common ground point, star ground.
No separate ground rod for a sub panel.
Stan, Like I said, you want to have a continuous ground from your new AC sockets > to your sub panel > to your main panel. If you put in a new ground rod 70' or more away from the one at the main panel, there could be a potential (voltage difference) between the two ground locations causing a current in the neutral leg -- you don't want that.
How does the 8AWG transition to the 10AWG -- through 4 breakers at the sub panel? I hope so! And do you have a 40A breaker (or fuse) at the main panel where the 8AWG takes off?
I think what people mean when they tell you to star ground your panel is slightly incorrect. I think what they really mean is to star wire it. That means that each duplex outlet in the wall should be wired directly back to the panel with a hot, neutral and ground wire, and not daisy-chained with other outlets like is done in cheap plug strips.
But electrical systems MUST BE GROUNDED (for fire insurance reasons if nothing else!) When you finally plug everything in, you will probably want to lift the ground pin on all the three prong plugs of all components except the one on the preamp, UNLESS -- you are A.) using all balanced interconnects, B.) a Balanced Power unit, or both.
If you have breakers at your sub panel (and I hope you do!) you can wire a little high speed diode between the equipment side of each breaker and ground. If there's a power surge or lightening strike, the diode will close, grounding and thus opening the breaker, before the surge can hurt your equipment (of course you'll have to replace the diode!) but it's a way of getting great protection without any sonic degradation -- which you do get from most power units that incorporate traditional surge protection circuits (the ones that can be reset :~)
Correction: My electrician said that I was okay with 8 gauge wire up to 80 feet- I'm pretty sure, (not 180 ft), after which time I need to use 6 gauge wire.I know that he said 8 gauge was the proper wire to run for code for my 65' run to sub panel. Nsgarch-----Yes, there are 4 20 amp breakers in my subpanel. And yes I've both heard and read that it can be dangerous to run a separate ground rod, which I haven't done. Also, the breaker at the main panel is a 40 amp breaker, which connects to the 8 gauge A/C wire that then goes to my sub panel.Thanks!
Gunbei, Well, I already have a dedicated romex A/C system that's been in place for about a year now and it seems that I'm already star grounded... So all that's left to do is change out the romex for Virtual Dynamics wire... 10 gauge-(short run) and probably 6 gauge armored for the long run-(If my electrician says that it's okay), get some cryoed breakers and I'm good to go. As always, Audiogon has proved to be an invaluble source of knowledge. Best! Stan