Ground bus question

[weinstro]

Hello,

I have a hum problem in a home theater installation when I connect two pieces of equipment on separate circuits.

In the sub-panel, the electrician added an additional ground bus. The original bus appears to have a separate non-insulated cable which presumably connects to the main service panel.

The second ground bus does not appear to have either a connection to the grounding point in the main panel, nor a connection to the original bus bar. It is bolted to the sub panel enclosure. Only 240v circuits appear to be connected to the secondary ground bus.

Is this a problem?

Thanks,

Rob

 

[mikehughes8]

If both ground buses are connected to the panel enclosure then electrically they represent the same point. Therefore the EGC on the first bus also connects the second bus to the main disco. To remove any doubt perhaps you could bond the buses.
The voltage of the circuits connected to each bus are irrelavant. The ground bus and EGC carry fault current.

 

[hurk27]

To address the hum problem, the cause of this is neutral current on the grounding, this could be caused by the way the sub panel is wired if one circuit is fed from it and the other is fed from the main panel, or a inadvertently grounded neutral at a receptacle or switch, or even at a light fixture. some times it can be found by turning off each breaker until it stops (leaving the two circuits on, that feed the sound system) then just take apart each outlet on the affecting circuit to find the problem. if the hum is still there then it could be on one of the circuits the sound system is on. when the circuit is found then there are many ways to find it and are much the same as trouble shooting GFCI or AFCI tripping problems.

Here are a few of the problems I have found to cause this.
besides a grounded neutral, I found in one case was where someone lost a neutral in a buried box and just used the ground for the neutral to get the circuit back running.
In a church sound system, the mixer was fed from a panel from a added new service, and the amps was fed from the existing service, which put the shielding parallel to the service neutral, not much you can do but run a separate circuit to feed the mixer from the existing service. the stage snake had about 32 channels, and the amp returns was another 12, so it wasn't cost worthy to use the isolating 1k ohm 1 to 1 transformers.

since current takes all paths to source, this current on the grounding will also be on the shield of the audio cable, and the audio amp in the sub woofer does what it is design to do, amplify it.

The easy way to eliminate the hum would be to install a 1k ohm 1 to 1 isolating transformer in the audio cable feeding the sub woofer from the main amp, these can be bought at car/home audio supply places like best buy, there called various name like hum busters.

 

[tryinghard]

...The second ground bus does not appear to have either a connection to the grounding point in the main panel, nor a connection to the original bus bar. It is bolted to the sub panel enclosure. Only 240v circuits appear to be connected to the secondary ground bus...

Rob, Just remember the grounded conductor (insulated neutral and usually white color) does not connect to the equipment grounding at any point past he service entrance, unless your at a transformer.

• 250.24(A)(5)
• 250.142(B)

This purpose is to eliminate parallel current returning to its source, to contain current on the intended insulated conductor the neutral only.
In the voltage world the equipment grounding has virtually nothing to do with the functionality of the circuit its only purpose is to enable an effective ground fault path to source. It may contain stray current but is never intended to carry parallel current. Again its only purpose is to carry fault current.
Correctly installed circuitry is the best way to insure clean sound.