[brian john]

Recording studio, then let's assume there are two transformers one for lighting another for critical receptacles utilized for sound equipment.

In this case you would have the current from the "bootlegged" lights having to return to the source transformer over the ground.

This ground current is the number one cause of 60 hz hum in sound systems

 

[brian john]

Is this what they would call "electrical noise?"

One type yes.

 

[brian john]

So in the scenario pictured, the circuits serving audio gear connect to the panel pictured, and the lighting circuits connect to a different panel not served by the transformer? Or vice versa?

I've heard of this, but I've never seen it, and presumed it wasnt common. Especially as studios are getting smaller, more numerous, and smaller in budget.

I'm still not following how, in Brian's post, the current from the "bootleg" light ends up traveling back via the ground rather than the neutral?

It would return to the panel/transformer neutral then at the neutral ground bond the current would travel on the ground (copper, water pipe, metal duct, grounding electrode conductor all metallic components in the path) to the neutral ground bond of the separate panel/transformer.

 

[brian john]

While not what we would call common, many of the better studios do have isolation transformers. See page 21 of this Middle Atlantic paper.

"Integrating Electronic Equipment and Power into Rack Enclosures"
'Optimized Power Distribution and Grounding for Audio, Video and Electronic Systems'

http://www.middleatlantic.com/pdf/PowerPaper.pdf

Couple issues with your drawing

As short as possible-LOGIC BEHIND THIS
Is the EGC Termination Bar in the panel isolated?
I contend double neutral is not necessary (NOW I SEE WHY 120/120)
I assume the they assume metallic conduit and do not require a copper EGC in the conduit?

 

[brian john]

Hypothetical, what's that? The OP is pondering the cause and effect of a
Code breaking application! There's nothing else to it, it's real!

Are you saying we should not explain why something is wrong and just tell him accept the NEC at face value with no explanation?

 

[brian john]

Do you have a part number on that isolation tranny. I want to see its specs. I havent seen a 120/120/120 transformer.

I missed the 120/120

 

[brian john]

Not only do some use isolation transformers, some studios use a balanced power system that has no neutral.
Just like in an operating room they have a 2 pole breaker for each 120 circuit and you measure about 60 volts to ground on each leg.

That is an ungrounded system.

 

[brian john]

Back to the isolation transformer in my post #8. The one in the drawing is a 240V input with dual 120V outputs. You really need to look at the white paper, not just one drawing.

The Middle Atlantic white papers with addendum are at:
http://www.middleatlantic.com/resources/whitepapers.htm

The:
"Integrating Electronic Equipment and Power into Rack Enclosures"
'Optimized Power Distribution and Grounding for Audio, Video and Electronic Systems'

white paper is at:
http://www.middleatlantic.com/pdf/PowerPaper.pdf

I have done quite a few studios, radio stations and TV stations and in my experience most of this junk is just that JUNK dreamt up by a engineers trying to sell a new product or magical fix a problem. A properly installed electrical distribution system will work without elaborate grounding schemes. IG's and the isolation transformers will buy you nothing if there are any neutral grounding issues in the facility.

I have installed many a shielded isolation transformer with IG's because if the customer wants it, pays for it, I'll install it.

 

[brian john]

I've heard "horror stories", if you will, about how poorly orchestrated shared neutrals can cause line noise and harmonic currents that can cause problems sensitive equipment elsewhere in the building. This doesnt wholly make sense to me, hence asking people smarter than me.

You heard horror stories from salesmen selling audio equipment or from electricians that do not know what they are doing?