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Discussion Starter · #1 ·
My company does audio visual services. When we use a single 120/208 VAC Three-phase "Y" power source for lighting, audio and video we get noise caused by the lighting dimmers injected into the audio and video. We purchased a 30 KVA Isolation transformer. We tie-in power and run it first to the lighting dimmers then put the isolation transformer in line before powering up the audio and video equipment. It eliminates buzz in the audio and interference lines in the video. The transformer has three hot and one ground connections on the input side (no neutral on the input side) and on the output side a ground, neutral and three hots. Is it correct for me to wire the input ground to the transformer case and wire the output ground to the output neutral and not connect the output neutral and ground to the case or input ground?
 

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Do you understand how the transformer works? It creates it's own neutral, so you don't need to bring one to it. So it needs bonded on the other end, somewhere. It is an isolation transformer, so there should be no real mechanical connection between sides except for the bonding.
 

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... Is it correct for me to wire the input ground to the transformer case and wire the output ground to the output neutral and not connect the output neutral and ground to the case or input ground?
That would be a code violation. The secondary neutral must be connected to the transformer frame and must be connected to a grounding electrode.
 

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RIVETER said:
Transformers don't "create" neutrals.
Well, it is a separately derived system which has its own neutral apart from the one associated with its feeders. 3 phase without a neutral input to 3 phase with a neutral output. So, it kinda does.
 

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Terminology is everything.

Maybe you could explain, in your own words, the essence of the secondary neutral point in a delta wye transformer. Being vague and uppity only gets you so far when you never actually contribute with actual knowledge.


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the transformer is delta wye, therefore the neutral is derived (ie. created) at the secondary. period.

Article 100. Definitions

Separately Derived System. A premises wiring system
whose power is derived from a source of electric energy or
equipment other than a service. Such systems have no direct
connection from circuit conductors of one system to
circuit conductors of another system, other than connections
through the earth, metal enclosures, metallic raceways,
or equipment grounding conductors.
 
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My company does audio visual services. When we use a single 120/208 VAC Three-phase "Y" power source for lighting, audio and video we get noise caused by the lighting dimmers injected into the audio and video. We purchased a 30 KVA Isolation transformer. We tie-in power and run it first to the lighting dimmers then put the isolation transformer in line before powering up the audio and video equipment. It eliminates buzz in the audio and interference lines in the video. The transformer has three hot and one ground connections on the input side (no neutral on the input side) and on the output side a ground, neutral and three hots. Is it correct for me to wire the input ground to the transformer case and wire the output ground to the output neutral and not connect the output neutral and ground to the case or input ground?
I have done many audio, visual projects (TV, recording, telco, Data Centers, large control centers..... (I trouble shoot for grounding issues)
If properly wired and installed per the NEC 99.9 0/0 of your noise issues are eliminated. And all the rest is smoke and mirrors generated by folks trying to sell product or they do not understand what they are doing.
 

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Terminology is everything.
So explain where a secondary side neutral comes from, using your terminology. You haven't yet.
If there wasn't a completely separate neutral point on the secondary side, an isolation transformer would be rather useless.
The transformer doesn't just pull a neutral out of the air.
 

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Discussion Starter · #11 ·
Thanks for the replies. I do know how transformers work, thank you. The transformer is mounted on wheels. There are Cam-Lock E-1016 single pole connectors on the input and output sides. It is used in concert arenas and hotel ballrooms to isolate the power that feeds audio and video from the lighting dimmers. A Silicon Controlled Rectifier dimming system creates spurious noise due to the saw-tooth waveform and the over-shoot and under-shoot dimming curve. And venues may have "dirty" power, i.e., non-smooth sinusoidal wave-forms.
The input is a three-wire plus ground DELTA and the output is a four wire plus ground WYE, so a neutral is "created."
The transformer is moved from job site to job site and may get its power from a "company switch" or disconnect in the back of a theater, convention center, hotel ballroom or from a portable generator. Therefore, it is not required to follow the same rules for grounding that a permanent installation does.
I am asking for input about the safest way to wire the transformer. When I acquired the unit the input ground went to a bolt through the steel case of the unit. The output ground is also attached to the same bolt. There is also a bond from the neutral on the output side of the transformer to that bolt. Therefore, the output of the transformer is not truly isolated from the input. The George R. Brown Convention Center in Houston is notorious for "noisy power." Audio and video often have buzzes and interference bars. I assume that these come from reactive lighting and H VAC loads and that all of the buildings many power transformers and sub systems share a common ground.
So, what are the safety issues that I should consider with removing the bond between the input ground and the output neutral and ground? Creating a "floating" neutral and ground on the output side of the transformer.
Thank you.
 

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So, what are the safety issues that I should consider with removing the bond between the input ground and the output neutral and ground? Creating a "floating" neutral and ground on the output side of the transformer.
Thank you.
That is what you cannot do.:no:

You're trying to apply a Band-Aid to a system which has likely not been implemented correctly to begin with.

Unfortunately for you, you would likely have to step it back to the power source and determine its health (PQA). The troubleshooting will likely lead to that power's source.
 
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I just installed a UPS System using an Isolated XFMR 480 delta primary and 120/208Y secondary. On secondary take XO to ground and also t get your neutral from XO.
 

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Isolated XFMRS are basically built to keep the noise between the windings minimal , to isolate the noise from your equiptment you need to isolate the equiptment using a triad grounding system apart from the existing ground rod your using(Per DCS proper grounding )
 

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...
The transformer is moved from job site to job site and may get its power from a "company switch" or disconnect in the back of a theater, convention center, hotel ballroom or from a portable generator. Therefore, it is not required to follow the same rules for grounding that a permanent installation does.
...
Can you cite the code section that says that?
 

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And that paper very clearly shows that the instrument "triad" grounding array is connected to the plant grounding grid as required by the NEC. Your previous statement implied otherwise.

Also there is zero evidence that installing the ground rods in a "triad" is any more effective than installing them in any other arrangement.
 
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