Installing a Transformer for Isolation

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cosmicelectric

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Hello all,

I was recently contracted to install a piece of equipment that needs to be protected by means of isolation. I plan to do this via a transformer. I have experience installing transformers for step up and step down applications, but have not installed one for an isolating scenario.

My question is, can I use a 240x480 - 120/240 step down transformer and wire it in such a way that is simply a 240 - 240 isolating transformer? So no step down occurs, it simply passes the same voltage through? I have never wired one like this.

I am looking at this transformer currently -


I have attached the wiring schematic, and have highlighted the way in which I believe I can achieve what I have explained. Wire the primary for the 240 configuration and the secondary also for the 240 configuration.

What do you guys think?
 

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cosmicelectric said:
Would you mind providing a brief field example of how I would achieve proper grounding in an application such as this?
Click to expand...
This is a system that is permitted to be, but not required to be a grounded system.

If you want to make it a grounded system, you make one of the two wires a grounded conductor, and provide a grounding electrode and grounding electrode conductor. Note that the grounded conductor must be identified as such per 200.7
 
220416-1332 EDT

I would ask the question "what does isolation mean"?

Does a transformer with a separate primary and secondary, and no resistive conductively between primary and secondary provide isolation?

Provide more details on the goal of the request.

.
 
don_resqcapt19 said:
This is a system that is permitted to be, but not required to be a grounded system.

If you want to make it a grounded system, you make one of the two wires a grounded conductor, and provide a grounding electrode and grounding electrode conductor. Note that the grounded conductor must be identified as such per 200.7
Click to expand...
I believe the type of transformer the OP has, two winding, would result in the system being required to be grounded.
 
wwhitney said:
Well, if the X2-X3 connection is inside the transformer and only X1 and X4 are brought out, would that not make it a 2-wire 240V "system" for applying 250.20(B)?

Cheers, Wayne
Click to expand...
Did you see the OP's attachment? It appears the X2 and X3 are available. This is a very common transformer design where you connect the two windings in parallel for 120V or series for 240V. X2 and X3 effectively become a center tap, thus the system "can be grounded such that voltage is less than 150v to ground."
 
cosmicelectric said:
My question is, can I use a 240x480 - 120/240 step down transformer and wire it in such a way that is simply a 240 - 240 isolating transformer? So no step down occurs, it simply passes the same voltage through?
Click to expand...
Absolutely.

cosmicelectric said:
Would you mind providing a brief field example of how I would achieve proper grounding in an application such as this?
Click to expand...
You could ground X2-X3 as a neutral to ground, even if you don't need 120v.
 
electrofelon said:
Did you see the OP's attachment? It appears the X2 and X3 are available.
Click to expand...
Sure, they are available at the transformer. But I guess I'm proposing that the "system" is determined by what conductors you choose to bring out of the transformer. Why does the existence of a secondary tap that I don't want to use as a circuit conductor force me into choosing a different wiring system?

I.e. does 250.20(B) really look inside the transformer, rather that at just which conductors are in the resulting feeder?

Cheers, Wayne
 
wwhitney said:
Sure, they are available at the transformer. But I guess I'm proposing that the "system" is determined by what conductors you choose to bring out of the transformer. Why does the existence of a secondary tap that I don't want to use as a circuit conductor force me into choosing a different wiring system?

I.e. does 250.20(B) really look inside the transformer, rather that at just which conductors are in the resulting feeder?

Cheers, Wayne
Click to expand...
I think so. Note we have had a similar discussion when discussing protecting the secondary conductors with the primary OCPD. The allowance for this requires a 2 wire secondary. Does that mean "used" or "available"? I see this a little differently though as you CAN ground this system to get less than 150V to ground.
 
electrofelon said:
I think so. Note we have had a similar discussion when discussing protecting the secondary conductors with the primary OCPD.
Click to expand...
But in that case, what does the physics say? (I forget).

If you have a 3-phase delta-wye transformer, but only bring out 3 circuit conductors on the secondary side (X0 has no connections at the transformer, not even to the case), are the primary currents in direct proportion to the secondary currents?

Similarly if you take a dual voltage single phase transformer like in the OP, and only bring out X1 and X4, and leave X2-X3 ungrounded, will the primary current be in direct proportion to the X1-X4 current? In this case I think so.

Which from my point of view just gives another reason that you should be allowed to ignore X2-X3 and use a 2-wire secondary, so you can take advantage of the allowance to protect the secondary conductors via the primary OCPD.

Also, it's worth looking 250.25 "Conductor to Be Grounded — Alternating-Current Systems." It starts off "For ac premises wiring systems, the conductor to be grounded shall be as specified in the following:" Here I would say that by specifically referring to the premises wiring system, it is clear that what matters it the choice of feeder conductors brought out, not the capability of the transformer. So I would suggest that is the intent of 250.20(B) as well.

Cheers, Wayne
 
..and just an interesting side note to post #10. There is a difference between the "/" and the "X" designation. Windings with the X are NOT suitable for three wire service, while ones with the / are. I do not understand why. There is a link to the document, ANSI c57.12 at the last post in this thread:

240 x 480-120/240 V Transformer Ratio

Greetings, Gents - I'm a little confused on '240x480' designation on the high side of the transformer. This is different from ' 480-120/240' designation. Does this mean that I can connect either 240V or 480V supply on the primary and get 240V/120V on the secondary side? What does "X' mean in...
forums.mikeholt.com
 
Jake@01 said:
I assumed they were 120v before u showed the wiring schematic.

Sent from my SM-S906U using Tapatalk
Click to expand...

cosmicelectric said:
Hello all,

I was recently contracted to install a piece of equipment that needs to be protected by means of isolation. I plan to do this via a transformer. I have experience installing transformers for step up and step down applications, but have not installed one for an isolating scenario.

My question is, can I use a 240x480 - 120/240 step down transformer and wire it in such a way that is simply a 240 - 240 isolating transformer? So no step down occurs, it simply passes the same voltage through? I have never wired one like this.

I am looking at this transformer currently -


I have attached the wiring schematic, and have highlighted the way in which I believe I can achieve what I have explained. Wire the primary for the 240 configuration and the secondary also for the 240 configuration.

What do you guys think?
Click to expand...
I think doing it that way has no problem coz you will get the 240/240 voltages hence the voltage flow will not be affected. But I would like to advise that we already have some isolation transformers that already exist in the market and you can try finding out about them. For me, I had such a problem but now in the area of the printed circuit board and I had to use a transformer from PCBway in the link https://www.pcbway.com/project/share/Isolation_transformer.html?
 
220416-1029 EDT

No one responded to my previous post.

Why is an isolation transformer requested? What problem is it to solve? What is an isolation transformer? There has to be adequate information to provide any reasonable answer.

For example, an ordinary transformer ( ordinary meaning no DC electrical conductivity from input winding to output winding ) will not prevent a large spike on the input waveform from passing thru, nor will an ordinary transformer prevent capacitive coupling from primary to secondary, nor will it prevent voltage variation on the input from passing thru, etc.

.
 
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