Isolation transformer

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dcv

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texas
I have a 277/120V 1 PHASE 3WIRE 10 kva Isolation Transformer feeding a Isolation panel in a cath lab which is a typical brown and orange wire from a 2 pole breaker feeding the receptacles .
Question please help me understand why the polarity is shown reverse in all outlets no matter how they are terminated.I'm using a plug in type polarity tester.
I'm really trying to understand how this transformer works 120v between phases, but not to ground.

Thank for the help
dcv
 

dereckbc

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Plano, TX
Re: Isolation transformer

Does the output have grounded circuit conductor? Your tester would be looking at voltage between N-G and seeing a voltage fooling it into a false reading
 

roger

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Fl
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Electrician
Re: Isolation transformer

Dcv, the isolation transformer is 60v - 60v which is 120v leg to leg, and has no reference to ground.

This would be the same as a residential transformer that is not grounded, only you would be 120v - 120v which is 240v leg to leg.

A receptacle analyzer will not see correct wiring and will show reversed polarity or even an open neutral.


Here is drawing from Square D

;)

Roger

[ September 21, 2004, 09:14 AM: Message edited by: roger ]
 

dereckbc

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Re: Isolation transformer

Roger, this looks a lot like a 60/120 technical power system, except it is not grounded. Interesting! Looks like it would accomplish the same thing, no grounded circuit conductor, low harmonics, clean power.

I assume they would have to provide some sort of GFP with this arrangement for protection since fuses or breakers would not work.
 

roger

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Fl
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Electrician
Re: Isolation transformer

Hello Dereck, these systems actualy use a Line Isolation Monitor to alarm at GFCI level.

To see some better info on the Panel and LIM's go to this PDF file.

The wiring diagrams start on page 17.

Here is the rest of the Square D article.

Roger

[ September 21, 2004, 12:24 PM: Message edited by: roger ]
 

brentp

Senior Member
Re: Isolation transformer

Roger,

Please explain the green, or ground, in your diagram. What exactly is the 'ground' that the system is now referencing to? Specifically, how is the current returning to its source?

Thanks,
Brent
 

roger

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Fl
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Electrician
Re: Isolation transformer

Hello, Brent, the green wire is the "Equipotential Bonding" conductor which is intentionally connected to a referenced ground be it a copper water line, panel ground bus, or another common connection to grounding system of the facility.

See the last sentence of 517.160(A)(2) but don't pay attention to the reference of NFPA 99 3.3.2.2.1, that is a pre 2002 NFPA 99, the correct reference will be NFPA 99 4.3.2.6.1.2 in the 2002 edition.

When the system becomes grounded at any point on the winding, it simply becomes a dual (or tri, depending on if it's center grounded or off center grounded) voltage system and returns to source the same way.

Roger

[ September 21, 2004, 07:45 PM: Message edited by: roger ]
 

dcv

Member
Location
texas
Re: Isolation transformer

Roger
Thank you for the diagram and the square D info ,I understand that because the shield is grounded on the primary side only and secondary has no ground reference causing the Polarity reversal.

Seems to me that a trade has been make here with the reduction of noise for the tripping of a breaker.
Am I correct in understanding that if a problem arises and If there is no Reference to ground only the alarm will sound the breaker "will" not trip?

Since noise reduction is the big Issue ,And I see where the noise is reduced with the transformer coil Isolation,but wouldn't the grounding of each outlet to a building ground have the potential to transfer building noise?

Thanks,
Craig
 

roger

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Location
Fl
Occupation
Electrician
Re: Isolation transformer

Craig, sorry I forgot about this thread. I have been swamped lately with all the flooding and associated work that has accompanied it.

The real reason originally for Isolation Power Systems was for reduced static and spark potential when flammable anesthesia was used.

These early systems used "ground detectors" the same as an ungrounded delta transformer system would use.

Todays LIM's are self diagnosing and very high impedance components which are altogether different than the old "Ground Detectors", or "Analogue Monitors"

The "Clean Power" provided by todays ISO Systems is simply a bi product of the intended use, and if the system could be fine tuned for residential use, it would eliminate GFCI's altogether, which is the only true purpose it serves in Health Care installations.

There is no trade off for an OVER CURRENT PROTECTION DEVICE tripping, the two pole breaker will trip at its manufactured setting with or with out a ground, and a leg to leg bolted fault will do the same thing.

Now if we could refine it with the proper safeguarding, (alarms or what have you) it would be great for Offices and even better for Marine installations.

Roger

[ September 23, 2004, 09:25 PM: Message edited by: roger ]
 

dcv

Member
Location
texas
Re: Isolation transformer

Roger
Thank you for explaining that ,Honestly I have been wandering around in disbelieve thinking these breakers would not trip.
I did not make the connection between reduced static and sparks and flammable gases , :)
 

roger

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Staff member
Location
Fl
Occupation
Electrician
Re: Isolation transformer

Craig, you are welcome. I'll try to find some more info I've got at work on Monday that I can send to you.

Roger

[ September 24, 2004, 07:51 PM: Message edited by: roger ]
 

dcv

Member
Location
texas
Re: Isolation transformer

Roger
I would greatly appreciate any material you have on the subject. :)
Thanks again
Craig
 
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