isolation transformers

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joe

Member
Location
Maryland
Can someone please explain to me the difference between a transformer and an isolation transformer.I read up on them but all I get out of it is that they are mainly for very sensitive equipment and they have a ratio of 1:1.
thanks,
Joe
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: isolation transformers

Joe there are multiple transformers out there including isolation transformers.

One of the most common ones you will find is the step-down transformer used in commercial and industrial applications. It is simular to most utility transformers. Commercial /industrial are gennerally served by 480/277Y VAC. A step down is used to transform this voltage down to 208/120Y for single phase loads. They still provide isolation, but also step down.

Some other types but not all inclusive, are constant voltage, ferroresonant, zig-zag, auto-transformer, current transformer, potential transformer, K-factor, signal, impedance matching, balum, filters, mixers, and others. Each has a specific service.

Is this what you are looking for?

[ March 25, 2003, 11:02 AM: Message edited by: dereckbc ]
 

charlie b

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Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: isolation transformers

Sending current through the primary of a transformer creates around the coils a magnetic field. The secondary coils detect that field, and respond by sending current out the secondary conductors. The voltage in the secondary will be Ns/Np times the voltage in the primary. The current in the secondary will be Np/Ns times the current in the primary. Here, ?Ns? is the number of turns in the secondary, and ?Np? is the number of turns in the primary. The power in the secondary will be the same as the power in the primary, except for internal power losses. That is the essence of a "transformer."

If Ns and Np are equal, there will be no change in voltage or current, but the primary and secondary circuits will be isolated from each other. That is the essence of an "isolation transformer."
 

mikeackley

Senior Member
Location
Washington
Re: isolation transformers

Joe:

Just to add to what Dereck has already stated (and this is a huge, varied subject your question touches upon) but simply stated...

Transformers are a major class of coils with two or more windings electrically isolated from each other. The ratio of primary to secondary turns of these windings determines the output voltage. Transformers that have the ability to transform voltage to higher levels (step-up) will have more turns on the secondary coil. Conversely, step-down transformers will have more turns on the primary coil. If the number of turns on the primary and secondary coils are equal, i.e., 1:1, the voltage and current is transferred unaltered from the primary to secondary -- these are often called Isolation Transformers.

Isolation transformers in turn have many applications; industrial, audio, telecom, utility, medical, military, etc., etc. I happen to have one on my boat, isolating the shore power connection to the boat's on-board electrical system to help prevent galvanic corrosion. You mentioned sensitive equipment which is another major use as isolation transformers can be very effective at cleaning up dirty street power, eliminating/reducing ground loop, ground line noise, and interference.

I'll stop at this point and let your questions dictate to what extent you want more info. or technical details. I know a little, but Dereck is extremely knowledgable, the real expert here, so you are in good hands.

Edit: Just after posting my reply I see charlie steped in which is fine. So some of my verbage unintentionally duplicates what he has said. Now Joe, you have two real experts to answer any additional questions you may have.

[ March 25, 2003, 12:30 PM: Message edited by: mikeackley ]
 

bennie

Esteemed Member
Re: isolation transformers

All the above information is excellent.

However the true purpose of an isolation transformer, is not to isolate the power wiring, it is to break the common ground connection, to the utility.

This establishes a separate neutral and ground reference.

Eliminate the circuit from a boat to the shore ground will prevent corrosion through electrolysis.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: isolation transformers

What code sections permit the installation of a truly isolated system, if the isolated system has a neutral?
Don
 

bennie

Esteemed Member
Re: isolation transformers

Health care isolation transformers.
480 ungrounded supply to a delta/wye transformer.

[ March 27, 2003, 11:32 AM: Message edited by: bennie ]
 

mikeackley

Senior Member
Location
Washington
Re: isolation transformers

Don: Great question - this one might top 50 posts.

My 2 cents -- answer to your question depends upon its context.

One answer might be the NEC does not permit isolation transformers to be installed in such a way to isolate the customers neutral from the utilities neutral.


Another answer, again depending upon the context of your question, might be the NEC does permit a Separately Derived System, which a isolation transformer may create, providing 250.30 provisions are met. Simple bonding of the chasis or enclosure of the SDS (e.g., the medical type bennie mentions) is acceptable.

[ March 27, 2003, 05:22 PM: Message edited by: mikeackley ]
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: isolation transformers

Bennie,
Yes, the Article 517 application is one case of a truly isolated system. The second one is also possible, but only if the 480 source is an ungrounded source. That is not very common in this area any more. My only point is that is very difficult to install a code compliant, isolated system where there is a neutral needed for the isolated system.
Mike mentioned the isolation transformer that he uses for his boat. If that transformer was requried to be installed per the NEC [boats are not in the scope, 90.2(B)(1)], it would not be able to be installed as an isolated system.
Don
 

bennie

Esteemed Member
Re: isolation transformers

This is my point about power transformers being called separately derived systems ;)
 

bennie

Esteemed Member
Re: isolation transformers

Here is a ridiculous method to make a compliant isolation system transformer.

Back feed a 480 to 208/120 transformer. Do not ground the now 480 secondary. Take two legs of the 480 to the boat 230/120 isolation transformer.

This is compliant with 277 to ground but not compliant with 120 to ground? :mad:
 

mikeackley

Senior Member
Location
Washington
Re: isolation transformers

I couldn't agree with you more Don. The installation of my on-board marine isolation transformer is not NEC compliant nor does it have to be as you point out.

However, this transformer does carry the Marine UL Listing. It also meets all the requirements of ABYC E-8.20.1 with a full current carrying shield and a totally encapsulated transformer.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: isolation transformers

Bennie,
I think that part of the problem is that I don't see "separately derived system" meaning the same thing as "isolated system".
Don
 

bennie

Esteemed Member
Re: isolation transformers

Don: I realize that.

I still believe a separately derived system is a premises wiring system without external connections. This is documented in the NEC archives.

This issue went sideways when Joseph McPartland ceased as author of the handbook.
 

bennie

Esteemed Member
Re: isolation transformers

A generator supplies a separately derived system when there is no solid electrical connection to the exterior supply.

A transformer supplying a separately derived system can only have magnetic coupling, no solid electrical connection to the MGN, this is indeed an isolation transformer.

A premises power transformer is a power transformer nothing else, it is a part of the premises wiring system. Why should it be anything else?
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: isolation transformers

If you look at 210.4(D) it is about Identification of ungrounded conductors where more than one voltage system exists.

This says to me the code considers each voltage in a building to be a system.

So when we change from 480 phase to 208/120 we have changed voltage systems.

As this system did not exist before wouldn't it have to be separately derived system?

It is not separately derived current or energy, but a separately derived voltage system
 

bennie

Esteemed Member
Re: isolation transformers

Review Article 100, definition of "Premises Wiring System".

It is clear that everything after the source of power constitutes a premises wiring system.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: isolation transformers

I think that these descriptions are both valid, the "Premises Wiring System" is all inclusive.

The "Voltage Systems" are just smaller parts of the whole.

The code is always changing and maybe this is another area that has changed, I am just going with what I am reading out of the current book.

I could be way off, it wouldn't be the first time. :)
 

bennie

Esteemed Member
Re: isolation transformers

Bob: Electrical engineering and applied technology dictate the definition of a separately derived system.

The explanation in the Handbook, and Soares Grounding Book, is putting a different spin on the definition.

The Soares illustrations are insinuating the neutral conductor is switched by the transfer switch. Anyone can see this is not true. This is an irresponsible and potentially dangerous schematic.

The Soares Book insinuates the green wire does not make an electrical connection and is not a circuit conductor, only the neutral (white) wire does this. Yet both terminate at the same place.

The Soares Book states that the only transformer configuration that is not a separately derived system is an auto-transformer. This is pure fantasy. No document available to substantiate this statement.

A grounded wye to grounded wye must have the neutral common to both. It didn't make the cut.

The schematics in the Soares Book are mostly of MGN system transformers, definitely not power sources for separately derived systems.

[ March 28, 2003, 11:14 AM: Message edited by: bennie ]
 

engy

Senior Member
Location
Minnesota
Re: isolation transformers

Hang in there Don...

“Bob: Electrical engineering and applied technology dictate the definition of a separately derived system.”

I thought you dictated definitions Bennie? ;)

“The Soares illustrations are insinuating the neutral conductor is switched by the transfer switch. Anyone can see this is not true. This is an irresponsible and potentially dangerous schematic.”

Wow

“The schematics in the Soares Book are mostly of MGN system transformers, definitely not power sources for separately derived systems.”

For an MGN the neutral has to be grounded more than once. A bunch of building distribution transformer secondaries grounded to the same grounding system is not a MGN system.

Mike
 
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