A transformer that doesnt step up or down?

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augie47

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Tennessee
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State Electrical Inspector (Retired)
Possibly a isolation transformer.
An isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. They also block interference caused by ground loops. Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers or laboratory instruments. Isolation transformers are different from auto transformers in which the primary and secondary share a common winding.
 
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iceworm

Curmudgeon still using printed IEEE Color Books
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EE (Field - as little design as possible)
Some of the people i worked with encountered a transformer that doesnt step up or down, whats the purpose?

There are only two reasons why I have ever speced/installed a 1:1 transformer (assuming you mean a power transformer)

D\ungrounded Y or D\ungrounded D -- Isolation for a piece of equipment such as a VFD.

480D\480grounded Y -- Establish a grounded Y system when the main power is Impedance grounded and some 277circuits are required. Generally a small xfm (75kva or so)

I'm sure there are other reasons - these are the only two I have ever needed.

ice
 

hurk27

Senior Member
I have used many isolation transformers for many different purposes, but the most common one I can think of is ground loop isolation in audio cable, back when I was doing sound for clubs, it was common for me to have several 1:1 600 ohm or 1k ohm audio transformers in my bag of tricks to eliminate ground loops between mixer boards and amp racks fed from different circuits.

On the power side, some hospital circuits, and some pool lights also require isolation transformers.

While these and a few other type of installs don't require the use of 1:1 transformers, I have seen the POCO remove the cross over neutral bond to isolate the power to dairy farms and Marina's to prevent stray voltage from causing the cows to stop producing milk, or in the case of Marina's to stop damage to aluminum hull boats from the power grid, but in these cases they install what is called a neutral block that while isolates the secondary neutral from the primary neutral, if there is a transformer fault from the high side to the low side it will still fault the high voltage to the MGN.
 

brian john

Senior Member
Location
Leesburg, VA
There are only two reasons why I have ever speced/installed a 1:1 transformer (assuming you mean a power transformer)



480D\480grounded Y -- Establish a grounded Y system when the main power is Impedance grounded and some 277circuits are required. Generally a small xfm (75kva or so)


ice

But that is not a 1:1
 

Besoeker

Senior Member
Location
UK
In the UK, do they use these shaver sockets with isolation transformers in place of GFCI? Just curious.
TBH, I don't know. Domestic/commercial isn't my field.
I think the philosophy is that, if the source is isolated from ground, there is no fixed potential to ground and no current path that could cause electrocution.
 

Besoeker

Senior Member
Location
UK
Some of the people i worked with encountered a transformer that doesnt step up or down, whats the purpose?
As it happens, this on isolation transformers just dropped into my business email today:

Transformers can be more than just static devices that transfer electrical energy. Separation transformers, isolation and extra isolation transformer play a major role in the protection of people and equipment. They come in all ranges, from very small (a few VA) to quite large (a few MVA), and although more expensive than autotransformers or transformers with simple separate windings, they are an easy way to solve problems that could arise concerning:

* Protecting individuals from electrical shock
* Avoiding losing power in the case of a first insulation fault
* Protecting sensitive equipment from electrical noise
* Creating a star point for equipment that require it

http://www.leonardo-energy.org/isolation-transformers

There's also an application note with it that goes into more detail.
 

AdrianWint

Senior Member
Location
Midlands, UK
In the UK, do they use these shaver sockets with isolation transformers in place of GFCI? Just curious.

Yes ..... and sorta no.

The only kind of outlet allowed in a bathroom is an isolated shaver outlet. This has a very low capacity, less than 30W, and provides a supply which is not earth referenced. The idea being, that since the shaver supply has no connection to earth, a shock between live & earth is not possible.

In Uk terminology the equivalent device to a GFCI is an RCD (Residual Current Device). General purpose socket outlets are not allowed in bathrooms, even if protected by an RCD.

RCD's are common (and under the 17th edition of our wiring regulations, compulsory) for socket outlet circuits & are usually rated at 30mA

Adrian
 

Besoeker

Senior Member
Location
UK
Yes ..... and sorta no.

The only kind of outlet allowed in a bathroom is an isolated shaver outlet. This has a very low capacity, less than 30W, and provides a supply which is not earth referenced. The idea being, that since the shaver supply has no connection to earth, a shock between live & earth is not possible.

In Uk terminology the equivalent device to a GFCI is an RCD (Residual Current Device). General purpose socket outlets are not allowed in bathrooms, even if protected by an RCD.

RCD's are common (and under the 17th edition of our wiring regulations, compulsory) for socket outlet circuits & are usually rated at 30mA

Adrian
Thanks Adrian.
My field is mostly industrial. The main use we have for BS7671 is conductor ratings. Bathrooms are generally not part of that.
I've browsed through that part a bit. Complex different zones but if you can stand with your feet in water and have eight foot long arms a bathroom is out of bounds for you....
:)
 

DetroitEE

Senior Member
Location
Detroit, MI
Yes ..... and sorta no.

The only kind of outlet allowed in a bathroom is an isolated shaver outlet. This has a very low capacity, less than 30W, and provides a supply which is not earth referenced. The idea being, that since the shaver supply has no connection to earth, a shock between live & earth is not possible.

In Uk terminology the equivalent device to a GFCI is an RCD (Residual Current Device). General purpose socket outlets are not allowed in bathrooms, even if protected by an RCD.

RCD's are common (and under the 17th edition of our wiring regulations, compulsory) for socket outlet circuits & are usually rated at 30mA

Adrian

Thanks for the info Adrian.

This topic intrigued me so I did a little more research...it seems like the RCDs are indeed intended to prevent electrocution, however they don't go by the 5mA standard like we do in the US. One explaination I read was that since their reaction time was less than that required to induce ventrical fibrillation, this is still acceptable. However, it still seems like a GFCI would be safer for personnel protection than an RCD. I'm curious why the trip current on RCDs isn't also near the 5mA threshold.
 

beanland

Senior Member
Location
Vancouver, WA
Grounding Transformer

Grounding Transformer

Are you sure it isn't a zig-zag or wye-delta grounding transformer. These only have wires to the "primary" and no secondary. It looks like it is not "doing anything" but it is.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110414-2352 EDT

ronaldrc:

Any transformer can limit fault current. The fault current will be defined by the series internal impedance as viewed from the secondary side and the load impedance on that transformer.

So when you short some point in a circuit the fault current will not exceed a current defined by the open circuit secondary voltage, and the combined impedance of the transformer internal impedance with the impedance of the circuit up to the point where the short occurs.

If the short is immediately at the transformer then it is only the transformer internal impedance that limits the short circuit current.

.
 
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