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Why did I buy an isolation transformer?


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So I bought a 2 kW isolation transformer (such things often happen at my house after a wine or two), based upon a recollection that some folks on the forum had successfully used such to improve the noise floor in their systems.  When the unit was delivered I realised there is no earth...YIIIIKES!!!

 

Some quick reading revealed that having no output earth connection indeed does reflect the normal purpose of isolation transformers (for operating DC lab equipment or performing testing/repairs on AC equipment). Fair enough.

 

So I seem to have an entirely useless item on my hands, or am I missing something else here?

 

Informed advice is most welcome 🙂

Thanks

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Not entirely sure what you are asking.

 

The whole point of " Isolation transformer " is to "isolate" the equipment, that means nothing goes to ground, so everything is kept in one circuit.

 

If you are asking if the unit is unsafe to use if you have a short, the answer is no.

It's actually safer, as long as you don't touch ground.

 

And the fuse will still burn if you have a short, so don't worry.

If you are asking about lightning strike , that will still work, as the input power still has ground.

Lastly, if theres a surge, the circuit breaker on your board will still break.

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On 10/02/2021 at 9:27 PM, tripitaka said:

So I bought a 2 kW isolation transformer (such things often happen at my house after a wine or two), based upon a recollection that some folks on the forum had successfully used such to improve the noise floor in their systems.  When the unit was delivered I realised there is no earth...YIIIIKES!!!

 

I find that bizarre; the 1kVA Sirios isolation transformer I bought a long time ago (which I no longer need to use, after changing house) passes the input mains earth to the 6x mains output sockets.  AIUI, for audio purposes (so not necessarily for lab purposes) "isolation" means:

  • delivering the same voltage on output as on input, but
  • passing the mains (ie' Active/Neutral') from input to output via magnetic coupling - rather than hard wires.  This cuts mains noise.

Andy

 

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I bought a Tortech isolation transformer, precisely because it provides unearthed mains power. I had planned to use it with mains-earthed components (when I had more than one of those) in order to prevent ground loops. I was also going to see whether it removes power amp transformer hum, by removing DC offset.

 

Alas, the Tortech hums very noticeably and much more than the amp’s transformer, even when idle. It is banned from the listening room now, but useful with the oscilloscope, for safe testing in earthed circuits.

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1 minute ago, Steffen said:

I bought a Tortech isolation transformer, precisely because it provides unearthed mains power. I had planned to use it with mains-earthed components (when I had more than one of those) in order to prevent ground loops. I was also going to see whether it removes power amp transformer hum, by removing DC offset.

 

Alas, the Tortech hums very noticeably and much more than the amp’s transformer, even when idle. It is banned from the listening room now, but useful with the oscilloscope, for safe testing in earthed circuits.

Tortech was the brand I was going to purchase. Humming, I can't have that....

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A isolation transformer is designed to couple AC active and Neutral.  Safety earth is to couple all your chassis to Earth. Your equipment should already have, each of its chassis's bonded to earth via IEC mains lead earth.   

 

The Isolation transformer if its a toroidal design, needs to isolate its central bolt from chassis, with what is referred to as otherwise a shorted turn.  See images for how this is done with a smaller toroidal. 

Avoiding Shorted Turn 2.JPG

Avoiding shorted Turn.JPG

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6 minutes ago, stereo coffee said:

Your equipment should already have, each of its chassis's bonded to earth via IEC mains lead earth.   

 

Unless it is double-insulated, like all except one of my components are.

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59 minutes ago, Steffen said:

 

Unless it is double-insulated, like all except one of my components are.

And properly complies with double insulation standards    https://sound-au.com/articles/mains-safety.htm#s1

 

Also precautions when using or testing equipment  where isolation transformers are used is detailed here:   https://sound-au.com/articles/iso-xfmr.htm

 

"Use of an isolation transformer should be limited only to equipment (or test procedures) where it's essential.  The rest of the time, equipment should always be connected to the mains in the same way that it is when being used normally.  While this means that there will be some parts at hazardous (mains) voltages, these are generally inaccessible in a lot of gear.  The exception is switchmode supplies of course, but it's common for them to be treated as exchange items only, and most are not designed to allow repairs."

 

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Thank you all so much for replies. 

 

Below is a diagram from Tortech, who manufactured the item in question.

It seems to me that for this particular equipment, there is no 'IEC Mains lead earth'.... and so whether that represents a safety concern is the essence of my question.  

 

I thought that an electrical fault, where active tripped to the external metal casing of the appliance, would make YOU the earth if the unit was not already externally earthed to the mains lead, no? But then, having said that, I can't see why using a powerdrill in the rain would be ok, as the text in the diagram below seems to imply 🙂

no-earth-isolated-diagram-annotated.jpg

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5 minutes ago, tripitaka said:

But then, having said that, I can't see why using a powerdrill in the rain would be ok, as the text in the diagram below seems to imply 🙂

 

Seems to be a lot of confusion about what an isolation transformer does. 

 

It provides an AC power supply (active and neutral wires) that is isolated, and not referenced to any existing voltages, including ground.  That last bit is important, because normal power from the wall is referenced to ground, by means of earthing the neutral wire at the distribution board and network. 

 

That means you can grab hold of the isolated active wire and touch a ground point (or get wet in the rain with a power drill in your hands), and NOT get electrocuted. No current can flow. Thus an isolation transformer is a safety device.

 

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Through an isolation transformer, there no longer is an “active” lead. When you touch it you will probably feel some tingle, as with floating voltages in general, but there will be no circuit through you to earth.

 

EDIT Beaten by @aussievintage;)

 

Edited by Steffen
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34 minutes ago, aussievintage said:

 

Seems to be a lot of confusion about what an isolation transformer does. 

 

It provides an AC power supply (active and neutral wires) that is isolated, and not referenced to any existing voltages, including ground.  That last bit is important, because normal power from the wall is referenced to ground, by means of earthing the neutral wire at the distribution board and network. 

 

That means you can grab hold of the isolated active wire and touch a ground point (or get wet in the rain with a power drill in your hands), and NOT get electrocuted. No current can flow. Thus an isolation transformer is a safety device.

 

 

Thanks Aussie, Steffen et al

I'll fire up this sucker and see what happens... if I disappear from Stereonet then draw your own conclusions 🙂 🙂 🙂

 

BTW, it doesnt seem to hum, so at least it ticks that box!

 

Edit: I'm still here 🙂 and I reckon I'll keep this unit.  

SQ-wise I'll confidently say it's... doing no harm haha

Thanks for all the advice from commenters, much appreciated

 

Edited by tripitaka
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Goodaye all

 

Been thinking about this recently.

l have two 2 amps which do not have grounds on the chassis.

Not double insulated,  both serviced and in excellent condition.

 

Now not too concerned because we have RCD's on the house and lets face it they are more reliable than a old fuse or circut breaker. (Sometimes a little too sensative)

 

So if you have RCD's on the house l cant see a problem with using a isolation transformer.

 

regards Bruce

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5 minutes ago, sloper said:

they (RCDs) are more reliable than a old fuse or circut breaker.

 

One does not replace the other.  They have a different purpose.  An RCD detects a current imbalance to ground, indicating a possible fault, and so it trips.  A circuit breaker or fuse, detects an over-current condition  and breaks the circuit to prevent destroying the wiring and a possible fire.

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4 minutes ago, aussievintage said:

 

One does not replace the other.  They have a different purpose.  An RCD detects a current imbalance to ground, indicating a possible fault, and so it trips.  A circuit breaker or fuse, detects an over-current condition  and breaks the circuit to prevent destroying the wiring and a possible fire.

  

... Yes and for that matter, my  isolation unit comes with a statement specifically warning that it is likely to trip upstream RCDs. 

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Just now, tripitaka said:

  

... Yes and for that matter, my  isolation unit comes with a statement specifically warning that it is likely to trip upstream RCDs. 

 

I would have thought it would prevent them doing their job, as there can't be any fault current to ground from the isolated side of the transformer.  Interesting,  I will ponder that a bit further :) 

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4 hours ago, aussievintage said:

 

I would have thought it would prevent them doing their job, as there can't be any fault current to ground from the isolated side of the transformer.  Interesting,  I will ponder that a bit further :) 

Your doubts do you credit, I reckon I more likely misunderstood the warning.  I now see that it related to the specific case of carrying out RCD testing downstdeam and then tripping upstream RCDs as a result.

 

But, as you say, the upstream RCD might not be fulfilling its intended purpose, regardless of what is going on downstream.

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