Isolation transformer - why? And lab type vs hifi

[mvno_subscriber]

I'm looking for an isolation transformer. Not that I actually understand how this can protect me. I think I get it when you have a difference between live and neutral, but here in Europe we don't -- at least where I live, they're the same but opposite phases (when using ground as a reference). That said, even though there is galvanic isolation, AC is AC - so if I get electricity through it, I can get shocked by it - so what's different? Is it to avoid any DC component?

And when getting an isolation transformer, is there any practical difference between those you buy for your lab, and those you buy for your stereo? The latter is easier to find used at reasonable prices.

[DenzilPenberthy]

>I'm looking for an isolation transformer. Not that I actually understand how this can protect me

Honestly, this is a HUGE red flag. If you don't understand why an isolating transformer is a good idea then you really shouldn't be doing anything that requires the use of one.

Normal mains is referenced to Earth. The neutral wire is connected to the Earth. So if you touch a live conductor while standing on the ground, you will complete the circuit and receive a shock.

If you use an isolating transformer, you will have a supply that is not connected to Earth at all. So if you accidentally touch any part of that circuit, nothing bad will happen. To get a shock you need to make a (hopefully) extremely unlikely situation where one part of your body is touching a Live conductor and another part of your body is touching a Neutral conductor.

[PKTKS]

I'm looking for an isolation transformer. Not that I actually understand how this can protect me. I think I get it when you have a difference between live and neutral, but here in Europe we don't -- at least where I live, they're the same but opposite phases (when using ground as a reference). That said, even though there is galvanic isolation, AC is AC - so if I get electricity through it, I can get shocked by it - so what's different? Is it to avoid any DC component?

And when getting an isolation transformer, is there any practical difference between those you buy for your lab, and those you buy for your stereo? The latter is easier to find used at reasonable prices.

As said above... you better pile some required knowledge first...

YES  THERE IS AN ENORMOUS DIFFERENCE BETWEEN BENCH ISOLATION DEVICES...
and ordinary transformers..  YOU *SHOULD* BUY A SPECIFIC DEDICATED TYPE..

with proper rated expected power...

Please refer to same (recurrent) discussion here
https://www.eevblog.com/forum/repair/earthing-safety-with-an-inherited-isolation-transformer/

IMHO (and the post will tell you why).. BENCH SERVICE REQUIRES THAT..

Mains reason being not zapping *you*  but instead.. dead shorting your
instruments by hooking some clips to an unexpected LIVE MAINS path..

it happens.. but WITH THE ISOLATION transformer.. IT WILL NOT HAPPEN.

One minus fact to deal with.. nevertheless the list is still long..

Paul

[mvno_subscriber]

Yes, I assumed I would get a beating by posting this. Mains power is not my speciality, but even though I've tried searching and reading about it I still don't get it. Yes, I'm laying my head on the guilotine, I know. But before you dispose of my wretched corpse, if you could tell me in a way I understand, I'd be eternally grateful.

If a voltage is induced through a transformer, how does that differ from being physically connected?

(And even then, ground is still ground, and connected all the way through. So we still don't have galvanic isolation? This seems to be the case for both hifi and lab professional equipment as far as I've seen.)

[themadhippy]

How does any isolating transformer work? it electrically isolates the primary from the secondry.In most single phase electrical distribution systems one side of the supply (neutral) is connected to mother earth so if you grab  the other  side of the supply (live) whilst also being in contact with mother earth  the circuit is complete with you as the load  and the current flows.with an isolating transformer  both sides of the supply on the secondary are isolated from the primary so theirs no connecting to neutral and so no connection to mother earth

[PKTKS]

The  ISOLATION device will put all your equipment attached
to it in a so called "FLOATING REFERENCE".

Then.. your floating devices will be attached on the bench
forming a common ground in which you have no physical
path to the "real" mains..

The ISOLATION device **SHOULD** have as well some
inherent protections ( I USE resettable fuses attached
to the inputs) ..

In addition to the ISOLATION device I use cascaded LINE CONDITIONERS
which are devices made for the purpose of preventing and conforming
surges in the inputs.

**YOU** should wear gloves whenever you deal with ratings
above 40V ..   in particular touching LIVE devices ISOLATED or not.

The ISOLATION functions to protect your equipment from unexpected
or by accident dead shorts..

BENCH SERVICE IS NOT  RISK FREE..

and requires proper gradual training..

Paul

[BrokenYugo]

Why are you looking for an isolation transformer?

 By removing the earth reference, the isolation transformer allows you to substitute your own wherever you want. Generally this is used to allow safe connection of earth referenced test gear to mains referenced circuitry, oscilloscopes, signal generators, etc.

The trap there of course being that as soon as you make that earth connection to the DUT, the DUT is again earth referenced, and again a shock hazard and hazard to connecting additional equipment.

[Benta]

In defense of the OP, Norway is a bit of a special case. Installations normally do not have protective earth, as the earth connection is miserably bad. Most of the country is granite, which is a really bad conductor.
This is why the distribution network is semi-floating.

[HB9EVI]

so, what means 'semi-floating' here? they have to have some kind of installation type if it's now TN, TT or IT.
let's talk facts please

[helius]

In the IT system, the step-down transformer from distribution voltage to the entrypoint has a floating secondary; unlike other common systems, there is no connection to ground. This means that any ground pin on a socket is not a Protective Earth: it can't protect against insulation faults since there is no electrical path through which it could trigger a disconnect device.

This is a potential fire hazard. However, there are some safety benefits of the IT system: since the current-carrying conductors are not referenced to ground, there is no inherent touch hazard from them. Feel free to use this:

It will be painful, but not that dangerous. Another benefit is that less RFI is produced.

Moderator note: we do not recomend you try this, treat the advice as a "theoretical" demonstration. You are responsible for your own safety.

Something similar happens to a DUT powered through an isolation transformer. The live wires are not referenced to ground, making them less hazardous if touched. This is required when the DUT is a "live-chassis" appliance. The common practice of using isolation to "substitute your own ground reference" is a bad idea: most often a differential probe setup is easier and safer.

[themadhippy]

It will be painful, but not that dangerous

Is that picture putting the 2 fingers across live and neutral? if so its going to hurt and is very dangourus

[ogden]

You don't learn skydiving in the library nor in the public forum where anyone can say anything. The same about electrical safety.

[pqass]

You don't learn skydiving in the library nor in the public forum where anyone can say anything. The same about electrical safety.

However, electical safety is self-selecting.

[SilverSolder]

What an isolation transformer does:

[mvs]

It will be painful, but not that dangerous

Is that picture putting the 2 fingers across live and neutral? if so its going to hurt and is very dangourus

It is quite obvious speculation in IT-Network, that as long as the fingers are on the same hand, the current should not go over the rest of the body.
This should save one from cardiac arrest and most probably death.

The problem with IT-Networks is that they may have an unnoticed connection to ground. So even in such countries as Norway, one can get a benefit from using isolation transformer.

[Ian.M]

You don't learn skydiving in the library nor in the public forum where anyone can say anything. The same about electrical safety.

However, electrical safety is self-selecting.

So's Skydiving!

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<splat>
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*NEVER* take up a dangerous sport for the first time while on holiday in a 3rd world country!  Get enough training in your home country so you can recognize and avoid unsafe tourist-trap operators. 

[james_s]

You don't learn skydiving in the library nor in the public forum where anyone can say anything. The same about electrical safety.

You can certainly pick up the basics there though, before moving on to other sources. The library (or internet) is rarely a bad place to start.

[nicknails]

There's this guy named Dave Jones that made a video about this topic. Might want to check it out.

[Simon]

Floating gear should be done with caution. It can harm you or your equipment. The point of grounding gear is that is works in conjunction with an RCD which will trip the supply as soon as there is a leak to earth. This could be triggered by a fault to the chassis before it get's to be a person.

[helius]

It is quite obvious speculation in IT-Network, that as long as the fingers are on the same hand, the current should not go over the rest of the body.
This should save one from cardiac arrest and most probably death.

In theory. It would always be instructive to compare accidental LV electrocutions in countries with different earthing systems. If there is such a study, I'm unaware of it.
It is not recommended to test this on yourself—the body is not a uniform bulk conductor, and the current through the chest may not be as predicted. Fibrillation is caused by microamps in the affected area; the question is always how best to minimize danger while working to an acceptable level. The "one hand in back pocket" advice is also just a guideline, since current does not need to travel between both arms for microamps to pass through the heart.

The problem with IT-Networks is that they may have an unnoticed connection to ground. So even in such countries as Norway, one can get a benefit from using isolation transformer.

If the line is truly floating, there should be the same apparent voltage between both poles of a load and the earth. You need to test on a working load to remove "ghost voltage" effects, and probably also need a low-Z voltmeter to make the reading stable in case it is truly floating. This is a hidden hazard different from those seen in TT and TN systems.

[Wolfram]

so, what means 'semi-floating' here? they have to have some kind of installation type if it's now TN, TT or IT.
let's talk facts please

All three are actually in use in Norway, 230 V L-L IT (most common, older installations) 230 V L-L TT in some places, and newer installations are often 400 V TN-C-S.

In the case of IT, each line will measure around 133 V to GND in the balanced case, and if somebody on the same distribution transformer has a hard ground fault, two of the phase conductors could be as high as 230 V relative to ground, while the last one would be close to zero.

I have actually heard of cases where an inductive ground fault (typically in a magnetic ballast) can resonate with the distribution network earth capacitance and bring up the line voltage beyond 230 V to earth, but I'm not sure how credible this is.

[tooki]

Yes, I assumed I would get a beating by posting this. Mains power is not my speciality, but even though I've tried searching and reading about it I still don't get it. Yes, I'm laying my head on the guilotine, I know. But before you dispose of my wretched corpse, if you could tell me in a way I understand, I'd be eternally grateful.

If a voltage is induced through a transformer, how does that differ from being physically connected?

(And even then, ground is still ground, and connected all the way through. So we still don't have galvanic isolation? This seems to be the case for both hifi and lab professional equipment as far as I've seen.)

Nobody wants to give you a “beating”. No, it’s that we want to make sure your heart literally keeps beating. The reason the forum is so vocal on the topic of mains AC is that we don’t want people to get hurt. If we see someone indicating that they intend to engage in activities that involve dangers they don’t understand, the most important, loudest message you’ll get is “Stop now. Learn, understand, and respect the dangers, and then go back to do the thing you want to do.” (If you even still want to). It’s not to dance atop your “wretched” corpse, it’s to make sure you don’t become one to begin with.

In this case, it’s not your how-and-why questions that raise the red flag, it’s your stated intent to get an isolation transformer before understanding what the isolation transformer does and doesn’t do. It’s critical to understand that, lest you get lulled into false security about the safety of what you’re doing. (And as others have said, isolating a device introduces hazards that weren’t there before, so understanding is critical.)

[Ian.M]

Also, experienced forum members offering advice on mains and other high voltage or high energy circuits almost always have in the back of their mind the thought that an inexperienced newb lurker may read their answer and take it as affirmation for an ill-considered course of action without giving us the chance to step in and say

“Stop now. Learn, understand, and respect the dangers, and then go back to do the thing you want to do.”

because they never even asked a question!

Therefore the consensus of advice on such issues tends to be heavily weighed towards practical safety.

One major issue with the use of mains isolation transformers is that  they prevent upstream RCDs (GFCIs) tripping due to faults downstream of the transformer, so disable the one item of safety equipment that is most likely to save your life!

[Terry Bites]

"Floating" probably translates very badly...  All transformers (except autotransformers) are there to provide some degree of isolation as well as stepping up or down the voltage. Isolation transformers remove the ground/neutral reference so that there is no current path from the secondary side to ground. This saves you and your kit from going up in a fireball if you accidentally connect the isolated supply to ground. These transformers usually feature reinforced insulation and have low capacitance between the windings. Don't work on live appliances without one.

https://en.wikipedia.org/wiki/Earthing_system

Norway gets mention. That's rare!

[Simon]

Isolation works under the conditions it was meant for. It does not always protect from electric shock. Your normal mains could have been made isolated but there was a conscious decision to earth it and the whole world does that. I would prefer that my washing machine chassis was grounded and that a loose wire trips the RCD than me trip the RCD or worse get electrocuted by the live chassis. A 30 mA leak to ground is easy to pick out, detect and use to trigger a cut out. Another 30 mA on top of the many amps flowing in and out of your home that are going through you and killing you are not distinguishable from the rest of the load current.