Isolating transformer principle

[KeenPensioner]

As can be seen from my profile I am not a qualified electrician but I'm studying in my own time just for the sake of it. Everywhere I look for info on isolating transformers I get something like this "This means that the isolation transformer secondary side must not be grounded. It would create a physical connection between the primary and secondary transformer side." or "no return path to earth from transformer secondary". I don't get that. If you look at the attached image, is there not a "return path" with 0.24A passing through the body? Perhaps I just don't understand how neutrals/earths work! Any help would be appreciated.

 

[Pete999]

As can be seen from my profile I am not a qualified electrician but I'm studying in my own time just for the sake of it. Everywhere I look for info on isolating transformers I get something like this "This means that the isolation transformer secondary side must not be grounded. It would create a physical connection between the primary and secondary transformer side." or "no return path to earth from transformer secondary". I don't get that. If you look at the attached image, is there not a "return path" with 0.24A passing through the body? Perhaps I just don't understand how neutrals/earths work! Any help would be appreciated.View attachment 43600

This may help, but it is one of many explanations

During my working time Isolation transformers in the context I was working in were used as a security means, equipment was not connected to the mains physically.

 

[KeenPensioner]

Thanks Pete, but I've viewed that from JW and many others...still don't get it! What about the return path in my image?

 

[Wilko]

Hi - my fractured way of thinking, for what it's worth
As the 2ndary winding is not attached to E within the transformer, it means that we can make either end into 0V. So in your pic, the end Mr Stick touches should drop to OV (ish) and not hurt him (much).

 

[KeenPensioner]

Thanks Wilko, but won't one of the "ends" be at 240V and the other at 0V as in the primary winding?

 

[Darkwood]

The isolating transformer induces the voltage from the primary into the secondary to give you 230v output between the end of the windings, there is no physical connection between primary and secondary in these kinds of transformers, the secondary has no relation with earth as you have induced and created the voltage separate from the primary side, the voltage difference (Potential Difference) only exists between the 2 wires so touching either wire doesn't give a shock as there exists no potential difference to earth, you can still receive a potentially fatal shock by grabbing both wires and thus making the circuit with your body but in normal use this is rarer than accidentally coming into contact with one wire alone.
If you strap one side of the secondary winding down to earth (which would defeat the object of having the isolating transformer) you would in effect create a similar set up to the power source on the primary where you now have one side strapped to earth and this has no potential difference zero volts (N) but in doing this you now create a potential difference from the other secondary wire of 230v to earth, thus touching it would give a path through your body to earth hence you receive a potentially fatal shock.

 

[Pete999]

Thanks Wilko, but won't one of the "ends" be at 240V and the other at 0V as in the primary winding?

Have you seen this one?

 

[Darkwood]

Thanks Wilko, but won't one of the "ends" be at 240V and the other at 0V as in the primary winding?

The secondary has no reference to earth and thus has no 0v wire and respective 230v wire, it is an AC waveform measured between the 2 wires so the voltage will rapidly rise and fall and reverse direction at a frequency of 50hz either wire will be peaking about +320v and falling to 0v then -320v and so on and so on, the 230v we often refer to is the Root Mean Square (RMS) 'a mean average' if these figures were confusing you.

 

[Taylortwocities]

Dear @KeenPensioner

You need to look at the WHOLE picture. ie you need to understand where the earth actually comes from.
It is provided at the electricity substation/transformer. Have a look at the first 3 minutes of this JW video

So, the mains suply into your house (Live and Neutral) are referenced to the earth connection. At the supply transformer, the neutral (0v) is connected to earth. There are various ways in which the earth arrives at your house. The video above shows the various methods.

When an isolating transformer is used, the live and neutal are totally disconected from the (supply) earth, so there will be no voltage reading between either of the output terminals of the isolating transformer and the supply earth. The only potential difference will be between the two output terminals of the transformer.

Hope this helps.

 

[static zap]

By eye --- Thinking Circuit ---
the black line from his hand --- finishes at his knee , there is no continuous path ,
(a small current could flow due to capacitive coupling across transformer-and that would ref to E--- will Be no where 240mA more like 0.24mA )

 

[KeenPensioner]

Thank you all for taking the time to reply. I've watched almost all of John Ward's videos (on this and other subjects). After reading all your replies and revisiting his videos I think I'm nearer to understanding what's going on. On the "unearthed" secondary winding, rather than thinking of L and N, think of L1 and L2 where L1 can be thought of of as oV or 240V with no potential difference with reference to earth.

 

[DPG]

Yes, that's the key - no reference to earth.

 

[Lucien Nunes]

Bear in mind that earth is just another conductor, not a magic hole that gobbles up current. We use it as a reference because it's available everywhere and it's the most likely thing for people to come into contact with. But unless it leads back to the source of potential, in this case the secondary winding, it's not going to complete a circuit. If you look at your proposed shock path, you'll see the circuit is incomplete. Secondary winding - man - earth - primary winding, but there it ends, because the primary winding is isolated from the secondary. Without a complete circuit, no potential exists across the man and no current flows.

There are situations where isolating transformers are used, with one end or the centre-tap of the secondary earthed. These do not prevent a shock to earth from the secondary, but usually this will be extra-low voltage so the shock current is not harmful.

 

[telectrix]

There are situations where isolating transformers are used, with one end or the centre-tap of the secondary earthed. These do not prevent a shock to earth from the secondary, but usually this will be extra-low voltage so the shock current is not harmful.

is that not "reduced low voltage" as in 55-0-55 site transformers?

 

[static zap]

There are situations where isolating transformers are used, with one end or the centre-tap of the secondary earthed. These do not prevent a shock to earth from the secondary, but usually this will be extra-low voltage so the shock current is not harmful.

is that not "reduced low voltage" as in 55-0-55 site transformers?

Just when you think you know " Isolating " transformers ....
The site transformer comes along -- as trusting floating is bad practice with multiple devices ...

 

[Lucien Nunes]

is that not "reduced low voltage" as in 55-0-55 site transformers?

Yes that too, I actually had in mind small ELV power supplies for electronic goods. E.g. sitting on my bench are half a dozen 12V SMPSUs of which some have earth-referenced outputs and some don't. I didn't want to over-complicate things by mentioning that it's not the transformer secondary itself but the rectified output that is earthed in those.

 

[Lucien Nunes]

is that not "reduced low voltage" as in 55-0-55 site transformers?

Yes that too, I actually had in mind small ELV power supplies for electronic goods. E.g. sitting on my bench are half a dozen 12V SMPSUs of which some have earth-referenced outputs and some don't.

 

[KeenPensioner]

Lucien Nunes explains it well to me as an amateur. If I get it now the image below shows what I think he means. Thanks again to everyone