Isolation Transformers and Electrical Safety

[temujin]

Hi,

An isolation transformer is used in order to "safely" separate one part of an electric circuit from the mains. (description in http://en.wikipedia.org/wiki/Isolation_transformer" )

Still I have problems grasping the important concept of such a transformer. I have spent some time googling, but I cannot find an answer as to why such a transformer makes the secondary side SAFER with respect to electrical shocks.

Does anyone want to share their insights?

regards
temu

 

[uart]

Hi temujin. The short answer is that most electric shocks do not occur because someone simultaneously touches two supply wires, rather they more frequently occur when someone touches a single live wire and a "ground". The isolation transformer has a secondary which is non-grounded, thus you can avoid being shocked by touching just one wire and "ground".

BTW. The isolation transformer has other uses as well, particularly in measurement and instrumentation systems.

 

[Pumblechook]

I think on UK building sites 230/110 transformers are used and the centre tap of the secondary is earthed so that each live wire is no more than 55 volts wrt to Earth.

 

[uart]

Yeah I guess you'd call that "low voltage two phase" rather than isolation.

I can understand why they'd do that on a building site. It's the same reason that most power systems use a ground connection, if you don't purposefully ground it then sooner or later someone with inadvertently ground one connection or other potentially making it more dangerous. If you did try to use ground isolation it would be pretty hard on a building site to prevent someone inadvertently grounding something somewhere or other.

 

[temujin]

Hi temujin. The short answer is that most electric shocks do not occur because someone simultaneously touches two supply wires, rather they more frequently occur when someone touches a single live wire and a "ground". The isolation transformer has a secondary which is non-grounded, thus you can avoid being shocked by touching just one wire and "ground".
.

Thanks...but then I have more questions...

With reference to the diagram I have attached, there is accidentally a connection at point "A" making the "guy" touch a wire or enclosure that is conductive.

Does the fact that the secondary part of the isolation transformer not beeing connected to Protective Earth help my friend on the drawing?

regards
temu

 

[uart]

Does the fact that the secondary part of the isolation transformer not beeing connected to Protective Earth help my friend on the drawing?

regards
temu

Yes it does. There is no completed circuit though the Earth so no current can flow.

 

[temujin]

Thanks uart, but still one thing is unclear to me...

If there is a connection at point A (in my previous drawing) wouldn´t there be a difference in potential between the metal enclosure and Protective Earth?

temu

 

[uart]

No not in the sense that I think you mean. (There certainly can be and usually is a "potenial difference" between any two objects that are isloated, but no current can flow if there is not a completed circuit).

I think you need to come to terms with the concept of a "floating" power supply. If this is a concept that you haven't previously come across or hadn't thought much about then this is where you need to start.

Imagine for example a series stack of 20 12volt batteries with just two external terminals that are not initially connect to anything (and neither grounded). Clearly there is a potential difference of 240VDC between these two terminals and if you touched both at the same time then you're going to get shocked.

What however if you only touched say the negative with one hand and ground with the other. There is no closed circuit so you don't get shocked. In this case you are actually grounding the negative terminal through your body so the positive terminal will "float" to 240 volts above ground.

Now let imagine that you released the negative terminal and touched both the positive terminal and ground at the same time. Again there is no completed circuit so you can't get shocked. In this case you are now grounding the positive terminal so the negative terminal will "float" down to 240volts below ground potential.

Apart for the fact that it's AC rather than DC, the output of an isolation transform can be though of a little like "floating" battery if that makes sense to you.

 

[temujin]

Thanks again, uart...it becomes clearer but not 100%.. I need to read some more before I ask again ... :-)

Another thing... with reference to my circuit in post #5, if there is a connection between any point on the secondary side of the circuit and Protective Earth, does the isolation transformer have any safety function at all?

regards
t.