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Old 30th May 2008, 06:48 AM   #1
atmars is offline atmars  United States
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Default isolation transformer

I have read in several places the importance of using an isolation transformer for transformerless amp or project for safety. Can anyone tell me how this works exactly. I understand the transformer isolates you from the mains, but there is still high voltage on the amp side, same as if the transformer wasn't there. How does the gap protect the user from electrocution?

Ignorant man thanks the more informed in advance.
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Old 30th May 2008, 08:25 AM   #2
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It's a matter of current, really, not voltage. The mains is a very low impedance source and can deliver high currents on demand. A typical isolation transformer can deliver much less current, owing to the resistance of its windings and the limited capacity of its core. That's not to say it's safe to get a shock from the isolation transformer secondary, it's just not as dangerous as the mains.

It's not only shocks that are important, there is also the risk of fire. The isolation transformer is less likely to cause a fire in the case of a short-circuit or a near-short, again because of its limted ability to deliver curent.
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Old 30th May 2008, 08:43 AM   #3
timpert is offline timpert  Netherlands
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Uhm.... No, it's not about the current at all. My safety isolation transformer is a hefty 2kW unit (I use it to debug anything mains connected) which would be useless if it really was about current. Anything that is capable of putting more than a few tens of milliamps through your body is lethal, so it must be something else.

The problem is, mains is wired as live, neutral and safety earth. The neutral line is connected to ground in the distribution station, so in theory safe to touch even if you stand barefoot in a puddle in your garden. It is still possible that there is something wrong with the neutral wire in your installation so DON'T EVER DO SOMETHING LIKE THIS!!! ALWAYS TREAT ANY MAINS WIRE AS LIVE except for safety ground of course. The live wire has the full AC on if, so it will put a current through anything that has a conductive path to ground. In the normal situation, that path is provided by neutral, but in unfortunate situations, that might be provided by you.

If you use an isolation transformer, the entire secondary circuit is isolated from ground, so if you (accidentally) touch something connected to it, a current to ground cannot flow. This adds a degree of safety, but a 1:1 isolation transformer can still kill of both secondary terminals are touched together.
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Old 30th May 2008, 09:43 AM   #4
ace7one is offline ace7one  Norway
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I found this link, here it stands that a isolation transformer will remove some noise. But that have nothing do to with safety.

http://blackmagic.com/ses/bruceg/EMC/isolatrans.html
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Old 31st May 2008, 12:24 AM   #5
AJT is offline AJT  Philippines
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Default Re: isolation transformer

Quote:
Originally posted by atmars
I have read in several places the importance of using an isolation transformer for transformerless amp or project for safety. Can anyone tell me how this works exactly. I understand the transformer isolates you from the mains, but there is still high voltage on the amp side, same as if the transformer wasn't there. How does the gap protect the user from electrocution?

Ignorant man thanks the more informed in advance.
to give you an idea, our mains supply is coming from a 3phase 400volt system ussually in a wye configuration, and with the neutral connected to earth..

so between a single phase and earth, we get 230volts, there is a danger of shocks if we are to touch the line (even the neutral wire in most cases)and we are barefooted standing on our cemented floor for example...

now, an isolation transformer is able to break this loop as neither of its secondary wires are connected to earth....

if i may say so, the power transformers used in our appliances can be construed as isolation transformers...
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