22-06-2015, 07:14 PM
There is much confusion around the subject of isolation transformers, and whenever the topic comes up on the forums, you discover that there are many views. A lot of them are wrong, unfortunately, but it's sometimes very hard to get people to accept that ("it's what I did for 25 years and never 'ad a problem, mate!"). So when deciding the pros and cons of using a transformer, try to take things back to first principles - sketch out an equivalent circuit if that helps - and be sure that you fully understand what you're trying to achieve.
In a workshop context, here's what you need to know:
There are other uses of "isolating transformers". Quotes, because they might contain a dual-wound transformer, but aren't actually isolated; the classic example of this would be the familiar yellow site transformers, which have a centre-tapped secondary with the centre-tap connected to mains earth - this means that the greatest potential between earth and a phase would be 55V. And discussed in the thread linked to, you can get 230V types with an earthed centre-tapped secondary winding to reduce the magnitude of an electric shock to 115V. These are not - I repeat - isolation transformers in the workshop sense.
Some people might use an isolation transformer as an expensive and heavy surge arrester (why not, if you already have it?). Others might use them to isolate leakage currents, preventing RCDs upstream from tripping (that's a bit specialist). I know someone who uses one on his hi-fi and believes it makes the audio better. The humble transformer can be put to use in many ways
In closing, remember again that an isolation transformer does not improve safety as such - it simply allows the connection of earthed test gear. In fact, there is actually a strong risk that it might impair safety through giving a false sense of security. An RCD is generally the safest way to work "live", with the isolation transformer only being used when absolutely necessary.
Hope this help,
Mark
Edit: Thanks to teatime, my post crossed with David's.
In a workshop context, here's what you need to know:
- isolation transformers are to allow you to connect earthed test equipment to a "DUT" (device under test). That is all.
- Only one DUT may be connected at once.
- The mains earth must not be connected through to the DUT.
There are other uses of "isolating transformers". Quotes, because they might contain a dual-wound transformer, but aren't actually isolated; the classic example of this would be the familiar yellow site transformers, which have a centre-tapped secondary with the centre-tap connected to mains earth - this means that the greatest potential between earth and a phase would be 55V. And discussed in the thread linked to, you can get 230V types with an earthed centre-tapped secondary winding to reduce the magnitude of an electric shock to 115V. These are not - I repeat - isolation transformers in the workshop sense.
Some people might use an isolation transformer as an expensive and heavy surge arrester (why not, if you already have it?). Others might use them to isolate leakage currents, preventing RCDs upstream from tripping (that's a bit specialist). I know someone who uses one on his hi-fi and believes it makes the audio better. The humble transformer can be put to use in many ways
In closing, remember again that an isolation transformer does not improve safety as such - it simply allows the connection of earthed test gear. In fact, there is actually a strong risk that it might impair safety through giving a false sense of security. An RCD is generally the safest way to work "live", with the isolation transformer only being used when absolutely necessary.
Hope this help,
Mark
Edit: Thanks to teatime, my post crossed with David's.
