Isolation/isolating transformers

[dave cox]

I can see that by using an isolation transformer you will bypass any existing RCD so you might consider it more dangerous. For sure, there are some scenarios where that would be true but conversely in some scenarios it might be safer with a energy limited / floating supply. If you ground some part of the circuit (for measurement or it happens accidentally) then this floating advantage is defeated.

An alternative would be putting an RCD on the secondary side of and isolating transformer but also ground the secondary BEFORE the RCD. This would give you a virtual live / neutral set up, with RCD protection, but with a lot less energy available. It would be possible to do earth referenced measurement (on ac side) as long as very little current flows into the earthed point, since it bypasses the RCD. Moving the earth point to a centre tap would half the voltage available for a shock.

dc

[GrimJosef]

Originally Posted by stuarth

... An RCD will limit the duration of an electric shock in some circumstances where in the same circumstances an isolation transformer will prevent the shock occurring at all. In other circumstances, neither will save you from a shock.

You have described two classes of circumstances there.

The other big advantage of an isolation transformer is that it allows you to connect earthed test gear to a live chassis set.

This generates a third class of circumstances. Now if you touch some parts of the set you can get an electric shock via the earth path that you have created with your test gear, and the RCD will not protect you.

Cheers,

GJ

[Brian of Romsey]

Further more, it is useful to remember (from a DIY & bodging perspective) at any pair of identical transformers can be back-to-backed to make a 1:1 isolation unit. It doesn't matter what the secondary voltage is really.

A second useful tip is that small low voltage transformers can be used in this sort of situation to tweak the output voltage up or down a small amount. This is done by putting the secondary in series with the main transformer's primary (and the little transformer's primary across the mains). Depending on the phase of the small transformer's secondary with respect to the main transformer's primary there is either an additive or subtractive effect. Important to ensure that the small transformer's secondary current rating is at or above the main transformer's primary current. Example: Main transformer is 500 VA so primary current will be a bit over 2 Amps, so a 6.3 V, 3 Amp transformer will do the job and will give +/- (6.3 / 230) which is about 2.7%. A 12 V transformer gives about 5%.
See https://www.youtube.com/watch?v=ZEFVNFyG9po
http://electricalacademia.com/transf...ing-principle/
https://www.mig-welding.co.uk/forum/...p-mains.38115/
http://www.dms-audio.com/bucking-transformer-diy
https://cdn.instructables.com/F12/9C...=webp&crop=3:2

[stuarth]

For the thirst class of circumstances you need the isolating transformer, otherwise the RCD will trip as soon as you connect you test gear earth anywhere in the live chassis circuit.

Stuart

[Sinewave]

Originally Posted by GrimJosef

You have described two classes of circumstances there.

This generates a third class of circumstances. Now if you touch some parts of the set you can get an electric shock via the earth path that you have created with your test gear, and the RCD will not protect you.

Cheers,

GJ

This is a very good point.

[usradcoll1]

Originally Posted by stuarth

For the thirst class of circumstances you need the isolating transformer, otherwise the RCD will trip as soon as you connect you test gear earth anywhere in the live chassis circuit.

Stuart

I understand an RCD can be located on the mains that feed the workshop alone, instead of depending on the one in the CU.
In the US, they're very reasonably priced, of course it doesn't eliminate the need of an isolation transformer. I use both! I float all my test equipment from earth by using a 3 to 2 prong adaptor, as I feel using an isolation transformer plus an RCD eliminates a great deal of hazard.
In the US, our RCD is known as a GFCI. Ground Fault Current Interrupter.
You can install as many as you want or need.

[GrimJosef]

Originally Posted by stuarth

For the thirst class of circumstances you need the isolating transformer, otherwise the RCD will trip as soon as you connect you test gear earth anywhere in the live chassis circuit.

Stuart

Yes. The point I was trying to make was that using the isolation transformer, which you have to if your test gear earth can't be lifted, can expose you to more danger than you were exposed to without it. It's convenient (you might say necessary for certain test gear). But it isn't a safety device - if anything it can be the opposite.

VB

[usradcoll1]

Originally Posted by Brian of Romsey

Further more, it is useful to remember (from a DIY & bodging perspective) at any pair of identical transformers can be back-to-backed to make a 1:1 isolation unit. It doesn't matter what the secondary voltage is really.

A second useful tip is that small low voltage transformers can be used in this sort of situation to tweak the output voltage up or down a small amount. This is done by putting the secondary in series with the main transformer's primary (and the little transformer's primary across the mains). Depending on the phase of the small transformer's secondary with respect to the main transformer's primary there is either an additive or subtractive effect. Important to ensure that the small transformer's secondary current rating is at or above the main transformer's primary current. Example: Main transformer is 500 VA so primary current will be a bit over 2 Amps, so a 6.3 V, 3 Amp transformer will do the job and will give +/- (6.3 / 230) which is about 2.7%. A 12 V transformer gives about 5%.
See https://www.youtube.com/watch?v=ZEFVNFyG9po
http://electricalacademia.com/transf...ing-principle/
https://www.mig-welding.co.uk/forum/...p-mains.38115/
http://www.dms-audio.com/bucking-transformer-diy
https://cdn.instructables.com/F12/9C...=webp&crop=3:2

Way back in the day, when my age was 8 or 9 YO, I'm 74 now, I had a Library book with all electrical experiments for beginners. It showed a hookup with two bell ringing transformers connected sec-to-sec. I didn't have two bell transformers, so I used an electric train transformer as the first transformer. A pleasing option is that it could be used as a form of variac. It couldn't be used for large loads, but it should've been enough for a 4 or 5 tube AC-DC set.
Dave, US radcoll1 .

[emeritus]

Interesting use of a train controller. I suppose it would have been for something like the Lionel O gauge system and would have been capable of delivering a couple of amps at up to 20V.

[6.3volts]

This may cause a few raised eybrows but I went down the route of two 110V equipment transformers back to back to give 240 and 110 both isolated from the grid and each other. Tons of power with no sag and about £100 spend.