Isolation transformers, use of.

[Mark Hennessy]

What I have tried to do is to highlight the bits that are most useful to us without the engineering bits that could confuse some users here whom are not used to reading engineering papers unlike us more experienced folk can.

Yes, ideally an article needs to be written that summarises everything - accessible enough for beginners, but one that covers all the facts. It's something that I've had in the back of my mind for many years now, but if someone wants to beat me to it...

The other useful bit I have tried to give is an example where the secondary of the isolating transformer finds its own virtual center tap when a live side circuit with a bridge rectifier is involved even though there is no physical electrical connection to it.

It's true that some people - beginners or otherwise - can be confused by bridge rectifiers.

During the power supply courses I run, we do experiments to demonstrate the possibilities. This is done at 12V for safety, but it gets the point across reasonably well.

It's colloquially known that a TV set chassis with a bridge rectifier and no isolation will be at "half-mains". What a proper analysis (or a simple experiment) shows is that from the point of view of Neutral/Earth, the chassis swings up and down between 0V and -325V. The average value of this is very roughly "half-mains".

The other point of view - as you say - is when you isolate the AC and see what happens w.r.t. to the earthed negative output of a diode bridge. Each of the AC inputs will alternately swing between earth and Vpeak (325V DC), ignoring the diode drops.

The OB truck set-up could be used to localize the RCCD operation to a workshop that is distant from the house RCCD but you would need a second isolating transformer with a floating secondary for the item under test.

That's a good point - although I don't think you mentioned that earlier? Yes, it's a way to provide discrimination between the house RCD and the workshop RCD, but potentially an expensive one. The additional transformer would need to be quite large, and as such, mechanical noise and inrush currents might cause problems. And transformers can run warm, even off-load. Obviously, the transformers for isolating "devices under test" can be the usual type (500VA is common).

In my case, the most sensible option would be to run a cable from my consumer unit that comes from the non-RCD protected section. My installation is old enough to have a "split" unit, where sockets are on an RCD, and lights and the immersion heater are on a double-pole isolator (and appropriate MCBs, of course). But like you, I'm careful not to trip the RCD. If the house one goes before the bench one, there's a fair few clocks and computers to put right afterwards.

When you say "RCCD", I presume you mean "RCCB"? Too many acronyms

Cheers,

Mark