How essential is an isolation transformer?

[AlanBeckett]

Lawrence, between that and the chainsaws, how come you are still alive

Don't forget the Pit Work as well.
Cats and nine Lives spring to mind.

I rely on an RCD, Safeblock, sometimes an Isolation Transformer and long experience.

Alan

[Mark Hennessy]

"So how does an isolation transformer isolate the operator, given that said operator must be assumed to be earthed because he or she will be reaching out to touch test gear, etc, or perhaps standing on the floor? Or am I confusing semantics?"

It depends how far you want to take it, it won't isolate you from earth but it will isolate you from live if the isolation hasn't  been compromised, live and neutral feeding a load is a simple circuit, if your not part of it then no shock.

I think I'd want a sketch to be sure I've totally understood you.

Yes, I think we should call you "NineLives" from now on

[SurreyNick]

This "...properly plan my workbench arrangement" is for post-(house)move. Right now my 'clean' workbench is the kitchen table and my 'dirty' workbench is a tiny school-type table on castors which I use out-of-doors. When we move I hope to have a proper workshop, or at least a dedicated space in the garage/shed where I can have a permanent bench set-up.

It may be that I will work on one or more of the AC sets before we move, in all likelihood though, I won't. What I have been doing in the interim is cobbling together the workshop equipment and test gear that I will need in order to work on these units. Up until now I have focussed exclusively on battery powered sets. Starting with safety stuff I knew a series lamp limiter would be useful, as will be a variac, so these two I have made and are now included in my 'gear'. I also have a non-contact voltage detector. The next item was going to be the isolation TX (if I needed one), hence today's question on this forum.

But what the responses to this thread and the posts in the link from David to the other thread showed me is that I am going about it piecemeal and without proper thought and planning in respect to what I will need. That's what I meant when I said "I should reconsider and properly plan my workbench arrangement". I was publicly chiding myself really. I usually spend time researching, reading and learning what I should know before going off ham fisted. That's how I approached things before messing about with the battery powered receivers and it's what I should have done for the AC ones, but I got carried away. So before doing anything else I first need to establish exactly what I need for working on pre-war (up to 1935) AC receivers and not continue charging ahead making and buying things on a whim and a fancy.

Advice along these lines will be extremely useful

Nick.

[pwdrive]

Here's a sketch but not by me, it shows what I was meaning Figs 3 & 4 (stray capacitance and other possible leakage paths not shown):

http://www.soundonsound.com/sos/nov06/ar...k_1106.htm

Lawrence.

[Mark Hennessy]

Honestly, I think you're being a bit hard on yourself

Starting with a lamp limiter and a variac is absolutely the right thing. The non-contact voltage detector is also useful (not just for old radios). And of course, you've got your new DMM with good test leads. So I think you're doing good.

Isolation transformers are more debatable. Sometimes they are essential; other times they do more harm than good. Perhaps put off the purchase until after the move, and use the time between now and then to get your head around all the issues that have come up in this thread (and countless threads before).

My aim was to explain that the issue is not "black and white", and that if very experienced engineers can be confused by isolation transformers, then you mustn't feel bad if you don't immediately get it. Please don't be intimidated by this - it's just a case of starting from first principles... Please ask for clarification if needed.

If/when you do get one, it's probably best to make it a "portable" unit. In quotes because it'll be heavy! Very useful for secondary purposes, like gluing things together and holding doors open

Unless you get into TV work, 100VA would probably be plenty. The ones I have here are 500VA (just over 2 amps) - and are fine for everything I've needed to do. As suggested earlier, two connected back-to-back might do the job, but whatever you do, please make it safe. You did such a good job with your lamp limiter and variac that I'm sure you'll have no problems - but if in doubt, please ask.

[SurreyNick]

Thanks Mark
I feel confident about asking questions here and not fearful of being put-down or derided for being a novice.
Questions will, no doubt, follow in due course.
Nick

[Refugee]

The isolation transformer I have is a 1000VA boat anchor and is seriously heavy and was designed to be bolted to a wall with conduit holes in the case. It stays under the bench and up at worktop height is a 1000VA Variac that remains plugged into the transformer all the time. I have a second pair of units that can be connected together consisting of a 250VA isolating transformer and Variac with 13A plugs and sockets on them. That big set up is staying as a fixed installation.
I got all the bits for free at various times over many years.

[Alan Douglas]

The "death lead" is usually called a suicide cord here; I have several, one with fuses in the AC plug.

The one at the motor shop was interesting: a 60W lamp mounted on a box on the wall, with two flexible wires coming out, terminated in "test prods" made from solid house wiring with the tips stripped to expose bare copper. One lead went to neutral; the other to the lamp socket, and the other side of the lamp went to 120V. You were presumed to know enough not to touch the bare wire while probing for grounds in motor windings.

And on the subject of "do as I say, not as I do": part of my bench is stainless steel, grounded.