How essential is an isolation transformer?

[SurreyNick]

Could I get some advice please?

I have so far assembled a series lamp limiter and a bench variac in advance of carrying out some repairs and restoration of three 1930s AC receivers I possess. I also have an RCD adaptor. I am now having an internal debate whether I should....

1. Just rely on the RCD adaptor.
2. Buy an isolation transformer, ready enclosed.
3. Buy the transformer and construct an enclosure around it.
4. Buy the transformer and construct a variable isolated power supply aka the Heathkit IP-5220.

Cost is an issue. It's unlikely I could afford a ready-enclosed isolation transformer, so would have to wait until a cheap second-hand one came up on eBay. The same goes for a ready-assembled Heathkit i.e. outside of my pocket - even tatty old ones on eBay are too pricey. I could probably afford a transformer on its own and make an enclosure to go around it, and I could also strip down my recently constructed bench variac and build an isolated variable power supply. The question is, how important is it to have an isolation transformer? Some advice would be most welcome.

Thank you

Nick.

[Refugee]

An isolation transformer is not vital for fixing prewar radios but is very useful all the same.
The main use for an isolation transformer is when you need to connect powered instruments to a set that does not have an internal isolation transformer.
Vintage TV is where an isolation transformer is most useful.

[tiv48]

awkward to answer really from a safety point of view I would recommend an isolating TX even I f its only two back to back transformers say from old battery chargers.
Then again I worked an radios and tv sets for over 20 years with out one.
I only bought one when tv sets started to use full wave rectifiers so the chassis was always at half mains voltage .
on an AC only set most (not all ) will have a mains isolating transformer .
if you intend to use scopes ect you do really need one as the negative os the probe will be connected to earth.
Please remember that high DC volts can also give you a nasty shock.
Always try and work on a set plugged into the mains with one hand in your pocket.
and stand on say a rubber mat.
in other words just be careful check the chassis with a neon screw driver ,try Not to hold the chassis while taking voltage readings
So a nice thing to have (isolating TX ) but not essential .
Rob T

[pwdrive]

If the chassis is not earthed and has the potential to become live then my take on it is to use one whether or not the mains supply is RCD protected, don't forget that the chassis of an AC/DC radio/record player/tv etc can become live due to a defective on/off switch even though the mains plug is wired correctly.

All the stuff I have at the moment is AC only and earthed, that means I don't use one at the moment but if I was working on AC/DC stuff I would have one installed.

Buy and make an enclosure for it (vented) don't carry the mains earth connection through to the outlet socket.

Lawrence.

[Nick]

The IP-5220 is basically a small 3A 120VAC variac followed by a 1:1 3A isolation transformer with a voltage and current meter bunged in.

i.e., its very crude, and no good in the UK

[SurreyNick]

Thank you for your assistance guys.
Although I conclude from your posts that I don't necessarily need one (the 3 sets do have mains transformers) I nevertheless think, "yes", I will get one anyway.
I am very safety conscious and the warning about the possibility of the chassis becoming live due to a defective on/off switch is enough to convince me of the sense in having one. The rest of the advice regarding safety is welcome too, thank you. Fortunately I was already familiar with it, having served a pseudo apprenticeship working on battery powered sets. But the advice is still very welcome and appreciated. One can never be too careful eh.

Now I just need to decide whether to build a 240V equivalent of the Heathkit, or buy and enclose a TX. Decisions, decisions, decisions...

Nick.

[Yorkie]

There was a thread earlier this year before you joined the forum Nick, (started by someone who is no longer a member), which resulted in a debate about the safety aspects of using isolation transformers.

A few of the posts generated more heat than light, but overall, there was a lot of useful info imparted which you might find  interesting  Nick.

You won't find it difficult to sort the wheat from the chaff:

http://golbornevintageradio.co.uk/forum/...ransformer

I always do as much testing on any equipment with the mains lead unplugged. A great deal of diagnostic testing can be done without the equipment powered up and it's good practice anyway. EG, the continuity of heater supplies, checking transformers for open circuit, checking the continuity of inductors, transformers, switches, checking if resistors are within tolerance. AC only equipment  has an inbuilt 'isolation transformer' in that only the primary is connected across the mains. When checking voltages I always clamp the ground lead to chassis, then I only need one hand to use the positive prod to poke around with, and concentrate on what I'm doing because it's so easy on say small valve-holders (B9A, B7G) to slip and short two pins together.

I do have an isolation transformer and use it when working on AC/DC sets, which isn't very often.

(There have been examples of restorers connecting earthed test gear such as a signal generator or scope to live chassis radios with interesting consequences).

[pwdrive]

It's a crazy old world, when I was in the tv trade as a bench engineer the service bench had a mains isolating transformer fitted, the soak bench behind me didn't (make sure the back's on lads)

Hanging along the side of the service bench was a death lead (amongst other leads)

Lawrence.

[Mark Hennessy]

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:

1. isolation transformers are to allow you to connect earthed test equipment to a "DUT" (device under test). That is all.
   
2. Only one DUT may be connected at once.
   
3. The mains earth must not be connected through to the DUT.
   

I have a PDF issued by the BBC that explains isolation transformers - see the attachement in this post: http://golbornevintageradio.co.uk/forum/...6#pid48916

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.

[Mark Hennessy]

It's a crazy old world, when I was in the tv trade as a bench engineer the service bench had a mains isolating transformer fitted, the soak bench behind me didn't (make sure the back's on lads)

Hanging along the side of the service bench was a death lead (amongst other leads)

Lawrence.

In fairness, the sets on soak probably wouldn't have had earthed test gear connected to them. I guess that brushing against the tube base PCB would have been the most likely way to get a bite from the back of a soaking TV? An isolation TX probably wouldn't have made much difference. Whereas an RCD would (though, depending on how far back we're talking, RCDs might not have been especially commonplace then?).

I'd love to know what the "death lead" was!