Isolation transformers : good idea or not?

[Daventry 5XX]

Hi all,

I've been looking at the idea of buying an isolation transformer. As a relative newcomer to vintage radio I'm unsure as to the pros and cons. Is it a good idea or not? If so where can I get one?

I've looked on eBay but there doesn't seem to be anything suitable.

Any advice very welcome.

Alan

[Craig Sawyers]

What is your main concern? Safety?

Craig

[Daventry 5XX]

Obviously, I don't want to accidentally fry myself.

I have a 30mA RCD on the supply board but I'm not sure that would be enough.

Alan

[paulsherwin]

You will find there is no consensus about this. There is a range of opinion - on one extreme there are people who believe it is dangerous to work on an AC/DC set without an isolation transformer, and at the other extreme people think isolation transformers are a waste of money which give a false sense of security.

Isolation transformers are most useful in a professional workshop environment where there are many items of earthed test equipment. These cannot be used with AC/DC appliances without a transformer.

If you are in good general health you will not be killed or seriously injured if you make contact with mains live, so long as no part of your body is earthed. Old school repairmen used to always work with one hand in their pocket to prevent shocks across the chest, and this remains good practice today.

[Steve_P]

Well I've never used one and I'm still here. If you have an RCD on your supply then that should open in time to prevent a dangerous shock.

They are nice to have, and if you feel you need one, then go ahead. But it does not make you 100% safe. Electricity is a good servant but a lousy master. Remember if you do get one that if the radio is plugged into it and some mains powered test equipment isn't, then you are in more danger, not less. All the bench should go through it.

NEVER put both hands in or near a live radio at the same time. One hand to make measurements, see if a part is hot, etc while the other one stays in your pocket.

Safety is not just things like this, it's common sense. In an AC/DC set, for example (dropper resistor rather than mains transformer) make sure the chassis is linked to Neutral and the On-Off switch does either both poles or at the very least just live.

Cheers,

Steve P.

[evingar]

I'm sure we have been here before, but I can't seem to find the relevant thread

An isolation transformer will prevent you getting a shock between a mains pole and earth, but you will still of course receive a nasty shock if you connected yourself between both poles of the transformer secondary.

My view is that is a good deal, especially if you are dealing with equipment where one side of the supply is connected directly to the chassis (AC/DC equipment). I (personally) power equipment under test through a combined variac and isolation transformer arrangement. The unit has 2 dials, one for current and one for voltage. It also has a current breaker fitted.

I do know (from past discussions on here) though that there are vastly differing views on isolation transformers (and even the use of variacs)

[Joe Ingle]

Like the original poster or this thread, I'm relatively new to valve radio, but had read that isolation transformers can be useful. (although reading this thread maybe now I'm not so sure...)

I couldn't find any to buy (or even any 1:1 transformers that I could use), so I made one for about £30 using two identical torroidal 250v:50v 80VA transformers back to back (step down, step up), mounted on a wooden board with a terminal block and mains socket.

I got them from Farnell:
http://uk.farnell.com/jsp/search/pro...sp?SKU=1675069

You need to make sure you wire all four secondaries in parallel, matching the wires marked with a dot on the schematic on the data sheet on the above link.

It seems to work fine though...

Joe

[ppppenguin]

Originally Posted by evingar

I'm sure we have been here before,....

Indeed we have. As others have said, safety is state of mind and good habits rather than a technical fix. FWIW my approach to the technical side is:

1. RCD on main supply to bench.
2. Isolating transformer when I have to connect earthed testgear to a live chassis set.

Please note that the RCD will not give any protection against you touching things on the secondary of an isolating TX. This also includes things such as the HT rail of a set with a double wound TX. The worst shock Iever had was from faulty insulation on a test probe while checking -1200V EHT on a 'scope. It hurt. A lot. No RCD or isolating TX could have helped at all. I was lucky to be thrown clear.

[John_BS]

Paul (in post 4) has summed things up nicely.

With the advent of reliable RCD's, you are normally protected against (lethal) live to earth shocks, but not, of course, against live-neutral or "DC" (HT to chassis) events. The hand-in-the-pocket is the only solution here.

Where the isolation transformer comes into play, therefore, is in instances where you need to use mains-powered class 1* test equipment on a piece of kit which has a "live" chassis. Note, however, that if you feed the transformer from your RCD, the latter becomes ineffective in protecting you from "live-earth" shocks emanating from the "Live chassis device under test" + "earthed test equipment" combination. Back to one-hand operation!

John

* Equipment which relies on earthing external/internal metal parts as a part of the overall protection scheme.

[oldticktock]

This is my set up maybe it helps visualise.

I use the isolated TX as Paul described earlier so I can use earthed test gear on my TV ( invariably AC/DC) restores, mainly the scope but other kit occasionally. Some lift the earth from their test gear, that's their choice.

The whole bench is supplied via RCD, all the above bench sockets and front panel except the main white socket on the front, which is the isolated TX socket.

Picture one is a close up of the front panel sockets, various vintage sockets and quick test, these can be routed to Lamp Limiter and variac. Also shows the Isolated T/X socket.

The second picture shows the large beefy isolation Transformer ( silver case on left just below bench)

I know the RCD works after making a silly mistake the the whole house went into darkness. The consumer unit has a built in RCD so perhaps the bench RCD is surplus to requirements.

Hope that helps
Chris

[Glowing Bits!]

Personally, I don't bother with isolation transformers, the only bit of advice I can give you is to go out and purchase a plug in circuit breaker, make a lamp limiter and make sure the set you're working on has a damn good neutral connection, both in the plug and on the chassis.

The neutral is pretty much an earth (return, to the substation), if this fails, the set will have a live chassis but may appear dead.

It may also be worth getting hold of a plug in energy monitor, this will let you know if the set has a runaway condition taking place.

Follow what the others have said about where to stick your hands, it's best to hold meter probes with your elbows either vertical or slightly upward, this makes sure no skin will touch the chassis.
Hope this little bit of info will prevent a premature death, funerals are not nice things to attend.

[Glowing Bits!]

Forgot to mention the carpet/rug/mat. Placing one of these on the floor will also help. Carpets are usually insulators, unless they're sopping wet!

[Top Cap]

What hasn't been mentioned is the fact that earthing the Neutral can cause a circuit breaker to trip. Therefore an isolating transformer will allow earthed test equipment to be connected to a chassis that would otherwise be connected to mains neutral. I find that a Variac and isolating transformer to be most beneficial when repairing switched mode power supplies.Isolating transformers do as it says on the tin, to isolate you from an earthed referenced mains supply. Even though my supply is protected by a RCD, the addition of an isolating transformer in my opinion steps up safety to another level which can't be a bad idea. At the end of the day though, one MUST maintain awareness when working on any high voltage circuitry. We used to have a golden rule in the workshop - only power one piece of equipment (what you are working on) from the isolated supply. Easy to see why if you had a TV and radio working together with cross coupled supplies as this would place full 240V potential between the two chassis.

[MrElectronicman]

Having recently received a belt from an earthed scope connected to an unearthed signal generator, which had the live connected to the chassis via a Y capacitor. I decided to invest in an isolation transformer.

I have all my bench electrical outlets connected by means of GFI's, except the one which gave me the shock and that has now been corrected.

Personally I have not given it much thought now that I am in North America because the voltage in much lower than that of Europe. After having gone through a rough time with my heart I am now more careful, but the incident above indicated to me that you can not be too careful, so the more bells and whistles the better.

[davep49]

An isolation transformer is a good idea if working on old equipment.
(1) AC/DC Radios and Most valve TVs have one side of mains connected to chassis.
(2) Older transformers may have faulty insulation.
(3) RF Mains filters may be faulty connecting either side of mains to chassis.
(4) You do not need to isolate earths on 'scopes, sig.gen etc - a dangerous practice as you can finish up with the whole test bench 'up in the air'.

There is a small one here:
http://www.vvttransformers.co.uk/vth..._datasheet.htm

RS components do a whole range for various powers and voltages.

Old school repairmen used to always work with one hand in their pocket

In the pocket is used because you can't quickly grab something else if you do get a shock, an earthed radiator springs to mind.

[Robert Darwent]

Originally Posted by Glowing Bits!

...it's best to hold meter probes with your elbows either vertical or slightly upward, this makes sure no skin will touch the chassis.

If it works for you then fine, but I can't imagine a more awkward (and possibly dangerous) thing to be doing whilst working on any chassis than to hold probes, etc with my elbows instead of my hands.

Personally, I make sure my arms are fully covered and wear a good close fitting, supple pair of rubber gloves if I've any doubts whatsoever that I may need to briefly use two hands when working on a live chassis.

Regards