Isolation transformer

[Negative-Bias]

I want  some isolation between me and a old TV I'm working on, I understand I can use 2x transformers 240/12v turn one around as a step up from the 12v  240/12 mains side on the step down what is taxing me is the Earth, I'm not sure if the earth in the UK is separate or it is connected to Neutral, some where down the line, UK  responses please , as I think we are different in the UK, but not really sure.

[Richard Crowley]

Does your "old TV" have a power transformer? 
Or is it the type that operates directly on mains power?
Does your "old TV" have a 2-wire power cord? 
Depending on what you mean by "old" perhaps it was made before 3-wire (grounded/earthed) mains cords were required?

I believe that mains plugs in many European countries are completely symmetrical. You don't know which lead is "neutral" (essentially 0 volts) and which is "hot" (full 230V).

Here in the US we had a whole generation of consumer radio and TV receivers which were "hot chassis". One side of the (symmetrical and indeterminate) power mains cord was connected directly to the chassis/ground.  They were required to be completely insulated from the user with non-conductive enclosure, plastic knobs, etc.  In the case of TV receivers, the antenna connection was transformer-isolated from the rest of the internal circuit.

Even with a "hot-chassis" device, I would strongly consider connecting the chassis directly to green-wire safety ground, and working out which side of the mains cord is "neutral" vs "hot".  That would provide protection to both you and your test equipment and the device itself.  And using an isolation transformer would be even better.

Yes, I don't see anything fundamentally wrong with using a pair of identical transformers like that wired back-to-back.  Just for safety, I would use transformers rated at least 2x the power expected to be drawn by the device under test.  And remember that the current in the low-voltage connection is much higher and should be wired accordingly.

[raresvintea]

Hello, from what i know, in residential arrea, the neutral is connected to the ground. I found a guide on youtube where was presented how to make a "isolation tranformer" and a big observation was that the grounds are connected togheter. And if the outside of the BNC oscilloscope probe is connected to the ground(like many standard oscillo) it will become dangerous to measure because you can touch ground (neutral) and other hight voltage and kaboom! The guy presented a method how you can isolate the ground on the output and everything will be ok!

I want  any other oppions, but we need to take precaucions or to make something that isn't like in the ISO standard. 
And because I am such a crazy i want to wuestion if a differenrtial protection on the output will add more safetly or will create problems.

Have a nice day!

[_Andrew_]

The earth connections in domestic supplies in the UK are connected back to the neutral star point on the substation. There are several ways this is achieved and the Wiring Regulations 17th Edition BS 7671:2008 incorporating amendment 3:2015 details this. There are diagrams in the guidance notes to help to decipher the blurb   

A dedicated 1:1 mains isolation transformer designed specifically for the task is are too difficult or costly to find.   

Although an isolation transformer, even one used correctly will not protect you from all the ways of getting or giving a life-threatening shock. Also plugging in to an isolation transformer does not necessarily guarantee eliminating the earth connection as earth paths can be inadvertently reintroduced or not apparent that they are present with in the design of connected equipment. Even old hand experienced engineers can get caught out and shocked.

Sadly as for guidance as to how to use an isolation transformer in a workshop environment this is something that you are un-lightly to get people telling you exactly how to do this on an open forum simply because of the risks.
This is not a case of people being rude, it is just that none of us can judge the competency in each other following advice, and like wise people giving giving correct advice in the first place when working with hazardous voltages.

[Negative-Bias]

Thank you all, this TV has sat here for Years because, part of me thinks yeah lets have a crack at it, and another part of me  thinks ,you've made  it to 61, walk away from it. Having read all the comments I've decided to walk away from it. Most things in my work shop run on batteries apart from the 2 scopes & a function generator. Even my Ham radio transceiver run on 240v or 12vDC.I don't feel confident messing with the TV. so the power cord has been cut off and it's in the skip, it's so old I don't think there's any useful parts in it I'll concentrate on Thanks again to all of you for contributing
Happy New Year

[Zero999]

I believe that mains plugs in many European countries are completely symmetrical. You don't know which lead is "neutral" (essentially 0 volts) and which is "hot" (full 230V).

Here in the US we had a whole generation of consumer radio and TV receivers which were "hot chassis". One side of the (symmetrical and indeterminate) power mains cord was connected directly to the chassis/ground.  They were required to be completely insulated from the user with non-conductive enclosure, plastic knobs, etc.  In the case of TV receivers, the antenna connection was transformer-isolated from the rest of the internal circuit.

Even with a "hot-chassis" device, I would strongly consider connecting the chassis directly to green-wire safety ground, and working out which side of the mains cord is "neutral" vs "hot".  That would provide protection to both you and your test equipment and the device itself.  And using an isolation transformer would be even better.

He's in the UK which has polarised three pin plugs.

The problem with connecting a hot chassis to the protective earth when it's also connected to neutral is it will most likely result in tripping of the RCD (GFCI). There will still be a few hundred mV to 1V between the earth and neutral conductor and connecting them together would cause a fairly high current to flow between them. The RCD would see a difference in the current passing through the live and the neutral conductors and trip as a result. Another thing to bear in mind is connecting the neutral to earth conductor, anywhere else, other than the substation or where it enters the property is prohibited under the wiring regulations.

There's a lot of myths surrounding isolation transformers. They can provide both a reliable and effective method of protecting against electric shock. The have advantages over RCDs/GFCIs, as they have no moving parts and equipment will carry on running, even under fault conditions which is essential for life support equipment. On this other hand they have disadvantages: the secondary can float at high voltages with respect to earth and RCDs/GFCIs have no effect when an isolation transformer is in the circuit. A fault indicator, a device which alerts you when once side of the secondary becomes earthed, is a good idea when working with isolation transformers and in some permanent installations is indeed mandatory.

Personally, I often like to work with an earth free workbench. I run the 'scope from one isolation transformer and the device under test from a different isolation transformer. Even when the 'scope's ground is connected to the device under test, I'd have to touch both the scope's case and the device's live conductor to get a shock.