Isolation Transformer setup.

[oldticktock]

Hi not sure if this is the right section.

I'm just want to make sure I have set up my ISO T/X correctly for using a scope on 405 TV's. The diagram shows how I have configured it and intend on using it.

Chris

[julie_m]

The 'scope should not be earthed. Otherwise, you are tying the TV chassis to earth (at least, as long as the probe's earthing clip is in place) and defeating the purpose of the isolating transformer -- which is to break the earth return path, so you can't get a shock from touching only one wire. (Mains neutral is tied to earth at the substation and, if you live in an urban area, again where the cable enters your home. Your body is capacitively coupled to earth.) The output of the isolating transformer is not referenced to earth; so if you touch just one wire, no current will flow.

Note that all cables downstream of the transformer need to be kept short, to minimise parasitic capacitive coupling to earth; otherwise you end up with the situation seen in some Continental countries, where the neutral point of the substation is not earthed and neither live nor neutral wire is safe to touch.

[oldticktock]

Hi AJS,

Oh my! I have confused myself on this I must admit.

I think on my other thread when I was seeking information about a scope, the subject of removing the earth lead from the scope, resulted in everyone saying "madness" get an ISO/TX so hence my confusion

https://www.vintage-radio.net/forum/...ad.php?t=56657

I'm confused as can been seen, so If I do have to remove the earth would it be ok for me remove the earth lead from the Isolated sockets and the safe side of the ISO, that way I can leave the scope with it's earth lead intact should I wish to use it elseswhere.

Chris

[mhennessy]

The 'scope should not be connected to the isolation transformer at all. Indeed, only one device should be connected to the isolation transformer; change the outlet for a single

Mark

[oldticktock]

Hi Mark,

Ok thanks for the advice. Glad I posted the query as I obviously got it totally wrong.

Chris

[mhennessy]

No problem - can't be too careful with this sort of stuff.

Incidentally, opinions vary about the earth connection in the isolated outlets, but FWIW, it's BBC practice to NOT carry the earth through. An earth connection is available if wanted in the form of a 4mm binding post adjacent to the outlet. For an example, see here, where I rescued a pair of isolating transformers and fittings, and built them into a 3U rack case following BBC practice.

Hope this helps,

Mark

[brianc]

Originally Posted by mhennessy

The 'scope should not be connected to the isolation transformer at all. Indeed, only one device should be connected to the isolation transformer; change the outlet for a single

Mark

I don't understand why only one piece of equipment should be connected to the isolation xfmr. Surely the reason for the transformer is to remove the connection of the neutral from "earth" so that the live is not 240VAC wrt earth. Once this isolation is achieved, then any number of things can be connected to the transformer (within the rating, of course). Am I missing something?

[oldticktock]

Hi,

I'm confused too but for differnent reasons mainly due too my inexperience in these matters

I thought the main mistake I made was to earth the Isolation sockets. I thought you still had to connect the scope and the TV to the isolatated double socket i had previously.

Sorry if i'm being thick here just very very confused, if I use a scope which is earthed on the normal mains does that not mean that I will be introducing and earthed device onto the Isolation side when I start probing the TV with the scope

I have removed the double as Mark advised

[mhennessy]

Originally Posted by brianc

I don't understand why only one piece of equipment should be connected to the isolation xfmr. Surely the reason for the transformer is to remove the connection of the neutral from "earth" so that the live is not 240VAC wrt earth. Once this isolation is achieved, then any number of things can be connected to the transformer (within the rating, of course). Am I missing something?

It's a good question. In the worst-case scenario, consider when one of the bits of kit has a fault (e.g. Live-Case short), then the case of the other bits of kit will be brought up to live potential. Obviously hazardous...

Or, consider fault-finding a switched-mode power supply where the mains is fed into a bridge rectifier. You will probably want to put a 'scope across the primary control circuit, meaning the negative output of the bridge is connected to earth via the 'scope. This means that the output of the isolation transformer is no longer floating with respect to mains earth, so any other kit connected is no longer "isolated".

HTH,

Mark

[Kat Manton]

Hi,

I only connect the equipment I'm working on via the isolation transformer. All test gear remains connected to the mains directly and I don't lift earths.

But, I don't regard an isolation transformer as a means for preventing me getting a shock, just a way to allow earthed test equipment to be connected up to equipment which lacks its own internal isolation transformer.

I think it works; I'm still alive and can count the number of shocks I've had on the fingers of one hand. Most occurred back when I was young and far less aware of my own mortality!

(Maybe building a linear amplifier with a pair of 4CX250B in my teens instilled a great respect for electricity in me. You only touch the HT supply for one of those once in a lifetime..!)

Regards, Kat.

[GMB]

if I use a scope which is earthed on the normal mains does that not mean that I will be introducing and earthed device onto the Isolation side when I start probing the TV with the scope

Yes, and that's why as Kat said it doesn't stop it being dangerous. Isolating the TV stops the blue flash when you attached the scope probe's earth connection.

So you still need the RCD on the supply.

You could put another isolation transformer on your scope, thereby making it into an unearthed one, which makes life even easier but isn't a substitute for isolating the TV too.

By the way, some test equipment with apparently earthed chassis sometimes has an earth strap somewhere that can be disconnected so as to float the signal earth (the box usually stays earthed).

[mhennessy]

Originally Posted by GMB

Yes, and that's why as Kat said it doesn't stop it being dangerous. Isolating the TV stops the blue flash when you attached the scope probe's earth connection.

So you still need the RCD on the supply.

Perhaps I'm missing something, but supply to what?

I should say that the general advice within the BBC is to use RCDs in preference to isolation transformers when working "live", but of course this advice can't always be followed (e.g. in the example I gave about working on a switched-mode power supply).

You could put another isolation transformer on your scope, thereby making it into an unearthed one, which makes life even easier but isn't a substitute for isolating the TV too.

Assuming you mean an isolation transformer that doesn't carry the earth through, this is not recommended. It doesn't take much leakage to lift the chassis of the 'scope to dangerous levels.

On the other hand, if your isolation transformer does carry the earth through, this achieves precisely nothing.

By the way, some test equipment with apparently earthed chassis sometimes has an earth strap somewhere that can be disconnected so as to float the signal earth (the box usually stays earthed).

Yes, but do be careful - this strap is often about removing earth loops at low signal levels; the kit I've seen with this facility doesn't allow large potential differences between signal earth and mains earth. Check the relevant manual carefully!

All the best,

Mark

[oldticktock]

Thanks to all for the valuable input, however it is getting somewhat confusing for me now.

From reading previous posts about ISO TX's I understood that one should not regard them as the ultimate safety device, but a mechanisum by which you could conduct tests with earthed test equipment.

What I have obviously been confused about, is how to wire the socket/s for the bench and on which side the scope should be placed.

Thankfully you have all provided the necessary information to enable me to move forward.

I've attached the final picky of how I've set it up. I think if we can basically agree that it is sound we can leave it there.

Once again many thanks to all contrubuters
Chris

[kalee20]

I'd go with that, OTT.

[mhennessy]

Originally Posted by oldticktock

I've attached the final picky of how I've set it up.

Chris

Looks good to me

Only thing I'd say is that I don't think you need the RCD on the 'scope. It obviously won't hurt, and I guess it might provide some protection in the event of the 'scope developing an internal fault, but it would be better to have the entire electrical supply to the bench going via an RCD - then that one RCD gives the same protection to all your kit...

Cheers,

Mark

[oldticktock]

Hi Mark,

That's my poor drawing the bench has 7 mains sockets all fed from one RCD all my test equipment, bench lights etc are supplied by it, including now the ISO. Then the ISO provides one socket as per your recommendation.

Chris

[mhennessy]

Perfect

[oldticktock]

Mods we can close up at this point