Isolation Transformer - a 2nd one for the Oscilloscope ?

[roli_bark]

When trouble shooting (testing) a Consumer Product - like TV, or its SMPS - we all know it is essential to hook it first up via an Isolation Transformer, before even trying to probe it with a Ground Referenced Test Equipment (like an Oscilloscope).

What I was wandering about is why some [info on the Net] recommends to use at the same time a 2nd Isolation Transformer for your Oscilloscope as well ?

[kripton2035]

isn't it easier to use a differential probe for that ?

[roli_bark]

Depending on whether you them probes, number of available Scope channels, and how many points you have to measure.

Still, why use a 2nd one remains interesting.

[pascal_sweden]

What I really don't get it that some people on this forum here buy a commercial isolation transformer from a well respected company, which is designed for medical industry, but that they then still have to do their own modifications to the isolation transformer to further "improve" it.

So the people who make these modifications believe that the company who designs the isolation transformer is stupid then?

Or in other words, why aren't there any commercial isolation transformers on the market which are good enough, and which require no further modifications?

The moment you do a modification to these devices, you are entering risk zone in my opinion, and most likely you don't even have a clue of what you are doing. You are just playing around, without realizing the risks you are building into the device, with your so called "improvement".

[roli_bark]

The issue of modifying [or not] your Isolation Transformer (whether Commercial/Industrial/Medical grade) - is not related to this thread.

Note:
[The only modification that is somehow related, is that some vendors tend to hook up the secondary winding to Ground (for better Noise attenuation issues). For testing & safety purposes with a Ground referenced Test Equipment, it is essential to disconnect that Ground wire from the secondary winding.]

[Falkra]

I think this is the topic about the modification :
https://www.eevblog.com/forum/testgear/teardown-and-modification-of-noratel-imed-e-150-isolation-transformer/

[Ijaz]

What I was wandering about is why some [info on the Net] recommends to use at the same time a 2nd Isolation Transformer for your Oscilloscope as well ?

It would be interesting to probe it further. Do you have any link ?

[roli_bark]

It would be interesting to probe it further. Do you have any link ?

http://electronics.stackexchange.com/questions/73991/why-do-we-need-an-isolation-transformer-to-connect-an-oscilloscope

[pascal_sweden]

The issue of modifying [or not] your Isolation Transformer (whether Commercial/Industrial/Medical grade) - is not related to this thread.

Note:
[The only modification that is somehow related, is that some vendors tend to hook up the secondary winding to Ground (for better Noise attenuation issues). For testing & safety purposes with a Ground referenced Test Equipment, it is essential to disconnect that Ground wire from the secondary winding.]

Isn't the whole point of an isolation transformer to have a secondary winding which is not connected to ground? I honestly don't get it. Why is it then called isolation transformer in the first place, and why do you need primary and secondary winding? Then it is better to just get an extension cable directly connected to the mains outlet

[roli_bark]

Isn't the whole point of an isolation transformer to have a secondary winding which is not connected to ground? I honestly don't get it. Why is it then called isolation transformer in the first place, and why do you need primary and secondary winding? Then it is better to just get an extension cable directly connected to the mains outlet

The point is this:
For some [especially Hi-Fi Audio] Isolation Transformers are intended to be used for attenuating the Electrical Mains Supply noise, and NOT for safety purposes. So for these types of applications the secondary is hooked up to Earth-Ground.

So people who use them for safety purposes, should be aware each time they get hands on an Isolation Transformers, and verify their secondary winding is DISCONNECTED from Earth-Ground.

[RGB255_0_0]

The issue of modifying [or not] your Isolation Transformer (whether Commercial/Industrial/Medical grade) - is not related to this thread.

Note:
[The only modification that is somehow related, is that some vendors tend to hook up the secondary winding to Ground (for better Noise attenuation issues). For testing & safety purposes with a Ground referenced Test Equipment, it is essential to disconnect that Ground wire from the secondary winding.]

Isn't the whole point of an isolation transformer to have a secondary winding which is not connected to ground? I honestly don't get it. Why is it then called isolation transformer in the first place, and why do you need primary and secondary winding? Then it is better to just get an extension cable directly connected to the mains outlet

[Medical] Isolating the equipment, not the safety of the end user - there's other sections of the equipment for that and home RCDs should still work with medical isolation transformers.

[nctnico]

When trouble shooting (testing) a Consumer Product - like TV, or its SMPS - we all know it is essential to hook it first up via an Isolation Transformer, before even trying to probe it with a Ground Referenced Test Equipment (like an Oscilloscope).

What I was wandering about is why some [info on the Net] recommends to use at the same time a 2nd Isolation Transformer for your Oscilloscope as well ?

Never ever do that! Use a differential high voltage probe instead. An isolation transformer is dangerous in many ways when used to make floating measurements.

[roli_bark]

Never ever do that! Use a differential high voltage probe instead. An isolation transformer is dangerous in many ways when used to make floating measurements.

You mean -
Never ever - for feeding the Test Equipment thru Iso-Xformer [I guess for safety reasons], but
Yes - for using an Iso-Xformer for feeding power to the product under test ?

[Andy Watson]

home RCDs should still work with medical isolation transformers.

Paths to ground exist via many routes and not necessarily via the RCDs - for this reason RCDs detect an imbalance in current between live and neutral (the clues is in the name - Residual Current Detector). Putting a transformer in the way (medical or otherwise) will balance the currents and null the effectiveness of the RCD.

[RGB255_0_0]

Yeah. I guess depends how sensitive the RCD is and where you were physically using it (probably not wise to run a long cable to your neighbour's house  ).

[nctnico]

Never ever do that! Use a differential high voltage probe instead. An isolation transformer is dangerous in many ways when used to make floating measurements.

You mean -
Never ever - for feeding the Test Equipment thru Iso-Xformer [I guess for safety reasons], but
Yes - for using an Iso-Xformer for feeding power to the product under test ?

No for both! Clipping the DUT to ground makes it dangerous to touch in an unknown way while defeating the RCD. With a differential probe you don't have this problem.

[madires]

Never ever do that! Use a differential high voltage probe instead. An isolation transformer is dangerous in many ways when used to make floating measurements.

You mean -
Never ever - for feeding the Test Equipment thru Iso-Xformer [I guess for safety reasons], but
Yes - for using an Iso-Xformer for feeding power to the product under test ?

Yup, isolation transformer only for the DUT, not for the scope. If feasable, use the poor man's differential probe (ch1 + ch2 inverted, ground clips removed). For regularly use get a proper differential probe. Those are expensive, but much cheaper than a funeral.

[TheAmmoniacal]

Wouldn't a battery-powered oscilloscope be the best option? Probably cheaper to do that modification than to buy differential probe. I bet it's cheaper even to get a pure sine inverter and a 12V battery!

[nctnico]

Wouldn't a battery-powered oscilloscope be the best option? Probably cheaper to do that modification than to buy differential probe. I bet it's cheaper even to get a pure sine inverter and a 12V battery!

But then every metal part of that battery powered scope would become a potential deathtrap 
A differential probe is the only sane way to go and they aren't that expensive compared to an isolation transformer anway.

[TheAmmoniacal]

Wouldn't a battery-powered oscilloscope be the best option? Probably cheaper to do that modification than to buy differential probe. I bet it's cheaper even to get a pure sine inverter and a 12V battery!

But then every metal part of that battery powered scope would become a potential deathtrap 
A differential probe is the only sane way to go and they aren't that expensive compared to an isolation transformer anway.

How do the handheld/portable scopes deal with that?

[roli_bark]

You mean -
Never ever - for feeding the Test Equipment thru Iso-Xformer [I guess for safety reasons], but
Yes - for using an Iso-Xformer for feeding power to the product under test ?

No for both! Clipping the DUT to ground makes it dangerous to touch in an unknown way while defeating the RCD. With a differential probe you don't have this problem.

Yup, isolation transformer only for the DUT, not for the scope. If feasable, use the poor man's differential probe (ch1 + ch2 inverted, ground clips removed). For regularly use get a proper differential probe. Those are expensive, but much cheaper than a funeral.

So, one says Nay and the other Yea - what gives ?
The Nay answer makes me a little nervous...

[madires]

The isolation transformer renders the RCD useless and is more reliable than a RCD, but it won't help when you touch both output potentials at the same time.

[tautech]

In fifty plus years scopes have been in common use only 2 recognized safe methods of scope isolation have evolved: isolated channel scopes and differential probes.
There are several methods that are often used in order to achieve the types of measurements that these safe methods provide, none of which are safe and shouldn't ever be discussed on a public forum for fear of the risk such methods wiil expose the less experienced user to.

So don't do it OP, take experienced users advice and put your hand in you pocket and get the correct tools for the job.

[roli_bark]

So don't do it OP ?

OK, let's not talk about the Do's"...

But "Don't do it OP" - what ?
Still it is OK to iso the DUT and NOT the Scope ?

Also what still makes me nervous is these responses:

1) ...An isolation transformer is dangerous in many ways when used to make floating measurements.
2) Clipping the DUT to ground makes it dangerous to touch in an unknown way while defeating the RCD

[pascal_sweden]

Get a differential probe from Pintek. They are very good quality, and very affordable!

This one for example: DP-30HS
http://www.pintek.com.tw/product_detail/landersound/index.php?Product_SN=97781&Company_SN=6002&Product_Site_Classify_SN=17072

Note that Pintek has recently launched the DP-60HS which goes up to 60 MHz.
They haven't added this model yet on their website.