How to Kill Yourself with an Isolation Transformer

[watchmaker]

I listened to those who put the utility of the CFCI over an isolation transformer.  And I grew up with isolation transformers.

One hand rule still applies.  GFCIs did not exist in the 1960s.  Polarized plugs are now standard. Differential probes are available.

I was troubleshooting a device under power and shorted a power transistor collector to ground.  The device was two prong power into a transformer.
GFCI tripped immediately and saved not only me but the device.

I am a reformed believer.

I do not know the use of iso xformers in vintage equipment today.  But I can see that they can invoke a false sense of security and I do not see the sense  in circumventing the GFCI as I think some have suggested.

[madires]

If you rely on a GFCI/RCD for your workbench make sure to get a type B or B+ to cover most fault current situations.

[pdenisowski]

Then why is Tektronix offering battery powered oscilloscopes for "floating oscilloscope measurements"?

If you mean the THS3000, it was discontinued some time ago.  We (R&S) do still manufacture and sell a battery powered, isolated oscilloscope (the RTH) - up to 600 V at CAT IV and 1000 V at CAT III

https://www.rohde-schwarz.com/us/products/test-and-measurement/oscilloscopes/rs-scope-rider-handheld-oscilloscope_63493-156160.html

Edit: No Seriously, unless I am missing something big here... They say in their webpage that using isolation transformers is dangerous and can kill you and all the nice stuff, but then tell you that using a TDS3000 series oscilloscope is ok, just use with caution... just a bit of caution, it´s all ok, it uses batteries and everyone is happy... just a bit of caution...

Damned if I ever seen some legal speech and marketing speech in my life combined...

The Tek app note on floating and isolated measurements is, in my opinion, very well written.  I have often been asked to do a video on floating and isolated measurements, and if I ever were to do one, it would start with a huge warning / disclaimer as well (in red, all caps, bold, and underlined).

I knew someone who used to joke that the reason he studied electrical engineering instead of civil engineering is that people wouldn't die if he made a mistake.  Floating / isolated scope measurements is a good example of where that (tongue-in-cheek) statement clearly isn't always true.

[Vovk_Z]

Edit: As a side note, if you use ethernet, take extra care that the cable is unshielded.

Ethernet twisted pairs are isolated at both ends with high frequency isolating transformers. So, unshielded twisted pair is more safe then shielded with grounding connectors. Basically, if you don't have a shield - you don't have a problem.

[madires]

In TP terminology 'shielded' refers to a single pair and 'screened' to the whole cable with multiple pairs.

[DavidAlfa]

WARNING: Don't put your dick inside bulb sockets, you might die!!

Same as you don't die while driving, your gas oven doesn't blow your home up, you don't suffocate with plastic bags...
People learn their skills so they don't die using isolation tranformers.

Five people might die using isolation transformers every year, but hundreds from electric shock and thousands due car crashes.
Accidents do always happen, same as the 1849813th time you cut chicken, but that day the knife slipped.
Let's ban chicken and knifes! And cars, electricity...

Calm down your overthinking!

[tggzzz]

Quite right.

Also: stop wearing seatbelts, and stop telling inexperienced (little) people not to run into the road without looking.

[DavidAlfa]

Don't run downstairs in darkness with a knife in your mouth, hands in the pockets! 

[tggzzz]

Don't run downstairs in darkness with a knife in your mouth, hands in the pockets! 

Some risks are worth taking.

Is that risk (or the subject of this thread) one of those?

[bdunham7]

I was troubleshooting a device under power and shorted a power transistor collector to ground.  The device was two prong power into a transformer.
GFCI tripped immediately and saved not only me but the device.

Hmmm.  Was this power transistor on the primary or secondary side of that transformer?  Assuming secondary--since a BJT on mains would be rather unusual--it seems to me that your GFCI likely tripped due to a surge and not an actual ground fault.  (No input fuse??)  In that case, while the GFCI tripping may have been fortuitous, your bacon and perhaps the device was actually saved by the isolation transformer.  Stepdown transformers are typically isolating as well and have the same protective characteristics--breaking ground reference, limiting surge voltage and limiting fault current.  Typically if you short something on the mains side, you'll get quite a spark and physical damage to your probes even with a GFCI.  No GFCI or MCB is fast enough to protect any normal-sized semiconductor from direct mains fault currents.  The bond wires would simply be vaporized.

[watchmaker]

I have to digest this.  Thanks. I am sure I will have followup questions.

This was a power transistor regulating the sweep voltage in a BK 501A.  It was on the secondary side and I knocked it into the frame (fucking essential tremors; what a joke).  It was the GFCI that tripped, not breakers.  Fuse on unit was not blown

It sounds like I do not understand what happened.

As far as risk; most of us are not risk averse.  We would not be doing what we do.  But we DO value risk mitigation.  I refuse to work with "Pennsylvania Mountain Men" who use a 24 inch bar with no chaps or hat and wear sandals while using a chainsaw.  But I have no problem approaching a 28 inch oak that needs to be made horizontal; always with two others (one to get help, the other to take care of injuries).

It is pretty easy to tell a pro from a wannabe.  The pro wants to come home with all their fingers.  The wannabe wants to prove he is fearless.

It this offends anyone, too bad.   At 71, I have to earned the right to speak my mind.

[tggzzz]

If that offends anybody, it says lots about them and nothing about you

It is very difficult to accurately assess all possible risk/failure mechanisms. That has to be factored into plans and procedures. Unfortunately some of the reasons behind some of the more obscure procedures is.easily omitted and forgotten.

Too many people think (I use that word loosely) that if they can't recognise/understand a risk, then it doesn't exist.

Beginners think about how things work. Professionals think about how things fail.

[TrickyNekro]

Edit: As a side note, if you use ethernet, take extra care that the cable is unshielded.

Ethernet twisted pairs are isolated at both ends with high frequency isolating transformers. So, unshielded twisted pair is more safe then shielded with grounding connectors. Basically, if you don't have a shield - you don't have a problem.

I do think that´s what I said 
Basically it´s a reference to NOT using CAT 7 or similar cables where the outside "sleeve" of the connectors is metal
connected to each other and usually grounded, or directed to ground through a snubber.

[TrickyNekro]

Floating / isolated scope measurements is a good example of where that (tongue-in-cheek) statement clearly isn't always true.

Indeed. Generally HV can be an expensive hobby... one way or the other...