I do not know if I belong but I have an affliction where I find myself just gazing at the VFDs and my blue backlit LCD on my 3478. I also find myself buying things "just because". Like an Agilent u1252A "just because" it was $125 (perfect except battery lock needed new plastic pin). Or a Phillips PM 2525 which I like because of the motorized switch.
I also love precision measurement ( I work down to 500 microMeters).
OK. My question is about isolation transformers (ad nauseum, I know). But when I was young we used em. But those were the same days in which I subscribed to subscription kits advertised in Pop Elec that had you build different tube ckts on a cardboard platform with perfboard (I wonder why they stopped making those?).
So I am aware accepted practice changes as a result of incident post mortems. I was taught you only needed to isolate the DUT. But I read a post here that even the TE to be used must be isolated. Then I read another post that said the use of isolation transformers is no longer an accepted industry practice.
I work in an area where risk mitigation is high on the list (tree felling). So I have no interest in being a "he-man" nor am I risk averse.
I am well and truly confused. 
Here is a scenario. Using a scope on an old TV (imaginary) by following the signal from the tuner. Good earth grounds on all equipment. Bench is covered ESD rubber surface. Rubber on floor.
Isolate the TV or no?
Isolate the TV AND the Scope?
No isolation at all and know what you are doing (differential probe is best, but maybe not available?)
Using a DMM
Isolate the TV?
Isolate TV and Bench mounted DMM? Leave the DMM earthed?
Use the Agilent 1252A?
Role of hand coverings? Nitrile, rubber, etc
What are the authoritative sources for best practices? Ideally from industry trade associations that derive these practices from existing regulatory bodies (code and workplace safety).
Thanks!
As you say practices change. In this case it is due to incidents with old practices and also the avaiability of new technology.
If you use an isolation transformer for shock protection then ALL equipment being used must be isolated from mains earth. Ideally there should not be any earthed conductive items in reach.
The new technology includes earth leakage circuit breakers and test equipment with fully isolated inputs e.g. Tek THS700 series 'scopes.
Very difficult to give specific advice without knowing the EXACT set-up i.e a site vist.
Generically use a earth leakage circuit breaker and take care. Do Nnot use an isolation transformer.
Robert G8RPI.
What really grinds my gears is that people think that using an isolation transformer is for personal safety reasons. It is not, and does not provide ANY safety for the person working on a device. It is PURELY and SOLELY to protect the scope inputs from mains ground referenced voltages, NOTHING ELSE. The RCD/GFCI/RCBO is what protects your person from the mains, if you are clumsy enough to do something stupid. NOTHING will protect you from, say, touching both rails of the primary side of an SMPS, other than good practice (one hand only), and careful, methodical working.
People need to get it through their thick heads that working on mains powered devices is DANGEROUS, and there is NO SAFETY DEVICE that will protect them in all circumstances. If you are not confident, or don't know what you're doing, DON'T work on it.
Back in the day, Mains operated was "the only game in town".
Yes, it
can be dangerous,
if you don't know what you are doing, but not near as dangerous as many things we do every day without a second thought, like driving to work.
There were no RCDs back then, & in some countries transformerless radios & TVs were common, which, at best case had the metal chassis connected to Neutral, & worst case to the Active line.
In Australia, due to a fairly strict licencing regime, & incidental things like transformers being relatively cheap & the locally built tubes being mainly 6v or 12 v heater types, with "series heater" types being expensive imports, meant that the overwhelming majority of radios & BW TVs used a power transformer.
The amount of wiring connected to the Mains in such equipment is minimal & easy to troubleshoot with an analog multimeter without the device being powered.
With the introduction of Colour TV in this country, SMPS became the norm, but even these had a fairly well-defined Mains referenced section, isolated from the rest of the circuitry by transformers at the SMPS operating frequency.
If there was no fault involving the SMPS, it was quite safe to work on such TVs without an isolation transformer, & many thousands of people did so.
If you need to critically probe the SMPS, the lack of a "common" point to connect the "ground" clip on the 'scope becomes a problem, so an isolation transformer does become useful.
Any kind of test equipment I ever worked on which had any pretensions of being the "real thing" either used a power transformer or a SMPS with a transformer at the switching frequency providing isolation.
Those of us brought up on valve/tube radios learnt the hard way to watch where we put our fingers, from receiving nasty but not usually dangerous (in domestic equipment) "zaps", from anode/screen supplies.
We always gave the Mains the greatest of respect, but weren't in a constant state of terror---
Unfortunately, those people who do work on electronics these days have become used to DC supply rails with voltages up to around 15v, where you can poke around without a care in the world---if you get across 15v most people won't even feel it.
Even worse, is the "newbie" who just bought a 'scope & wants to look at something with it (they never seem happy with just looking at the calibration output).
At one time there were a plethora of signal sources in most homes, from such things as analog VCRs, DVD players, set top boxes, etc.
Even later, there were "AC adaptors" of which, at least some had a low voltage ac output.
Nowadays. the "Noob" looks around, sees nothing & "zooms in" on the power point for a convenient source of a 50/60Hz signal.
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