I agree. Problem is, many shops wont buy you one. If you buy your own,
you can bet it will go missing. Even with an isolation transformer,
you really need one for each piece of equipment that is connected to
an AC source. It's not only the potential between pieces of equipment
that is the danger, it's the potential between equipment and ground.
The electrical codes in North America are set up to limit voltages to
ground to 150 volts or less. The 150 volts is not as likely to kill
you, although it can quite easily.

If you have a hot chassis plugged into an isolation transformer,
you've disconnected the chassis from building ground, which is earth
ground. The secondary of the transformer is it's own system, and the
120 volts across it are not related to building ground. You wont get
a shock from touching the chassis if you're standing in a puddle, or
touching a grounded piece of equipment. The 120 volts will be between
the chassis and the other end of the transformer secondary. There's no
need for current to travel from the chassis to earth ground. It has no
circuit there. Unless, of course, the isolation transformer develops a
short between primary and secondary, :-(

If you understand why a hot chassis is dangerous, you can prepare
yourself. I used to measure between each piece of equipment with a
meter to be sure I had no potential between them. I was also very
careful about where I placed my probe ground lead. Also, I tried to be
vigilant about the rule of keeping one hand in my pocket. I work as an
electrician now and I still obey that rule. If I have to get two hands
in th circuit while it's hot, I make sure one is well insulated, or
isolated with well-insulated tools.


This isn't a matter of "if you can afford one", when working on hot chassis
equipment an isolation transformer is *absolutely* nessesary, if you can't
afford to spend $50 or do some scrounging and cobble one together for free,
you shouldn't be poking around in the equipment under test in the first
place. If you try working on something without isolation you *will* fry your
scope sooner or later if not yourself in the process.