From a safety url someone posted today in the thread on fixing a
* Use an isolation transformer if there is any chance of
contacting line connected circuits. A Variac(tm) is not an isolation
transformer! (See the next section with regards to isolation
transformers and microwave ovens.) The use of a GFCI (Ground Fault
Circuit Interrupter) protected outlet is a good idea but will not
protect you from shock from many points in a line connected TV or
monitor, or the high voltage side of a microwave oven, for example. A
circuit breaker is too slow and insensitive to provide any protection
for you or in many cases, your equipment. A GFCI may, however, prevent
your scope probe ground from smoking should you accidentally connect
an earth grounded scope to a live chassis.
* Don't attempt repair work when you are tired. Not only will you
be more careless, but your primary diagnostic tool - deductive
reasoning - will not be operating at full capacity.
* Finally, never assume anything without checking it out for
yourself! Don't take shortcuts!
As noted, a GFCI (Ground Fault Circuit Interrupter) will NOT protect
you from the high voltage since the secondary of the HV transformer is
providing this current and any current drawn off of the secondary to
ground will not be detected by the GFCI. However, use of a GFCI is
desirable to minimize the risk of a shock from the line portions of
the circuitry if you don't have an isolation transformer.
An isolation transformer is even limited value as well since the
chassis IS the HV return and is a large very tempting place to touch,
lean on, or brush up against.