Re: Problem with common leads in dual channel scopings?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br> 

Tesla list wrote:
 >
 > Original poster: Harvey Norris <harvich-at-yahoo-dot-com>

 > This brings me to a question of scope operation: can
 > anyone simply say why grounding is necessary?

Oscilloscopes usually have the ground connection of all probes
connected to the metal case of the oscilloscope, and also to the
ground pin on the power plug. So, the ground probes can only be
connected to ground.
It is possible, for low-frequency measurements, to disconnect the
ground pin of the power plug using a suitable adaptor and then
float the ground connection of the oscilloscope. But the ground
connections of all the probes must be connected to the same
point, and the case must be insulated from the ground. Of course,
in a high-voltage (>30 V) measurement it's not a good idea to
touch the oscilloscope during the measurement, and in really
high voltage measurements (>500 V) the insulation of the power
transformer inside the oscilloscope, or of the power cable, may
fail, possibly leading to extensive destruction.

Antonio Carlos M. de Queiroz