I am quite aware of the reasons why people want to "float" scopes but there is always a way to do the measurement without floating the scope. For example, you could use a differential amplifier in the case where you would like to connect a probe ground lead to a point that is not at zero volts. In this case you, would use TWO probes . . . one connected to the + input of the differential amp and the other connected to the - input of the differential amp. Both probe ground leads would be connected to chassis (earth) ground and so would the ground pin on the scopes power cord. The scope display will show you what is on the + input relative to what is on the - input which is what you were trying to see when you wanted to float the scope.

What makes it so dangerous to float the scope is that it is very easy for you to come in accidental contact with the floating scope chassis and receive a very bad shock, possibly lethal. One of my first customer contacts as a Sales Engineer for Tektronix was to call on the Sylvania Lighting Center in Danvers, MA and investigate a rumor about an engineer working there that was killed while using a Tek scope. I found it it was true. During lunch, one of the engineers was working alone in the lab on a lighting experiment that was using some 3 phase, 220 volt power. He needed to make some measurements between points none of which were at earth ground. So, he floated the scope . . . He even has the scope sitting on a scope cart with a sheet of insulation material between the bottom of the scope and the metal tray it normally sits in so the scope cart would not be "hot" with the scope. He also had a "tunnel" of plexiglas on both sides and over the top of the scope in a crude attempt to prevent anyone from accidentally touching the hot scope. The back was not covered with plexiglass in order to allow the fan to do its job and the front was not covered so the engineer could access the scope controls. This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway.

Anyone who teaches you that floating a scope by defeating the power cord ground lead is a very poor teacher, indeed. They simply do not know enough about making SAFE measurements to be in a position to teach electronics.

Most of the time, just using a differential amplifier to make your measurements is an improved technique that will work fine. Craig also suggested the A6201 (?) Isolator as another useful item for measurements like this.

Stan
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