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When it is necessary to use an oscilloscope with isolated grounds?
For nearly 5 years now, I have been using oscilloscopes with a common ground. But I have no experience in using one with isolated ground.

The question popped when I was going through a spec-sheet for a DSO that the sales person had shared.

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The obvious answer is: when you want to do a floating measurement; i.e. not referenced to ground. Now with a common ground oscilloscope, this is a dangerous thing to do as you have to detach the ground within the oscilloscope from the mains ground. This could result in dangerous voltages on the casing of the oscilloscope, and should never be done. The right way to do is to use a oscilloscope specifically designed to be able to do floating measurements; thus being safe from any high voltages.

...And why do you want to do a floating measurement you ask? In some fields of engineering, you sometimes want to measure two machines or circuits with respect to each other, but the grounds are not connected. In that case, you don't want to connect the grounds with eachother through your oscilloscope, that would cause large currents to flow.

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An isolated o-scope is used when you can't isolate the ground on the equipment and its chassis ground is connected to the AC power's neutral (directly or through a capacitor). There is no difference in function between the two scopes. In the past, I have used an o-scope with an isolation transformer that didn't have its fg (3rd prong) connected to the wall outlet's 3rd prong to make my scope isolated. However, I still use a neon test lamp or a meter and see if I am getting medium voltages (5-500V) between the grounds of the unit and neutral and fg of the wall outlet. Just to know if I am working with a "hot chassis". As a tv repairman, I had to deal with hot chassis forever. But the isolation transformer is the standard equipment used just as much as the soldering iron in that field. I remember working at shops where they wired one on every bench.

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