Specifications

Floating a scope

Some people choose to "float" a scope so they can make a differential measurement with a probe. This

is done to remove the connection between the scope's chassis (and the outside of the BNC jacks) to

the power line ground. Then the user reasons that they can connect the probe and the probe's ground

lead into a circuit to e.g. measure the voltage across a resistor (a common technique to look at the

current in a circuit). Normally, the probe's ground wire is at power line ground potential and this would

cause a short unless the point the probe ground was connected to was already at ground potential. If

the scope is floated, then this eliminates the problem with the short. Isolating the scope from the power

line's ground conductor is done with a 3-to-2 wire adapter or an isolation transformer.

Floating a scope is not recommended because it can put the user at a safety risk. The risk is that

dangerous voltages can appear on the scope's chassis and the metal of the BNC connectors. It can

happen that the user only learns about this problem when they try to make a differential measurement

and end up with a blown circuit breaker, damaged circuit, or damaged probe -- or worse, getting a

shock. While the principle will be obvious to a user who sees a schematic of the situation, the exercise

drives the point home in a way that theoretical knowledge sometimes doesn't, especially when you

realize that the exercise's innocuous small battery voltage could have been a 120 or 240 VAC line

voltage -- and you may have inadvertently exposed yourself to these dangerous voltages.

If you need to make measurements that require a floating scope, either use an appropriately-isolated

differential amplifier or a battery-operated scope that is specified for such floating measurements.

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