Why can’t an oscilloscope directly measure mains power?
To ensure personal safety during testing and to achieve reliable measurement results, the ground leads of all oscilloscope probes are typically connected to the instrument’s chassis and to the ground wire of the power cord. Consequently, using a standard passive probe to measure mains voltage can result in a ground fault; at best, this will trip the circuit breaker, while at worst, it may pose a serious risk to human safety. The solution is to use a differential probe.
