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Oscilloscope probing mains AC DUT questions

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calzap:
Martin,

In Scenario #1, you’re making an assumption that point A remains connected to earth ground when the DUT is powered-up.   All it would take is the operation of a DPDT relay in the DUT to swap point A and the point where the probe is attached.  There are lots of other circuit scenarios where things change a wee bit when power is applied.

If the DUT is mains-powered, at least, power it via an isolation transformer.  Oh, and check with a DMM that the secondary of the isolation transformer really is isolated from the primary before you use it.  Not every transformer sold as an isolation transformer is designed for true electrical isolation.  Some are sold to isolate the output from noise on the input lines, but the secondary is not electrically isolated from the primary.  One of the secondary lines is electrically connected to the metal case which is connected to mains ground, which in turn, is connected to the neutral in the service entrance panel.

Mike in California

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