Unearthed Cathode Ray Oscilloscope (C.R.O.)

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Why unearthed CRO is used in power circuit measurements ??
What is the concept behind unearthing of CRO ???
 

Why unearthed CRO is used in power circuit measurements ??
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Unless you utilize an isolation transformer or a battery powered CRO, you should not use an unearthed instrument. It would be only used by people, who ignore existing safety rules. A differential probe with sufficient voltage rating is the safe way to connect an oscilloscope to power circuits.
 

It was a fairly common practice in small TV repair shops in the past where the chassis metal were floating at 1/2 mains potential. This was before sets became equipped with switching supplies which offered insulation. The correct method as mentioned should be via isolation transformer.
 

To FvM & E-design
Thanks a lot for showing interest !!

I want to know that how unearthed C.R.O has advantage over earthed C.R.O. for power circuit measurements ?
I know that unearthed C.R.O protects C.R.O.s internal circuits and also user from the high currents of device under test (power circuit).But i am not getting it fully....as unearthed instrument is not safety to use. for example we get a shock when we touch unearthed computer. i m confused over this point !!!
 

It is not possible to make power circuit measurements with an earthed CRO because the one of the probes is connected to earth. Connecting that earthed side of probe will cause current to flow from the circuit to the earth damaging the components associated with it.
 

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