Technically cutting the ground pin is wrong, but at least here in the
USA it is how MOST small shops do it.  Assuming the "ground" of your
circuit is safe to touch there is no safety hazard.

A better way is to run your bench off of an isolation transformer.
Ground the test gear and the circuit ground at one point isolated from
the mains ground.  This reduces most measurement hum, and only costs
$100 or so, but still has a problem if the circuit "ground" is at a
dangerous potential.

Good isolated probe adapters are very expensive, and cheap ones have
lots of artifacts that corrupt your readings.  IMHO they are only for
special circumstances.  You have to work with them to learn what the
artifacts are and how to compensate for them.

On the other hand the description suggested the spark was not from the
ground clip, but from the probe TIP!  The only way you should get a
spark from the probe tip is by exceeding the probe voltage (400V?) so
that the probe breaks down internally.  Also if it was a 1X probe and
the scope input was set to 50 ohms you could get a spark, but I think
you said it was a 10X probe.

Sherpa Doug

> -----Original Message-----
> From: David Duffy [mailto:piclist@AUDIOVISUALDEVICES.COM.AU]
> Sent: Wednesday, February 13, 2002 11:54 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: bad oscilloscope?
>
>
> Giles wrote:
> >Rodrigo,
> >Just convert the 3 pin wall plug to a 2 pin wall plug.
> (lift the ground
> >pin)
>
> NO !  NO  ! NO !   THIS IS NOT THE RIGHT WAY TO DO THIS !
> Rodrigo has said he is a beginner - don't let him fry himself.
> We don't need dead PicListers - they don't contribute to the list. :-)
> Isolated probe adapters are the safe way to do these measurements.
> Some of the TEK LCD 'scopes have isolated probes IIRC.
>
> >Believe it or not, you will be able to check almost any
> voltage then.  But
> >keep in mind that when you connect the ground of the probe
> to a circuit,
> >that the entire chassis of that scope will be at that potential.
> >Just a side note:
> >Be careful when using different wall plugs.  I have seen a
> situation that
> >one scope was plugged in on one wall, and we were measuring
> a circuit on
> >another wall.  The 2 walls were wired to different legs of
> the 220. (US wall
> >outlets are made up of 110 to ground of each leg of a 220
> line)  Anyway,
> >with a simple wiring mistake in one outlet of neutral and
> ground getting
> >swapped (usually no problem)  when the probe touch the
> circuit, very big
> >sparks flew!!
>
> This is exactly why you don't do this sort of thing.
> Regards...
>
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