On Nov 30, 9:51*am, "Wild_Bill" wrote:
It's not clear what you were trying to say.. "THIS? is a dangerous
practice".

Floating a scope plugged into a utility line power source (and omitting the
earth ground connection) is hazardous because the chassis (possibly the case
and other exposed hardware) of the scope will be at the same potential as
the probe ground clip while testing.
Also, it's possible to exceed the scope input's safe limits.

When using a proper isolation transformer, the equipment/device being tested
is commonly plugged into the I-T, not the scope (some exceptions apply,
in-place industrial equipment which can't be powered by a servicing I-T, for
example).

Some I-Ts have a pass-thru earth ground connection, and others may bond the
line Neutral or one side of the line power (which isn't any type I've ever
used, just read about here).

When servicing gear, I want the DUT device under test to have no paths at
all, to the utility service/line power Hot(s) or Neutral connections.
The DUT is then as safe as if it were powered by battery (except for only a
miniscule uA leakage at the I-T transformer).

FWIW..
In the U.S. the AC line Neutral is often bonded to earth ground at the
service panel).

Common Variacs or autotransformers do not provide line isolation, and
shouldn't be considered safety equipment.

A 2 channel/input scope with a Differential Mode selection and proper
isolation of the channel circuits can generally be used for safe testing,
but only if this procedure is included in the scope's operating manual.

--
Cheers,
WB
.............

"pfm" wrote in message

...



Many advocate the use of an isolating transformer. The oscilloscope
probe and ground are then used like multimeter leads. Tektronix and
others maintain this is a dangerous practice and may result in
cumulative damage to the oscilloscope. Differential probes are one
answer, though quite expensive.

Comments please. Thanks

BobH

Agreed on the dangerous aspect but I have had issues with ground loops
between an audio generator and the scope when testing audio
amplifiers. I carry a ground lifter in the scope probe bag and use it
when needed - not very often. I NEVER connect the scope ground lead to
any elevated supplies when using a ground lifter. There are other
better and safe ways to measure high Voltage circuits.

G²