Bad to connect scope leads to AC line voltage?

I seem to recall reading somewhere that one is supposed to use a 1:1
isolation transformer when wanting
to scope the hot side of line AC powered equipment.

Other than the obvious safety issue, is this bad for the scope? Will it
damage the scope somehow?

Thanks,

Joe McGuckin

 

[mike]

joe@via.net wrote:

I seem to recall reading somewhere that one is supposed to use a 1:1
isolation transformer when wanting
to scope the hot side of line AC powered equipment.

Other than the obvious safety issue, is this bad for the scope? Will it
damage the scope somehow?

Thanks,

Joe McGuckin

Depends....
on where you put the transformer
The overvoltage limts of your scope
Whether you have line transients
What probes you use

The safe way to do it is to use a probe isolator.
There's one for sale at the link in my sig.

Next best is to use a differential scope with probes rated for the
voltage in use + any transients etc.

Don't float the scope. Aside from the obvious safety issues...
there are some not obvious safety issues.
mike



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[Walter Harley]

<joe@via.net> wrote in message
news:1125989293.533764.18880@g49g2000cwa.googlegroups.com...

I seem to recall reading somewhere that one is supposed to use a 1:1
isolation transformer when wanting
to scope the hot side of line AC powered equipment.

Other than the obvious safety issue, is this bad for the scope? Will it
damage the scope somehow?

If you connect the scope ground lead to the line, it'll short the line to
ground through the scope, which will certainly fry something.

If the scope and probe are rated for 120vrms (or 240, etc., depending on
where you live), then there is nothing whatsoever different about the mains
than any other ground-referenced signal. Most common scope applications
deal with ground-referenced signals.

Where things get tricky is when you want to measure a signal that is
relative to the hot side of the line; for instance, you want to measure the
drop across a fuse. Not thinking clearly, you might decide to connect the
probe tip to one side of the fuse, and the probe ground lead to the other.
Kaboom.

So, the isolation transformer thing is really just a trick, a "poor man's
differential probe". It lets you reference your scope signal to anything
you want. But the cost is that the scope chassis is then hot with respect
to earth ground, and you're liable to zap yourself. Better to use a
differential probe, or two probes and the scope's differencing function if
it has one.