Re: Scope grounding and generators

Chuck Harris

Isolating the scope may protect the scope from catastrophic
currents, but it does nothing to protect the scope's operator.

The front panel on most lab grade scopes is grounded to
the same ground as the BNC's, and as such the scope probes.

If you connect the scope probe's ground to a voltage that
is other than earth ground, the scope's front panel and
chassis will be elevated to the same voltage as the scope's
probe's ground clip lead.

Don't do it!

If your intention is to connect anything to *ANY* pin on
a running generator, the generator must have its grounding
point connected to a good earth ground.

If you do not ground the generator any single point fault
in the generator or load that shorts the hot lead to earth
ground will elevate the generator's case 120 or 240V above

If your intention is to use a 465M, it too must have its
chassis grounding point connected to a good earth ground.

The only valid exceptions involve a whole lot of thinking,
and isolating of the operator, scope, and generator from
dangerous contact.

Isolating grounds on test equipment and generators is a
very dangerous "rookie" mistake.

Read the "OPERATORS SAFETY SUMMARY" that is in some form
in every Tektronix scope's manual since Tektronix stopped
being rookies themselves.

Why do I harp on such stuff?

Because I made that rookie mistake when I was a teenager
working alone. I was tuning up a transmitter, and for
stability, I had the hand holding the diddle stick on the
chassis of the transmitter, when I smelled smoke. Wanting
to avert a catastrophe, I quickly reached over with my
other hand and flipped the metal bat handled power switch
on my intentionally ungrounded (for "safety" reasons) power

I woke up in a very battered state crumpled on the floor,
up against a cement block wall.

You don't have to repeat this experiment. It has already
been done too many times.

-Chuck Harris

Jean-Paul wrote:

Robert: Most generators have a distorted sine output, depends on rating, and design.
Some have quite high THD. Other have terrible transients and EMI. Normally the mfg will have complete specs.

Rule of thumb: Larger size and high cost - (10..50 kVA) are closer to sinewave, low THD and transients
Cheaper consumer generators eg from Costco, under a few KVA, worse in every respect!

What is your intended use, and what power levels, make and model of generator?

I have used battery operated portables like TEK 212, rather than line powered scopes like 465M.

But you can float the 465 with an isolation trsf.

Good luck,


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