DIYbanter - View Single Post - "Floating Ground"

Ralph Mowery

In article ,
says...

I'm looking at a repair manual for a high powered transistorized audio
amplifier. It repeatedly says:

DO NOT use any test equipment to test or evaluate this amplifier, which
does not have floating grounds.

If the DC voltmeter is AC powered, Float the AC ground wire.

I (sort of) understand what they mean by a "Floating Ground", but I'm
not exactly sure what they mean. And, if I was to use a VTVM which is
plugged into the AC line, or an Oscilloscope (also plugged into an
outlet), how do I achieve a "floating ground"?

Now, lets say I use a pocket battery operated VOM. It's not plugged in,
so there is no ground. Is that safe to use on this device? (Seems to me
that a pocket VOM is NOT actually grounded to earth, so that WOULD be a
floating ground. (I think)....

There are 2 possibilities. The first one that comes to mind is the 'hot
chassis' where one side of the AC line is connected to the chassis.
Usually an isolation transformer is used, or battery powered instruments
that do not have a ground or conductive case.

The other is for mainly transistor audio output amplifiers. The speaker
has both leads floating above ground. If a meter is used that one side
of the test leads go to the chassis and the chassis of the meter is
grounded by the 3 wire AC plug, the amp will be dammaged.

The same for the battery powered meter applies. If it does not have a
conductive chassis and no AC line cord then you can use it.

Another rare,but possiable case is that a 'ground loop' can be made
where a lot of hum or other undesired signals can be made.