"David" wrote in message
...
I'm a bit confused about the ground clip on oscilloscope probes. I have
read that it can cause a ground loop when the measurement circuit is
grounded to the same ground as the oscilloscope itself. Is the ground
clip only to be used when there is an isolated ground then?

I have been taking measurements from 12V car batteries and DC power
supplies. Should the ground clip always be used in these cases?

If I am correct the ground clip should not be used to measure between two
voltages as one would do with a multimeter (referencing a different
voltage in the circuit instead of its ground). It confuses me why I would
not be able to use the ground clip to do that. An explanation/link would
be appreciated.

Line operated equipment and bench oscilloscopes often lead to ground loops.
A big spark erupts when the ground clip touches the chassis! Has probably
happened to most workers at least once. Often, the circuit breaker trips
and that is the end of the matter (other than a burnt clip end and soiled
underwear). The reason is that the clip is grounded through the 3-wire
power cord on the scope. If the equipment is not line isolated, it is
possible for the chassis to be hot with respect to ground. The short
circuit that results is called a ground loop.

Solutions:

1/ Isolation transformer for the equipment (best) or the scope (shaky).
2/ Use the differential mode on a dual channel scope (be careful about
common mode voltage ratings).
3/ Use a separate differential amplifier before the scope.
4/ Use a battery-operated scope (again, one must observe the scope's voltage
ratings).
5/ Make a floating measurement (use an adaptor to defeat the scope ground
.... this is dangerous but is often done ... please AVOID this method).

Hope that helps.