"Phil Allison"

** Typical ground loops can be eliminated by fitting a resistor between
the scope chassis ground and safety ground of about 10 to 22 ohms. For
obvious safety reasons this resistor must be protected by shunt diodes in
each polarity.

A simple way is to use a 25 amp rated bridge rectifier and connect the
resistor across the AC input terminals while shorting the " + " and " - "
terminals - giving two diode drops in each direction with current
capacity enough to trip a 20 amp supply breaker if necessary without
damage.

The bridge and resistor can go inside the scope.


** Just to spell it out, the surge current rating of a 25 amp bridge
rectifier is several hundred amps - adequate to survive repeated dead
shorts across the AC supply with no damage.

So, if the earth clip of your probe contacts mains active - the only result
will be a tripped breaker, just as it would be without the above
modification.

Also, diode bridges fail short when overloaded so in this case would fail
safe.



.... Phil