:Always use both hands when carrying the microscope, one hand at the base and the other holding the arm.
Both hands were always used when carrying.....
PAST because of the WERE, PASSIVE because the sentence never tells you WHO was using both hands..
Walk with it close to your body.
It was carried close to the body.
This example is almost impossible to make passive. A PASSIVE sentence does not answer the question WHO did something... but if a microscope is carried close to YOUR body, well, um.... I think YOU did the carrying. So "It was carried close to the body" is -almost- not really passive.
: Set the microscope at least 5 cm from the edge of the table.
The microscope was set at least 5 cm....
(This is a much clearer example. This is PAST because of the WAS... it it PASSIVE because the sentence never says WHO set the microcope there... the microscope was there, but the sentence does not tell you who put it there.)