".....the resistance between the two conductors shall be higher than 20MOhm measured at 100 VDC during 1 minute."

...for 1 minute.
And I know that 1 should be written in letters(one)

Which is correct?

..for one minute is okay.
...over one minute is also okay.

In technical writing of this type, it is very acceptable to use written figures(i.e. 1).

For many, it is okay to use them in all types of writing. This is just style preference. It's completely up to you.


Write whatever is best for your reader.