Hi,
in the system menu of the BK Precision 2831E, there is an entry for "ZERO CAL".
To perform this, the entry of a numeric key is required.
Through trial and error, I found, that the key is 6666.
The volt in socket has to be shorted to the ground socket.
After confirmation with ENTER, the selfcal runs automatically.
I have tested this with the 2831E and this will most likely also work with the sister model 5491B.
Chris