User`s guide

Remarks
The instrument multiplies the period of the reference frequency by the
specified number of power line cycles ([SENSe:]<function>:NPLC
commands) to determine the actual integration time.
The reference frequency setting is stored in volatile memory and will be
lost when power is turned off. The instrument automatically detects the
power-line frequency (50 Hz, 60 Hz, or 400 Hz) at power-on.
If the instrument detects a 400 Hz power line frequency, the 50 Hz
reference frequency value is actually used (a subharmonic of 400 Hz).