User`s guide

Remarks
If the detected power line frequency is 400 Hz, the 50 Hz reference value
is actually used (a subharmonic of 400 Hz).
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 measures the
power-line frequency (50 Hz, 60 Hz, or 400 Hz) at power-on.