6517B

I would like to use the 6571B to measure leakage current in a high voltage DC line. What is the maximum voltage that the 6517B can withstand without breakdown?

The data sheet indicates:  
Maximum input: 250 Vpeak DC to 60 Hz sine wave; 10 s per minute at maximum input on mA ranges (one-sixth
duty cycle)

Maximum common mode voltage (DC to 60 Hz sine wave): Electrometer, 500 Vpeak; V-source, 750 Vpeak

You may want to mount some protective diodes.  See attached from our Low Level Measurements Handbook.
Or download a copy:  Low Level Measurements Handbook