The safety failure of the SD card slot was not actually at 600V, the issue was that for a 600V CATIII rating you need to withstand a 4000V impulse on the terminals with the whole meter wrapped in grounded foil. This is to simulate a lightening strike on a transmission pole while the user is measuring a household voltage with the meter in hand. In that case there was arcing through the SD card slot. Just wanted you to know the exact failure mode in case you hypothetically were going to make dangerous alterations to your meter :)