Why do I get negative current values for main channel?

When there is a current increase, Arc and Ace increases the voltage internally due to voltage drop over the internal shunt resistors, to keep the set voltage on the banana connectors. The voltage regulation is very fast, but it is not infinite fast, so when the Device Under Test (DUT) has a very fast increase of current consumption, there will be a short voltage drop over the cables and internally Arc/Ace, creating a current rush out from Arc/Ace due to capacitances. The opposite happens when the DUT changes from higher to lower current consumption, then the voltage externally Arc/Ace is higher than internally Arc/Ace, creating a short current rush into the Arc/Ace due to capacitances in the system. To get the correct average current consumption and energy measurement, both of the overshoot and undershoot must be taken into consideration.