I have seen some arc's from much larger helo's (CH-53), and although they can be a hazard of several thousand volts that one would not want to have a step potential, or re-fueling fume across - they were nowhere near that large... I'll make the assumtion that the arc's seen in this video are a combination of both the aircraft and more so the lines. The other side of the main rotor is upwards of 30'+ away - and figure that is the (much lower) potential that needs to be equalized.
