Semi blanket statement here but all electronics pcb's which have an emi filter section have a current path to earth as there always caps that bleed to earth in this part of the board. Do some Googling on X-Y capacitors and emi suppression and you will find a lot of stuff. This page is good reading and the circuit diagrams make it easier to understand.
http://www.justradios.com/safetytips.html

Sometimes it's just the aircon gets blamed for being the straw that breaks the camels back when there are numerous devices in the circuit as all things with emi filters have these caps. I remember commissioning a VRF system a while back that had 16 Indoors on the one power circuit that was fed from a standard domestic 16A 30mA MCB/RCD. Most times when the indoors were powered up the RCD would trip but a bit of creative work with the power connectors on the pcb's (leaving half unplugged initially, then plugging them back in 1 by 1 after flicking the breaker on) would allow us to get the indoors powered up and the job commissioned before the site electrician could get back to change the RCD to one with a slower trip time to allow for the inrush as all indoor pcb caps charged up and also with a higher mA tripping capacity.

That was after they had already tried to get us to commission the system when it was powered by a gen set (A big sucker of a machine), that thing just spat the dummy the moment either the outdoors or indoors were energized as its controls were uber sensitive.