I looked it up in the manual and it said the power of the EMP had to be greater then the armour of the drone, which to me means if it is the same then the emp would do nothing.

here recently I fired a power 3 emp drone that caught a lvl 2 armour gravstar, lvl 3 beholder, and lvl 4 armour attack drone, thinking it would only destroy the gravstar, but the next turn came and both gravstar and beholder was destroyed

oh, and I did check to see if it wasn't just another emp drone fired and no, it said in tha tdrones list that it destroyed both.

I'll update the rules...