Maybe because Enel is always electricity? Even when he got hit when he was sleeping, he still turned into electricity. That is just what he is. Kinda like Buggy, Buggy can't turn his power on and off, because Zoro cut him, but his body just turned into seperate pieces. His fruit is similar to Luffy's. So, I guess I agree with Aldrich. Robin is sweating as well when she is wearing the handcuffs. It seems that it if effecting her, but not like it effects Luffy.