Fairies will become weak when exposed to electricity, which is a manmade substance.
Not... really. Electricity is a consequence of the laws of physics. As electrons move, they produce a charge, that will release when 2 opposite charges come into close enough proximity. That's why lightning happens -- friction in the clouds produces a charge, which discharges when it connects to an opposite charge, usually through a tree, or a building, or a even a person. Humans just happen to have found electricity supremely useful, and as such have woven it into basically every aspect of our lives.
I'd also question the notion that if it's man made, it's automatically harmful to a fairy. Depending on your source, a fairy can be harmed by all metals, or just by Iron specifically. I don't know any sources where a fairy is harmed specifically by metal that a human has worked, and for that specific of a concept I'd ask you for a source. That said, it's almost exclusively going to be due to human activities that a fairy will be exposed to metals of any variety, which might be the cause of the confusion. All metals exist in nature, they are simply buried in the ground where, typically, a fairy won't have any occasion to interact with it.
And with regards to Fire type, do note that Fire is resistant to Fairy. It's not the same as Fire being super effective against Fairy, but it does pay homage to the relationship.