When Mia eventually reaches the Railyard, she first tells Leo and they both quickly unplug each Synth one by one, thereby saving several. She or Leo should have just tripped the MCB or taken out the main fuse, then it will be like power cut, and the next surge wouldn't have reached any single Synth who was charging and all would have been saved. So much about Mia being a Synth having higher intellect and all.